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Geocaching IPhone App Review

Fri, 13 Nov 2009 07:18:30 +0100

by Chiffan from touchmyapps

Ever since I read Treasure Island, my mind has been full of dreams of finding buried treasures and going on pirate adventures. Well, that was when I was but a wee little lad – since then I’ve cleared my head of such nonsense, but still, the idea of how great it would be to go on a real treasure hunt haunts me. And then I heard of Geocaching…

Ok, to start it off, Geocaching is kind of a game, akin to 150-year-old letterboxing, and appeared shortly after the removal of “Select Availability” from GPS receivers back in 2000. It involves using a GPS receiver to hide and seek containers (“Geocaches”) anywhere in the world. So think of it as extremely advanced treasure hunting. The first cache was laid by Dave Ulmer and as of today, Nov 2 2009, there are over 933 160 active geocaches all over the world. And it was him who formulated the basic rule of the game, used to this day: You’ve got to log your find in the logbook in the cache and if you take something, you must leave something of equal or greater value. And that’s it.

Talking about the app, Geocaching, it is actually basically an interface to the site Geocaching.com (the largest and oldest of the geocaching sites) but with a lot of great features thrown in. The app requires a valid Geocaching.com account and the info is synced between the app and website.

Talking about features – you can do a full-fledged search using the app, including a quick search for nearby caches, or searching by zip code, address or cache code. You can also view the whole list of results on the map. What is really great is that you can save the results for offline use. This is extremely useful, since you can mark what caches interest you and not waste any time looking them up each time. And it’s also invaluable if you’re abroad. If you want, you can also hide already discovered caches from the search.

After you select the cache you’re going to seek out, you can view the route to it on the map. The map is pulled from Microsoft Virtual Earth or OpenStreetMap.org (configurable in settings). Personally I would recommend the Microsoft one. The app can also display the topographic (via OpenCycleMap.org) or satellite view (also from MS Virtual Earth). You can also place waypoints to ease navigation. The downside is that the map is not available in offline mode, which is odd, considering you can save the cache info from offline use. This made it extremely difficult for me to find the cache when I was on my vacation abroad and should definitely be addressed in the next update.

Once you find a cache, you’re supposed to post this on the website. You can do this using the app, as well as mark the cache found. If you don’t have Internet access, the post is stored and can be later sent out from the settings tab. Another thing, useful for hardcore geocachers is the ability to follow their trackables (special items, placed in caches to be moved to other ones by friendly geocachers).

The settings are quite simple, allowing you to toggle Metric or Imperial units, choose the data sources for the maps and filters. The help section features an extremely simple tutorial, which outlines the process and rules of geocaching in simple statements and comic-style pictures.

All in all, if you haven’t yet joined the geocaching community, you should definitely try it out – it is a really fun way to spend the day. It is a real adventure and even my wife loved it (which says more than you can possibly imagine). I would recommend for you to try out the activity using the website or the Geocaching Intro (Access three geocaches near your current location only) in advance though, since it is free and will give you a feel for whether you’re ready to turn out $9.99 for the app. But for any geocacher, this is definitely a must. And the iPhone is perfect for this activity!

Geocaching GPS Treasure Hunt Review Source

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A great Geocaching IPhone App review from touchmyapps. It is a good deal, good value for the experience. And they're right "the IPhone is perfect for this activity".

GPS, how about Longitude...

Fri, 13 Nov 2009 07:08:30 +0100

Longitude (pronounced /ˈlɒndʒɨtjuːd/ or /ˈlɒŋɡɨtjuːd/),^[1] identified by the Greek letter lambda (λ), is the geographic coordinate most commonly used in cartography and global navigation for east-west measurement. The line of longitude (meridian) that passes through the Royal Observatory, Greenwich, in England, establishes the meaning of zero degrees of longitude, or the prime meridian. Any other longitude is identified by the east-west angle, referenced to the center of the Earth as vertex, between the intersections with the equator of the meridian through the location in question and the prime meridian. A location's position along a meridian is given by its latitude, which is identified by the north-south angle between the local vertical and the plane of the equator.

History

Mariners and explorers for most of history struggled to determine precise longitude. Latitude was calculated by observing with quadrant or astrolabe the inclination of the sun or of charted stars, but longitude presented no such manifest means of study. Amerigo Vespucci was perhaps the first to proffer a solution, after devoting a great deal of time and energy studying the problem during his sojourns in the New World:

By comparing the relative positions of the moon and Mars with their anticipated positions, Vespucci was able to crudely deduce his longitude. But this method had several limitations: First, it required the occurrence of a specific astronomical event (in this case, Mars passing through the same right ascension as the moon), and the observer needed to anticipate this event via an astronomical almanac. One needed also to know the precise time, which was difficult to ascertain in foreign lands. Finally, it required a stable viewing platform, rendering the technique useless on the rolling deck of a ship at sea.

Unlike latitude, which has the equator as a natural starting position, there is no natural starting position for longitude. Therefore, a reference meridian had to be chosen. It was a popular practice to use a nation's capital as the starting point, but other significant locations were also used. While British cartographers had long used the Greenwich meridian in London, other references were used elsewhere, including: El Hierro, Rome, Copenhagen, Jerusalem, Saint Petersburg, Pisa, Paris, Philadelphia, and Washington. In 1884, the International Meridian Conference adopted the Greenwich meridian as the universal prime meridian or zero point of longitude.

Noting and calculating longitude

Longitude is given as an angular measurement ranging from 0° at the prime meridian to +180° eastward and −180° westward. The Greek letter λ (lambda),^[3]^[4] is used to denote the location of a place on Earth east or west of the prime meridian.

Each degree of longitude is sub-divided into 60 minutes, each of which divided into 60 seconds. A longitude is thus specified in sexagesimal notation as 23° 27′ 30" E. For higher precision, the seconds are specified with a decimal fraction. An alternative representation uses degrees and minutes, where parts of a minute are expressed in decimal notation with a fraction, thus: 23° 27.500′ E. Degrees may also be expressed as a decimal fraction: 23.45833° E. For calculations, the angular measure may be converted to radians, so longitude may also be expressed in this manner as a signed fraction of π (pi), or an unsigned fraction of 2π.

For calculations, the West/East suffix is replaced by a negative sign in the western hemisphere. Confusingly, the convention of negative for East is also sometimes seen. The preferred convention—that East be positive—is consistent with a right-handed Cartesian coordinate system with the North Pole up. A specific longitude may then be combined with a specific latitude (usually positive in the northern hemisphere) to give a precise position on the Earth's surface.

Longitude at a point may be determined by calculating the time difference between that at its location and Coordinated Universal Time (UTC). Since there are 24 hours in a day and 360 degrees in a circle, the sun moves across the sky at a rate of 15 degrees per hour (360°/24 hours = 15° per hour). So if the time zone a person is in is three hours ahead of UTC then that person is near 45° longitude (3 hours × 15° per hour = 45°). The word near was used because the point might not be at the center of the time zone; also the time zones are defined politically, so their centers and boundaries often do not lie on meridians at multiples of 15°. In order to perform this calculation, however, a person needs to have a chronometer (watch) set to UTC and needs to determine local time by solar observation or astronomical observation. The details are more complex than described here: see the articles on Universal Time and on the equation of time for more details.

Plate movement and longitude

The surface layer of the Earth, the lithosphere, is broken up into several tectonic plates. Each plate moves in a different direction, at speeds of about 50 to 100 mm per year.^[5] As a result, for example, the longitudinal difference between a point on the equator in Uganda (on the African Plate) and a point on the equator in Ecuador (on the South American Plate) is increasing by about 0.0014 arcseconds per year.

If a global reference frame such as WGS84 is used, the longitude of a place on the surface will change from year to year. To minimize this change, when dealing exclusively with points on a single plate, a different reference frame can be used, whose coordinates are fixed to a particular plate, such as NAD83 for North America or ETRS89 for Europe.

Elliptic parameters

Because most planets (including Earth) are closer to ellipsoids of revolution, or spheroids, rather than to spheres, both the radius and the length of arc varies with latitude. This variation requires the introduction of elliptic parameters based on an ellipse's angular eccentricity, (which equals , where and are the equatorial and polar radii; is the first eccentricity squared, ; and or is the flattening, ). Utilized in creating the integrands for curvature is the inverse of the principal elliptic integrand, :

Degree length

The length of an arcdegree of north-south latitude difference, , is about 60 nautical miles, 111 kilometres or 69 statute miles at any latitude. The length of an arcdegree of east-west longitude difference, , is about the same at the equator as the north-south, reducing to zero at the poles.

In the case of a spheroid, a meridian and its anti-meridian form an ellipse, from which an exact expression for the length of an arcdegree of latitude is:

This radius of arc (or "arcradius") is in the plane of a meridian, and is known as the meridional radius of curvature, .^[6]^[7]

Similarly, an exact expression for the length of an arcdegree of longitude is:

The arcradius contained here is in the plane of the prime vertical, the east-west plane perpendicular (or "normal") to both the plane of the meridian and the plane tangent to the surface of the ellipsoid, and is known as the normal radius of curvature, .^[6]^[7]

Along the equator (east-west), equals the equatorial radius. The radius of curvature at a right angle to the equator (north-south), , is 43 km shorter, hence the length of an arcdegree of latitude at the equator is about 1 km less than the length of an arcdegree of longitude at the equator. The radii of curvature are equal at the poles where they are about 64 km greater than the north-south equatorial radius of curvature because the polar radius is 21 km less than the equatorial radius. The shorter polar radii indicate that the northern and southern hemispheres are flatter, making their radii of curvature longer. This flattening also 'pinches' the north-south equatorial radius of curvature, making it 43 km less than the equatorial radius. Both radii of curvature are perpendicular to the plane tangent to the surface of the ellipsoid at all latitudes, directed toward a point on the polar axis in the opposite hemisphere (except at the equator where both point toward Earth's center). The east-west radius of curvature reaches the axis, whereas the north-south radius of curvature is shorter at all latitudes except the poles.

The WGS84 ellipsoid, used by all GPS devices, uses an equatorial radius of 6378137.0 m and an inverse flattening, (1/f), of 298.257223563, hence its polar radius is 6356752.3142 m and its first eccentricity squared is 0.00669437999014.^[8] The more recent but little used IERS 2003 ellipsoid provides equatorial and polar radii of 6378136.6 and 6356751.9 m, respectively, and an inverse flattening of 298.25642.^[9] Lengths of degrees on the WGS84 and IERS 2003 ellipsoids are the same when rounded to six significant digits. An appropriate calculator for any latitude is provided by the U.S. government's National Geospatial-Intelligence Agency (NGA).^[10]

Latitude	N-S radius of curvature	Surface distance per 1° change in latitude	E-W radius of curvature	Surface distance per 1° change in longitude
0°	6335.44 km	110.574 km	6378.14 km	111.320 km
15°	6339.70 km	110.649 km	6379.57 km	107.551 km
30°	6351.38 km	110.852 km	6383.48 km	96.486 km
45°	6367.38 km	111.132 km	6388.84 km	78.847 km
60°	6383.45 km	111.412 km	6394.21 km	55.800 km
75°	6395.26 km	111.618 km	6398.15 km	28.902 km
90°	6399.59 km	111.694 km	6399.59 km	0.000 km

Ecliptic latitude and longitude

Ecliptic latitude and longitude are defined for the planets, stars, and other celestial bodies in a broadly similar way to that in which terrestrial latitude and longitude are defined, but there is a special difference.

The plane of zero latitude for celestial objects is not parallel to the plane of the celestial and terrestrial equator: it is the plane of the ecliptic. This is inclined to the equator by the "obliquity of the ecliptic", currently about 23° 26'. The closest celestial counterpart to terrestrial latitude is declination, and the closest celestial counterpart to terrestrial longitude is right ascension. These celestial coordinates bear the same relationship to the celestial equator as terrestrial latitude and longitude do to the terrestrial equator, and they are also more frequently used in astronomy than celestial longitude and latitude.

The polar axis (relative to the celestial equator) is perpendicular to the plane of the equator, and parallel to the terrestrial polar axis. But the (north) pole of the ecliptic, relevant to the definition of ecliptic latitude, is the normal to the ecliptic plane nearest to the direction of the celestial north pole of the equator, i.e. 23° 26' away from it.

Ecliptic latitude is measured from 0° to 90° north (+) or south (−) of the ecliptic. Ecliptic longitude is measured from 0° to 360° eastward (the direction that the Sun appears to move relative to the stars), along the ecliptic from the vernal equinox. The equinox at a specific date and time is a fixed equinox, such as that in the J2000 reference frame.

However, the equinox moves because it is the intersection of two planes, both of which move. The ecliptic is relatively stationary, wobbling within a 4° diameter circle relative to the fixed stars over millions of years under the gravitational influence of the other planets. The greatest movement is a relatively rapid gyration of Earth's equatorial plane whose pole traces a 47° diameter circle caused by the Moon. This causes the equinox to precess westward along the ecliptic about 50" per year. This moving equinox is called the equinox of date. Ecliptic longitude relative to a moving equinox is used whenever the positions of the Sun, Moon, planets, or stars at dates other than that of a fixed equinox is important, as in calendars, astrology, or celestial mechanics. The 'error' of the Julian or Gregorian calendar is always relative to a moving equinox. The years, months, and days of the Chinese calendar all depend on the ecliptic longitudes of date of the Sun and Moon. The 30° zodiacal segments used in astrology are also relative to a moving equinox. Celestial mechanics (here restricted to the motion of solar system bodies) uses both a fixed and moving equinox. Sometimes in the study of Milankovitch cycles, the invariable plane of the solar system is substituted for the moving ecliptic. Longitude may be denominated from 0 to radians in either case.

Longitude on bodies other than Earth

Planetary co-ordinate systems are defined relative to their mean axis of rotation and various definitions of longitude depending on the body. The longitude systems of most of those bodies with observable rigid surfaces have been defined by references to a surface feature such as a crater. The north pole is that pole of rotation that lies on the north side of the invariable plane of the solar system (near the ecliptic). The location of the prime meridian as well as the position of body's north pole on the celestial sphere may vary with time due to precession of the axis of rotation of the planet (or satellite). If the position angle of the body's prime meridian increases with time, the body has a direct (or prograde) rotation; otherwise the rotation is said to be retrograde.

In the absence of other information, the axis of rotation is assumed to be normal to the mean orbital plane; Mercury and most of the satellites are in this category. For many of the satellites, it is assumed that the rotation rate is equal to the mean orbital period. In the case of the giant planets, since their surface features are constantly changing and moving at various rates, the rotation of their magnetic fields is used as a reference instead. In the case of the Sun, even this criterion fails (because its magnetosphere is very complex and does not really rotate in a steady fashion), and an agreed-upon value for the rotation of its equator is used instead.

For planetographic longitude, west longitudes (i.e., longitudes measured positively to the west) are used when the rotation is prograde, and east longitudes (i.e., longitudes measured positively to the east) when the rotation is retrograde. In simpler terms, imagine a distant, non-orbiting observer viewing a planet as it rotates. Also suppose that this observer is within the plane of the planet's equator. A point on the equator that passes directly in front of this observer later in time has a higher planetographic longitude than a point that did so earlier in time.

However, planetocentric longitude is always measured positively to the east, regardless of which way the planet rotates. East is defined as the counter-clockwise direction around the planet, as seen from above its north pole, and the north pole is whichever pole more closely aligns with the Earth's north pole. Longitudes traditionally have been written using "E" or "W" instead of "+" or "−" to indicate this polarity. For example, the following all mean the same thing:

−91°
91°W
+269°
269°E.

The reference surfaces for some planets (such as Earth and Mars) are ellipsoids of revolution for which the equatorial radius is larger than the polar radius; in other words, they are oblate spheroids. Smaller bodies (Io, Mimas, etc.) tend to be better approximated by triaxial ellipsoids; however, triaxial ellipsoids would render many computations more complicated, especially those related to map projections. Many projections would lose their elegant and popular properties. For this reason spherical reference surfaces are frequently used in mapping programs.

The modern standard for maps of Mars (since about 2002) is to use planetocentric coordinates. The meridian of Mars is located at Airy-0 crater.^[11]

Tidally-locked bodies have a natural reference longitude passing through the point nearest to their parent body.^[12] However, libration due to non-circular orbits or axial tilts causes this point to move around any fixed point on the celestial body like an analemma.

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As with my previous post, have you ever wondered what is that longitude in your GPS? Why did you even bother? Haha!

GPS, Latitude...

Fri, 13 Nov 2009 07:05:43 +0100

Latitude, usually denoted by the Greek letter phi (φ) gives the location of a place on Earth (or other planetary body) north or south of the equator. Lines of Latitude are the imaginary horizontal lines shown running east-to-west (or west to east) on maps (particularly so in the Mercator projection) that run either north or south of the equator. Technically, latitude is an angular measurement in degrees (marked with °) ranging from 0° at the equator (low latitude) to 90° at the poles (90° N or +90° for the North Pole and 90° S or −90° for the South Pole). The latitude is approximately the angle between straight up at the surface (the zenith) and the sun at an equinox. The complementary angle of a latitude is called the colatitude.

Circles of latitude

All locations of a given latitude are collectively referred to as a circle of latitude or line of latitude or parallel, because they are coplanar, and all such planes are parallel to the equator. Lines of latitude other than the Equator are approximately small circles on the surface of the Earth; they are not geodesics since the shortest route between two points at the same latitude involves a path that bulges toward the nearest pole, first moving farther away from and then back toward the equator (see great circle).

A specific latitude may then be combined with a specific longitude to give a precise position on the Earth's surface (see satellite navigation system).

Important named circles of latitude

Besides the equator, four other lines of latitude are named because of the role they play in the geometrical relationship with the Earth and the Sun:

Arctic Circle: 66° 33′ 39″ N
Tropic of Cancer: 23° 26′ 21″ N
Tropic of Capricorn: 23° 26′ 21″ S
Antarctic Circle: 66° 33′ 39″ S

Only at latitudes between the Tropics is it possible for the sun to be at the zenith. Only north of the Arctic Circle or south of the Antarctic Circle is the midnight sun possible.

The reason that these lines have the values that they do lies in the axial tilt of the Earth with respect to the sun, which is 23° 26′ 21.41″.

Note that the Arctic Circle and Tropic of Cancer are colatitudes, since the sum of their angles is 90°—similarly for the Antarctic Circle and Tropic of Capricorn.

Subdivisions

A degree is divided into 60 minutes. One minute can be further divided into 60 seconds. An example of a latitude specified in this way is 13°19'43″ N (for greater precision, a decimal fraction can be added to the seconds). An alternative representation uses only degrees and minutes, where the seconds are expressed as a decimal fraction of minutes: the above example would be expressed as 13°19.717' N. Degrees can also be expressed singularly, with both the minutes and seconds incorporated as a decimal number and rounded as desired (decimal degree notation): 13.32861° N. Sometimes, the north/south suffix is replaced by a negative sign for south (−90° for the South Pole).

Effect of latitude

A region's latitude has a great effect on its climate and weather (see Effect of sun angle on climate). Latitude more loosely determines tendencies in polar auroras, prevailing winds, and other physical characteristics of geographic locations.

Researchers at Harvard's Center for International Development (CID) found in 2001 that only three tropical economies — Hong Kong, Singapore, and Taiwan — were classified as high-income by the World Bank, while all countries within regions zoned as temperate had either middle- or high-income economies. ^[1] The validity of the Harvard report may be questioned because a different threshold is used for the tropical regions and the World Bank list fails to include Qatar's, United Arab Emirates', and Kuwait's economies. Further, countries such as Brazil have far better incomes than much of the Former Soviet Union and Iron Curtain states^{[citation needed]}.

Elliptic parameters

Because most planets (including Earth) are ellipsoids of revolution, or spheroids, rather than spheres, both the radius and the length of arc varies with latitude. This variation requires the introduction of elliptic parameters based on an ellipse's angular eccentricity, (which equals , where and are the equatorial radius (6378137.0 m for Earth) and the polar radius (6356752.3142 m for Earth), respectively; is the first eccentricity squared, ; and or is the flattening, ). Utilized in creating the integrands for curvature is the inverse of the principal elliptic integrand, :

Degree length

On Earth, the length of an arcdegree of north–south latitude difference, , is about 60 nautical miles, 111 kilometres or 69 statute miles at any latitude. The length of an arcdegree of east-west longitude difference, , is about the same at the equator as the north-south, reducing to zero at the poles.

In the case of a spheroid, a meridian and its anti-meridian form an ellipse, from which an exact expression for the length of an arcdegree of latitude difference is:

This radius of arc (or "arcradius") is in the plane of a meridian, and is known as the meridional radius of curvature, .^[2]^[3]

Similarly, an exact expression for the length of an arcdegree of longitude difference is:

Along the equator (east-west), equals the equatorial radius. The radius of curvature at a right angle to the equator (north-south), , is 43 km shorter, hence the length of an arcdegree of latitude difference at the equator is about 1 km less than the length of an arcdegree of longitude difference at the equator. The radii of curvature are equal at the poles where they are about 64 km greater than the north-south equatorial radius of curvature because the polar radius is 21 km less than the equatorial radius. The shorter polar radii indicate that the northern and southern hemispheres are flatter, making their radii of curvature longer. This flattening also 'pinches' the north-south equatorial radius of curvature, making it 43 km less than the equatorial radius. Both radii of curvature are perpendicular to the plane tangent to the surface of the ellipsoid at all latitudes, directed toward a point on the polar axis in the opposite hemisphere (except at the equator where both point toward Earth's center). The east-west radius of curvature reaches the axis, whereas the north-south radius of curvature is shorter at all latitudes except the poles.

The WGS84 ellipsoid, used by all GPS devices, uses an equatorial radius of 6378137.0 m and an inverse flattening, (1/f), of 298.257223563, hence its polar radius is 6356752.3142 m and its first eccentricity squared is 0.00669437999014.^[4] The more recent but little used IERS 2003 ellipsoid provides equatorial and polar radii of 6378136.6 and 6356751.9 m, respectively, and an inverse flattening of 298.25642.^[5] Lengths of degrees on the WGS84 and IERS 2003 ellipsoids are the same when rounded to six significant digits. An appropriate calculator for any latitude is provided by the U.S. government's National Geospatial-Intelligence Agency (NGA).^[6]

Latitude	N-S radius of curvature	Surface distance per 1° change in latitude	E-W radius of curvature	Surface distance per 1° change in longitude
0°	6335.44 km	110.574 km	6378.14 km	111.320 km
15°	6339.70 km	110.649 km	6379.57 km	107.551 km
30°	6351.38 km	110.852 km	6383.48 km	96.486 km
45°	6367.38 km	111.132 km	6388.84 km	78.847 km
60°	6383.45 km	111.412 km	6394.21 km	55.800 km
75°	6395.26 km	111.618 km	6398.15 km	28.902 km
90°	6399.59 km	111.694 km	6399.59 km	0.000 km

Types of latitude

With a spheroid that is slightly flattened by its rotation, cartographers refer to a variety of auxiliary latitudes to precisely adapt spherical projections according to their purpose.
For planets other than Earth, such as Mars, geographic and geocentric latitude are called "planetographic" and "planetocentric" latitude, respectively. Most maps of Mars since 2002 use planetocentric coordinates.

Common "latitude"

In common usage, "latitude" refers to geodetic or geographic latitude and is the angle between the equatorial plane and a line that is normal to the reference ellipsoid, which approximates the shape of Earth to account for flattening of the poles and bulging of the equator. This value usually differs from the geocentric latitude.

The expressions following assume elliptical polar sections and that all sections parallel to the equatorial plane are circular. Geographic latitude (with longitude) then provides a Gauss map. As defined earlier in this article, is the angular eccentricity of a meridian.

Reduced latitude

On a spheroid, lines of reduced or parametric latitude, , form circles whose radii are the same as the radii of circles formed by the corresponding lines of latitude on a sphere with radius equal to the equatorial radius of the spheroid.

Authalic latitude

Authalic latitude, , gives an area-preserving transform to the sphere.

Rectifying latitude

Rectifying latitude, , is the surface distance from the equator, scaled so the pole is 90°, but involves elliptic integration:

Conformal latitude

Conformal latitude, , gives an angle-preserving (conformal) transform to the sphere.

Geocentric latitude

The geocentric latitude, , is the angle between the equatorial plane and a line from the center of Earth.

It is the size of the central angle between the equator and the point of interest, as measured along a meridian. This value usually differs from the geographic latitude, as so:

Astronomical latitude

A more obscure measure of latitude is the astronomical latitude, which is the angle between the equatorial plane and the normal to the geoid (ie a plumb line). It originated as the angle between horizon and pole star. It differs from the geodetic latitude only slightly, due to the slight deviations of the geoid from the reference ellipsoid.

Astronomical latitude is not to be confused with declination, the coordinate astronomers use to describe the locations of stars north/south of the celestial equator (see equatorial coordinates), nor with ecliptic latitude, the coordinate that astronomers use to describe the locations of stars north/south of the ecliptic (see ecliptic coordinates).

Palaeolatitude

Continents move over time, due to continental drift, taking whatever fossils and other features of interest they may have with them. Particularly when discussing fossils, it's often more useful to know where the fossil was when it was laid down, than where it is when it was dug up: this is called the palæolatitude of the fossil. The Palæolatitude can be constrained by palæomagnetic data. If tiny magnetisable grains are present when the rock is being formed, these will align themselves with Earth's magnetic field like compass needles. A magnetometer can deduce the orientation of these grains by subjecting a sample to a magnetic field, and the magnetic declination of the grains can be used to infer the latitude of deposition.

Comparison of selected types

The following plot shows the differences between the types of latitude. The data used are found in the table following the plot. Please note that the values in the table are in minutes, not degrees, and the plot reflects this as well. Also observe that the conformal symbols are hidden behind the geocentric due to being very close in value. Finally it is important to mention also that these differences don't mean that the use of one specific latitude will necessarily cause more distortions than the other (the real fact is that each latitude type is optimized for achieving a different goal).

Approximate difference from geographic latitude ("Lat")
Lat	Reduced	Authalic	Rectifying	Conformal	Geocentric
0°	0.00′	0.00′	0.00′	0.00′	0.00′
5°	1.01′	1.35′	1.52′	2.02′	2.02′
10°	1.99′	2.66′	2.99′	3.98′	3.98′
15°	2.91′	3.89′	4.37′	5.82′	5.82′
20°	3.75′	5.00′	5.62′	7.48′	7.48′
25°	4.47′	5.96′	6.70′	8.92′	8.92′
30°	5.05′	6.73′	7.57′	10.09′	10.09′
35°	5.48′	7.31′	8.22′	10.95′	10.96′
40°	5.75′	7.66′	8.62′	11.48′	11.49′
45°	5.84′	7.78′	8.76′	11.67′	11.67′
50°	5.75′	7.67′	8.63′	11.50′	11.50′
55°	5.49′	7.32′	8.23′	10.97′	10.98′
60°	5.06′	6.75′	7.59′	10.12′	10.13′
65°	4.48′	5.97′	6.72′	8.95′	8.96′
70°	3.76′	5.01′	5.64′	7.52′	7.52′
75°	2.92′	3.90′	4.39′	5.85′	5.85′
80°	2.00′	2.67′	3.00′	4.00′	4.01′
85°	1.02′	1.35′	1.52′	2.03′	2.03′
90°	0.00′	0.00′	0.00′	0.00′	0.00′

Corrections for altitude

When converting from geodetic ("common") latitude to other types of latitude, corrections must be made for altitude for systems which do not measure the angle from the normal of the spheroid. For example, in the figure at right, point H (located on the surface of the spheroid) and point H' (located at some greater elevation) have different geocentric latitudes (angles β and γ respectively), even though they share the same geodetic latitude (angle α). Note that the flatness of the spheroid and elevation of point H' in the image is significantly greater than what is found on the Earth, exaggerating the errors inherent in such calculations if left uncorrected. Note also that the reference ellipsoid used in the geodetic system is itself just an approximation of the true geoid, and therefore introduces its own errors, though the differences are less severe. (See Astronomical latitude, above.)

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Have you really ever wondered? "Hey, what is this latitude in my GPS? I use it for geocaching treasure hunts. What is it?" Then you wonder, "Why did I even ask it?". Haha!

A little Geocaching Humor

Fri, 13 Nov 2009 07:02:48 +0100

"Geocachers, please don’t take offensive to this t-shirt"

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Hahahaha! This is a good one from Crunchgear. But for the geocachers, it's probably more like "Thank you for paying tax for the satellites that we use for our hobby". Hahaha!

IPhone Apps for the Geocaching enthusiast

Fri, 13 Nov 2009 06:51:13 +0100

Geocaching is becoming more a popular hobby for the outdoor activities seeker and "treasure hunting" enthusiast. Most often people are unfamiliar with the concept, and some assume that you need expensive equipment to have an enjoyable geocaching experience. Thankfully, you can go geocaching with your iPhone! without having to spend too much. Yahoo! As long as you are not addicted to your couch, you are going to enjoy experiencing different geocaching missions.

Let us check out these some useful Apps:

Geosphere: Geosphere is yet another cool geo-caching iPhone app that enables you to get outdoors and have fun. What I like about this app is the fact that it lets you search for missions based on difficulty and other factors.

iGeocacher: a sophisticated geo-caching app for iPhone that makes it so easy to go outdoors without having to use a single piece of paper. The interface is easy to use and the app works offline as well.

Geocaching: one of the most popular geo-caching apps for iPhone. Comes with 895,000 geo-caches that can keep you entertained forever.

Geocaching Buddy: more of an add on for iGeocacher. It integrates well with that app and makes it easier to complete your missions. It helps you remember your clues and get more done outdoors.

GPS Mission Pro: a cool app that lets you play games all around your neighborhood and quit your couch addiction.

MotionX GPS Sport: a complete GPS app that not only lets you get around using your phone, it makes it easy to go geocaching as well. Gives you great bang for your buck.

Geocaching GPS Treasure Hunt Information Source

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This are real great tips on IPhone Apps for the geocaching experience. Thanks! GPS Mission Pro sounds fun.

Hiking, Exercise can be so Rewarding: Geocaching

Fri, 13 Nov 2009 06:42:35 +0100

Cache In, Trash Out: The New Geocaching Mantra of Eco-Resorts

Take a hike; get a gift-not a bad way to spend a green day.

By Laurel House

Forget about golf, hotels are getting savvy when it comes to keeping their guests engaged in green activities that not only help clean up the environment, but give them a gift in return. Yes, there are two things that the average human being loves: the "hunt," and presents. Mix the two with the great outdoors and exercise and you've got GeoCaching--a form of high-tech treasure hunting that's inviting a childlike exuberance to emerge from even the most stale adults.

The outdoor game has players using GPS tracking systems to coordinate hidden treasure caches hidden in little boxes in obscure spots all over the world. From rarely explored waterfalls to city street corners, the world-wide scavenger hunt is getting people of all ages and abilities outside and into this greener exercise activity. Each cache consists of a visitor's log so that gamers can mark their territory and even leave a note for the next cache guest. Many caches also have little gifts with a take-one, leave-one concept that can be anything from dice to paper airplanes, jewelry and even tickets to sporting events.

No geocaches in your immediate area? Create, hide and register one yourself http://www.wikihow.com/Create-and-Hide-a-Geocache! It's a great way to change up your work out and even get kids to exercise without knowing it! With over 933,945 active geocaches around the world, you are almost certain to have a green adventure in your area. Or take a trip to one of these resorts to learn how the game works first:

Where Can You Learn to GeoCache?

Ojai Valley Inn and Spa, an environmentally conscious resort in Southern California, is a great place to learn the basics of geocaching with a guide-led hike complete with all of the accouterments including a backpack, GPS unit, energy bars, water, and maps. Highfalutin guests scour the surrounding mountains for secret caches while encouraging each person to remove any spotted trash. The idea is "cache in, trash out"--essentially incentive-driven trail clean-ups. The resort's geocaching crew makes sure that caches are stocked with fun little gifts like locally made bracelets and crafts to up the ante and keep the crowds scouring the trails for cache and trash.

Red Mountain Spa in Utah provides GPS units to interested guests then teaches the newbie geo-adventurers how to log and read coordinates (longitude and latitude), and finally lead them on a search of hidden caches throughout the 55-acre property. The tough terrain offers a very active trek, as buccaneers forge through streams, jump ravines and scale rocky hillsides all for the sake of treasure hunting! Since the caches are contained on the Red Mountain property and its surrounding, caches can include gift certificates to the spa, resort skincare products, and fitness gear!

Who knew exercise could be so rewarding!

Geocaching GPS Treasure Hunt Information Source

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"Who knew exercise could be so rewarding" indeed. This could make hiking even more an enjoyable experience. This is great for the young and even greater for the young at heart and spirit.

Trackable Treasures Collectors: Geocoins

Fri, 13 Nov 2009 06:33:05 +0100

A geocoin is a metal or wooden coin minted in similar fashion to a medallion, token coin, military challenge coin or wooden nickel, for use in geocaching. Some geocoins are trackable on the internet using a serial number and website address engraved on the coin.

Personal geocoins are a personal signature item bearing the geocacher's handle and personal design, similar to a heraldic device. Geocoins are often minted by caching organizations and as fund-raisers for geocaching events.

Geocoins with tracking numbers that have been registered on their associated websites are said to be "activated", whereas geocoins that are still unregistered are termed "unactivated". Activated geocoins that have been left in a cache are meant to be moved from cache to cache like a Travel Bug, whereas unactivated geocoins may be placed in geocaches to be found by others and kept as trophies. Unactivated coins may be also traded or given to other cachers like a calling card, as prizes, as awards, or merely sold and collected.

Trackable Geocoins

When a cache listed at geocaching.com contains a geocoin, an icon (often unique to the coin) is shown on the cache page's "Inventory" section. This icon will also appear in the inventory of any cacher holding one as well as in each cacher's historical trackable item listing. Icons will also remain in the inventory of cachers who log the 'discovery' of a geocoin's number without physically removing it from a cache. "Icon collecting" -- the act of having these icons listed in one's trackables listing -- is an associated hobby. Many people bring geocoins with unique icons to geocaching events so that others may see the coins and use the tracking number to collect the icons online, and it is not uncommon for collectors to activate some or all of the coins in their collections in order to have a matching online collection of icons associated with their geocaching.com accounts.

Geocoin Theft

It is not uncommon for activated, released geocoins to go missing, whether because a cacher is unfamiliar with the logging and tracking process or due to outright theft. Some geocoin owners will purposefully attempt to destroy the resale value of the coin by drilling and tagging it with an extra tag, marker, or other item that is intended to underscore the fact that the geocoin is meant to travel, not to be kept^[1]. Another somewhat controversial anti-theft measure is to create a copy of the geocoin, releasing the copy and keeping the original^[2].

History of the Trackable Geocoins

September 30, 2001 The first trackable geocoin released was the Moun10Bike Version 1 Geocoin #002.^[3] The Moun10Bike Version 1 Geocoins are the most sought-after geocoins in existence. They are all displayed on geocaching.com as owned by Moun10Bike and he has strictly forbidden their sale; however, unscrupulous or ignorant persons may list the coin for sale on auction sites such as eBay, fetching prices in the thousands of USD. When he finds that a coin has been sold without permission, he has the coin locked so it is no longer trackable.^[4]

2003 The first USAGeocoin was released for sale, making it one of the first geocoins that one can buy, release, and track online without minting an exclusive personal design. The proposed 2002 design was never made. ^[5]

2005 Geocaching.com permitted any geocaching.com user to purchase tracking numbers for approved designs, fueling a sudden surge in number of coins. Minimum purchase was initially set at 1000 tracking numbers.

Fall 2006 Groundspeak reduced the minimum purchase of tracking numbers, and the minimum number of coins minted to obtain a unique icon, to 250. The drop in the cost to create a geocoin with a unique icon fueled a 'geocoin craze' with hundreds of new personal, group and increasingly purely commercial designs minted.^[6]

February 17, 2007 The First Annual Geocoinfest was held in Temecula, CA. This event brought hundreds of geocoin collectors together for the first time in a mass event, with many exclusive coins being given away or traded.^[7]

March 4, 2009 Geocaching.com further reduced the minimum number of codes for purchase to 50, and the minimum number of coins eligible for a custom icon to 50. ^[8]

Geocoin Terminology

Coin Finishes

Antique Finish: A finish applied to copper, gold, or silver to give it a darker look. This finish is used often to have the fine details in a coin stand out more clearly.
Foggy Painting: Paint finish simulating metal applied surface. Can has gloss, shine or lustre but lacking definition (foggy details).
Matte: see Satin Finish.
Misty: Silver or gold finish simulating effect of unpolished areas of a proof coin.
Proof-like: Effect attained on high quality die stuck coins by high pressure and multiple strikes producing mirror finish background with satin finish relief areas
Satin Finish: A finish giving a matte (non-glossy) look to the metal. A misused term as traditionally (in fabric) a satin finish often has a level of gloss associated with it.
Silver: 1. plated with silver 2. .999 solid silver 3. silver-like nickel plated (shiny nickel).

Collecting Terms

AE: Artists Edition: type of SE; a version of a retail commercial geocoin only made available to the designer of the coin.
LE: Limited Edition: Typically a different version (color, metal, etc.) than the main run of coins. Produced in a limited quantity one time only.
RE: Regular edition not produced in a limited in number thus may be reprised according to demand as dies are held by the mint for minimum of three years.
SE: Special Edition: Typically a different version to the main run of coins but unlike LE, XLE or XXLE no limit on the number minted, and they may be reminted at any time.
XLE: Extra Limited Edition: Same as LE only fewer.
XXLE: Extremely Limited Edition: Same as XLE only fewer.
HTF: Hard to Find; refers to ease of acquiring through purchase or trade not total mint numbers
VHTF: Very Hard to Find; refers to ease of acquiring through purchase or trade not total mint numbers
Proof Coin: 1.the sample coins provided by the mint 2.Effect attained on high quality die stuck coins by high pressure and multiple strikes producing mirror finish background with satin finish relief areas.
Sample Coin: The sample coins provided by the mint. Some sample geocoins do not have tracking numbers.

Geocaching Terms

Custom Icon: 16 x 16 px or 32 x 32 px GIF files that are associated to the coin or coin series by geocaching.com trackable coins.
Micro: A geocoin that is smaller than ~1.25 inches in diameter. Generally recognized as a coin that would fit inside a 35mm film canister.
Non-trackable: A geocoin produced without a tracking number.
Personal: A geocoin produced or designed by an individual or team of geocachers featuring team or cacher's nickname ("geonick") prominently on the coin.
Pathtag: Pathtags are a geocoin about the size of a U.S. quarter. They are used for trading, collecting and personal signature items. They are trackable at the www.pathtags.com website but not at geocaching.com.

Geocaching Challenge in Molalla!

Fri, 13 Nov 2009 06:29:14 +0100

Molalla to launch geocaching challenge

By Pioneer Staff

The Molalla Geocaching Challenge launches this weekend, sponsored by the city of Molalla and Molalla Area Chamber of Commerce.

Geocaching is a high-tech treasure hunt that has become a popular hobby worldwide, with participants using GPS devices and clues posted online to locate hidden containers (caches).

“It’s a fun way to attract visitors to Molalla,” Mayor Mike Clarke said in a press release. “While they’re here looking for hidden caches, they may discover our elk farm, our train park, our BMX track, or our nature park and want to come back for another visit.”

The first 250 participants who complete the challenge will receive a free Molalla medallion at the chamber of commerce office.

Ten caches have been hidden as part of the challenge and the locations of each cache, along with clues for tracking them down, will be posted on www.geocaching.com by Friday, Nov. 13. Each cache will include a code word, which participants must correctly identify if they wish to receive the medallion.

Those who find all 10 code words will be entered in a drawing for a $100 gift certificate at the Farmstead Restaurant and Pub.

“Here’s yet another way to celebrate Oregon’s 150th birthday with a Molalla spin,” Molalla City Manager John Atkins said in a press release. “We’re giving people the chance to see where we have come from and where we are now—the hunt should prove as gratifying as the coins.”

Each medallion, also known as a geocoin, is inscribed with a unique code, allowing the owner to register the coin online if they choose and hide it in another cache. The coin’s whereabouts can then be tracked online as geocachers move it from cache to cache.

Brochures about the treasure hunt will be available at Molalla City Hall, the Molalla Area Chamber of Commerce and on the city’s Web site, www.cityofmolalla.com.

Funding for the coins was provided by the Clackamas County Tourism and Cultural Affairs Department, Atkins said.

Geocaching GPS Treasure Hunt News Source

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This is great news for the tachie in Oregon! Thank you Molalla!
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Molalla is hosting a Geocaching Challenge

By Vickie Kavanagh, The Oregonian

The City of Molalla and its chamber of commerce have teamed up to host a Geocaching Challenge -- a high-tech treasure hunting game designed for Global Positioning System device owners.

People are challenged to locate 10 caches -- each with a unique code word inside -- hidden in and around Molalla. The prize for seven out of 10 is a commemorative medallion or "geocoin." Geocoin owners can register the coins on an international site and, if they choose, hide them somewhere else and track their position as they are found and rehidden by other geocachers.

Those who identify all 10 code words correctly will be entered in a drawing for a $100 gift certificate to the Farmstead Restaurant and Pub at the Arrowhead Golf Club, 28301 S. Oregon 213.

The locations and clues will be revealed on or before Friday. For details, pick up a brochure at city hall, 117 N. Molalla Ave., or the chamber, 105 E. Main St.; visit the Web site; or call John Atkins at 503-829-6855.

Geocaching GPS Treasure Hunt News Source

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Geocoins! Geocoins! Good luck to those who will be participation in these. Find 7 of 10 hidden caches you get a Geocoin.

Geocaching sounds fun, what is GPS anyway?

Fri, 13 Nov 2009 06:20:05 +0100

The Global Positioning System (GPS) is a U.S. space-based global navigation satellite system. It provides reliable positioning, navigation, and timing services to worldwide users on a continuous basis in all weather, day and night, anywhere on or near the Earth.

GPS is made up of three parts: between 24 and 32 satellites orbiting the Earth, four control and monitoring stations on Earth, and the GPS receivers owned by users. GPS satellites broadcast signals from space that are used by GPS receivers to provide three-dimensional location (latitude, longitude, and altitude) plus the time.

GPS has become a widely used aid to navigation worldwide, and a useful tool for map-making, land surveying, commerce, scientific uses, tracking and surveillance, and hobbies such as geocaching and waymarking. Also, the precise time reference is used in many applications including the scientific study of earthquakes and as a time synchronization source for cellular network protocols.

GPS has become a mainstay of transportation systems worldwide, providing navigation for aviation, ground, and maritime operations. Disaster relief and emergency services depend upon GPS for location and timing capabilities in their life-saving missions. Everyday activities such as banking, mobile phone operations, and even the control of power grids, are facilitated by the accurate timing provided by GPS. Farmers, surveyors, geologists and countless others perform their work more efficiently, safely, economically, and accurately using the free and open GPS signals.

History

The first satellite navigation system, Transit, used by the United States Navy, was first successfully tested in 1960. It used a constellation of five satellites and could provide a navigational fix approximately once per hour. In 1967, the U.S. Navy developed the Timation satellite which proved the ability to place accurate clocks in space, a technology that GPS relies upon. In the 1970s, the ground-based Omega Navigation System, based on phase comparison of signal transmission from pairs of stations, became the first worldwide radio navigation system. Friedwardt Winterberg^[1] proposed a test of General Relativity using accurate atomic clocks placed in orbit in artificial satellites. To achieve accuracy requirements, GPS uses principles of general relativity to correct the satellites' atomic clocks.

The design of GPS is based partly on similar ground-based radio navigation systems, such as LORAN and the Decca Navigator developed in the early 1940s, and used during World War II. Additional inspiration for the GPS came when the Soviet Union launched the first man-made satellite, Sputnik in 1957. A team of U.S. scientists led by Dr. Richard B. Kershner were monitoring Sputnik's radio transmissions. They discovered that, because of the Doppler effect, the frequency of the signal being transmitted by Sputnik was higher as the satellite approached, and lower as it continued away from them. They realized that since they knew their exact location on the globe, they could pinpoint where the satellite was along its orbit by measuring the Doppler distortion (see Transit (satellite)).

After Korean Air Lines Flight 007 was shot down in 1983 after straying into the USSR's prohibited airspace,^[2] President Ronald Reagan issued a directive making GPS freely available for civilian use, once it was sufficiently developed, as a common good.^[3] The first satellite was launched in 1989 and the 24th and last satellite was launched in 1994.

Initially the highest quality signal was reserved for military use, and the signal available for civilian use intentionally degraded ("Selective Availability", SA). Selective Availability was ended in 2000, improving the precision of civilian GPS from about 100m to about 20m.

Basic Concepts

A GPS receiver calculates its position by precisely timing the signals sent by the GPS satellites high above the Earth. Each satellite continually transmits messages which include

the time the message was sent
precise orbital information (the ephemeris)
the general system health and rough orbits of all GPS satellites (the almanac).

The receiver measures the transit time of each message and computes the distance to each satellite. Geometric trilateration is used to combine these distances with the satellites' locations to obtain the position of the receiver. This position is then displayed, perhaps with a moving map display or latitude and longitude; elevation information may be included. Many GPS units also show derived information such as direction and speed, calculated from position changes.

Three satellites might seem enough to solve for position, since space has three dimensions. However, even a very small clock error multiplied by the very large speed of light^[15]—the speed at which satellite signals propagate—results in a large positional error. Therefore receivers use four or more satellites to solve for the receiver's location and time. The very accurately computed time is effectively hidden by most GPS applications, which use only the location. A few specialized GPS applications do however use the time; these include time transfer, traffic signal timing, and synchronization of cell phone base stations.

Although four satellites are required for normal operation, fewer apply in special cases. If one variable is already known, a receiver can determine its position using only three satellites. (For example, a ship or plane may have known elevation.) Some GPS receivers may use additional clues or assumptions (such as reusing the last known altitude, dead reckoning, inertial navigation, or including information from the vehicle computer) to give a degraded position when fewer than four satellites are visible (see,^[16] Chapters 7 and 8 of,^[17] and ^[18]).

Position calculation introduction

To provide an introductory description of how a GPS receiver works, errors will be ignored in this section. Using messages received from a minimum of four visible satellites, a GPS receiver is able to determine the times sent and then the satellite positions corresponding to these times sent. The x, y, and z components of position, and the time sent, are designated as where the subscript i is the satellite number and has the value 1, 2, 3, or 4. Knowing the indicated time the message was received , the GPS receiver can compute the transit time of the message as . Assuming the message traveled at the speed of light, c, the distance traveled, can be computed as .

A satellite's position and distance from the receiver define a spherical surface, centered on the satellite. The position of the receiver is somewhere on this surface. Thus with four satellites, the indicated position of the GPS receiver is at or near the intersection of the surfaces of four spheres. (In the ideal case of no errors, the GPS receiver would be at a precise intersection of the four surfaces.)

For automobiles and other near-earth-vehicles, the correct position of the GPS receiver is the intersection closest to the earth's surface. For space vehicles, the intersection farthest from Earth may be the correct one.^[20]

The correct position for the GPS receiver is also the intersection closest to the surface of the sphere corresponding to the fourth satellite.

System segmentation

The current GPS consists of three major segments. These are the space segment (SS), a control segment (CS), and a user segment (US).^[21]

Space segment

The space segment (SS) comprises the orbiting GPS satellites, or Space Vehicles (SV) in GPS parlance. The GPS design originally called for 24 SVs, eight each in three circular orbital planes,^[22] but this was modified to six planes with four satellites each.^[23] The orbital planes are centered on the Earth, not rotating with respect to the distant stars.^[24] The six planes have approximately 55° inclination (tilt relative to Earth's equator) and are separated by 60° right ascension of the ascending node (angle along the equator from a reference point to the orbit's intersection).^[25] The orbits are arranged so that at least six satellites are always within line of sight from almost everywhere on Earth's surface.^[26]

Orbiting at an altitude of approximately 20,200 kilometers (about 12,550 miles or 10,900 nautical miles; orbital radius of approximately 26,600 km (about 16,500 mi or 14,400 NM)), each SV makes two complete orbits each sidereal day, repeating the same ground track each day.^[27] This was very helpful during development, since even with just four satellites, correct alignment means all four are visible from one spot for a few hours each day. For military operations, the ground track repeat can be used to ensure good coverage in combat zones.

As of March 2008^[update],^[28] there are 31 actively broadcasting satellites in the GPS constellation, and two older, retired from active service satellites kept in the constellation as orbital spares. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements. With the increased number of satellites, the constellation was changed to a nonuniform arrangement. Such an arrangement was shown to improve reliability and availability of the system, relative to a uniform system, when multiple satellites fail.^[29] About ten satellites are visible from any point on the ground at any one time (see animation at right).

Control segment

The flight paths of the satellites are tracked by U.S. Air Force monitoring stations in Hawaii, Kwajalein, Ascension Island, Diego Garcia, and Colorado Springs, Colorado, along with monitor stations operated by the National Geospatial-Intelligence Agency (NGA).^[30] The tracking information is sent to the Air Force Space Command's master control station at Schriever Air Force Base in Colorado Springs, which is operated by the 2nd Space Operations Squadron (2 SOPS) of the United States Air Force (USAF). Then 2 SOPS contacts each GPS satellite regularly with a navigational update (using the ground antennas at Ascension Island, Diego Garcia, Kwajalein, and Colorado Springs). These updates synchronize the atomic clocks on board the satellites to within a few nanoseconds of each other, and adjust the ephemeris of each satellite's internal orbital model. The updates are created by a Kalman filter which uses inputs from the ground monitoring stations, space weather information, and various other inputs.^[31]

Satellite maneuvers are not precise by GPS standards. So to change the orbit of a satellite, the satellite must be marked 'unhealthy', so receivers will not use it in their calculation. Then the maneuver can be carried out, and the resulting orbit tracked from the ground. Then the new ephemeris is uploaded and the satellite marked healthy again.

User segment

The user's GPS receiver is the user segment (US) of the GPS. In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly-stable clock (often a crystal oscillator). They may also include a display for providing location and speed information to the user. A receiver is often described by its number of channels: this signifies how many satellites it can monitor simultaneously. Originally limited to four or five, this has progressively increased over the years so that, as of 2007, receivers typically have between 12 and 20 channels.^[32]

GPS receivers may include an input for differential corrections, using the RTCM SC-104 format. This is typically in the form of a RS-232 port at 4,800 bit/s speed. Data is actually sent at a much lower rate, which limits the accuracy of the signal sent using RTCM. Receivers with internal DGPS receivers can outperform those using external RTCM data. As of 2006, even low-cost units commonly include Wide Area Augmentation System (WAAS) receivers.

Many GPS receivers can relay position data to a PC or other device using the NMEA 0183 protocol, or the newer and less widely used NMEA 2000.^[33] Although these protocols are officially defined by the NMEA,^[34] references to these protocols have been compiled from public records, allowing open source tools like gpsd to read the protocol without violating intellectual property laws. Other proprietary protocols exist as well, such as the SiRF and MTK protocols. Receivers can interface with other devices using methods including a serial connection, USB or Bluetooth.

Further information: GPS navigation device

Navigation

Aspects of navigation are discussed in this section. The subsection on navigation signals discusses details of the message content. Carrier frequencies for the messages are stated. Demodulating the carrier and decoding to separate the signals from the satellites is described. The position calculation subsection does not require an understanding of the other subsections. Basic equations describing the geometry of the sphere and the fundamental concept that the satellite message travels at the speed of light are used in the subsection. The subsection on multidimensional Newton-Raphson may be of interest only to those readers who want a more detailed understanding on how an algorithm might be written and is unnecessary for the reader who is uninterested in this amount of detail.

Navigation signals

Transmission of each 30 second frame begins precisely on the minute and half minute as indicated by the satellite's atomic clock according to Satellite message format. Each frame contains 5 subframes of length 6 seconds and with 300 bits. Each subframe contains 10 words of 30 bits with length 0.6 seconds each.

Words 1 and 2 of every subframe have the same type of data. The first word is the telemetry word which indicates the beginning of a subframe and is used by the receiver to synch with the navigation message. The second word is the HOW or handover word and it contains timing information which enables the receiver to identify the subframe and provides the time the next subframe was sent.

Words 3 through 10 of subframe 1 contain data describing the satellite clock and its relationship to GPS time. Words 3 through 10 of subframes 2 and 3, contain the ephemeris data, giving the satellite's own precise orbit. The ephemeris is updated every 2 hours and is generally valid for 4 hours, with provisions for updates every 6 hours or longer in non-nominal conditions. The time needed to acquire the ephemeris is becoming a significant element of the delay to first position fix, because, as the hardware becomes more capable, the time to lock onto the satellite signals shrinks, but the ephemeris data requires 30 seconds (worst case) before it is received, due to the low data transmission rate.

The almanac consists of coarse orbit and status information for each satellite in the constellation, an ionospheric model, and information to relate GPS derived time to Coordinated Universal Time (UTC). Words 3 through 10 of subframes 4 and 5 contain a new part of the almanac. Each frame contains 1/25th of the almanac, so 12.5 minutes are required to receive the entire almanac from a single satellite.^[35] The almanac serves several purposes. The first is to assist in the acquisition of satellites at power-up by allowing the receiver to generate a list of visible satellites based on stored position and time, while an ephemeris from each satellite is needed to compute position fixes using that satellite. In older hardware, lack of an almanac in a new receiver would cause long delays before providing a valid position, because the search for each satellite was a slow process. Advances in hardware have made the acquisition process much faster, so not having an almanac is no longer an issue. The second purpose is for relating time derived from the GPS (called GPS time) to the international time standard of UTC. Finally, the almanac allows a single-frequency receiver to correct for ionospheric error by using a global ionospheric model. The corrections are not as accurate as augmentation systems like WAAS or dual-frequency receivers. However, it is often better than no correction, since ionospheric error is the largest error source for a single-frequency GPS receiver. An important thing to note about navigation data is that each satellite transmits not only its own ephemeris, but transmits an almanac for all satellites.

All satellites broadcast at the same two frequencies, 1.57542 GHz (L1 signal) and 1.2276 GHz (L2 signal). The receiver can distinguish the signals from different satellites because GPS uses a code division multiple access (CDMA) spread-spectrum technique where the low-bitrate message data is encoded with a high-rate pseudo-random (PRN) sequence that is different for each satellite. The receiver knows the PRN codes for each satellite and can use this to reconstruct the actual message data. The message data is transmitted at 50 bits per second. Two distinct CDMA encodings are used: the coarse/acquisition (C/A) code (a so-called Gold code) at 1.023 million chips per second, and the precise (P) code at 10.23 million chips per second. The L1 carrier is modulated by both the C/A and P codes, while the L2 carrier is only modulated by the P code.^[36] The C/A code is public and used by civilian GPS receivers, while the P code can be encrypted as a so-called P(Y) code which is only available to military equipment with a proper decryption key. Both the C/A and P(Y) codes impart the precise time-of-day to the user.

Demodulation and Decoding

Since all of the satellite signals are modulated onto the same L1 carrier frequency, there is a need to separate the signals after demodulation. This is done by assigning each satellite a unique binary sequence sequence known as a Gold code, and the signals are decoded, after demodulation, using modulo 2 addition of the Gold codes corresponding to satellites n₁ through n_k, where k is the number of channels in the GPS receiver and n₁ through n_k are the PRN identifiers of the satellites. Each satellite's PRN identifier is unique and in the range from 1 through 32.^[38] The results of these modulo 2 additions are the 50 bit/s navigation messages from satellites n₁ through n_k. The Gold codes used in GPS are a sequence of 1023 bits with a period of one millisecond. These Gold codes are highly mutually orthogonal, so that it is unlikely that one satellite signal will be misinterpreted as another. As well, the Gold codes have good auto-correlation properties.^[39]

There are 1025 different Gold codes of length 1023 bits, but only 32 are used. These Gold codes are quite often referred to as pseudo random noise since they contain no data and are said to look like random sequences^[40]. However, this may be misleading since they are actually deterministic sequences.

If the almanac information has previously been acquired, the receiver picks which satellites to listen for by their PRNs. If the almanac information is not in memory, the receiver enters a search mode and cycles through the PRN numbers until a lock is obtained on one of the satellites. To obtain a lock, it is necessary that there be an unobstructed line of sight from the receiver to the satellite. The receiver can then acquire the almanac and determine the satellites it should listen for. As it detects each satellite's signal, it identifies it by its distinct C/A code pattern.

The receiver uses the C/A Gold code with the same PRN number as the satellite to compute an offset, O, that generates the best correlation. The offset, O, is computed in a trial and error manner. The 1023 bits of the satellite PRN signal are compared with the receiver PRN signal. If correlation is not achieved, the 1023 bits of the receiver's internally generated PRN code are shifted by one bit relative to the satellite's PRN code and the signals are again compared. This process is repeated until correlation is achieved or all 1023 possible cases have been tried.^[41] If all 1023 cases have been tried without achieving correlation, the frequency oscillator is offset to the next value and the process is repeated.

Since the carrier frequency received can vary due to Doppler shift, the points where received PRN sequences begin may not differ from O by an exact integral number of milliseconds. Because of this, carrier frequency tracking along with PRN code tracking are used to determine when the received satellite's PRN code begins.^[41] Unlike the earlier computation of offset in which trials of all 1023 offsets could potentially be required, the tracking to maintain lock usually requires shifting of half a pulse width or less. To perform this tracking, the receiver observes two quantities, phase error and received frequency offset. The correlation of the received PRN code with respect to the receiver generated PRN code is computed to determine if the bits of the two signals are misaligned. Comparisons with correlation computed with receiver generated PRN code shifted half a pulse width early and half a pulse width late (see section 1.4.2.4 of ^[17]) are used to estimate adjustment required. The amount of adjustment required for maximum correlation is used in estimating phase error. Received frequency offset from the frequency generated by the receiver provides an estimate of phase rate error. The command for the frequency generator and any further PRN code shifting required are computed as a function of the phase error and the phase rate error in accordance with the control law used. The Doppler velocity is computed as a function of the frequency offset from the carrier nominal frequency. The Doppler velocity is the velocity component along the line of sight of the receiver relative to the satellite.

As the receiver continues to read successive PRN sequences, it will encounter a sudden change in the phase of the 1023 bit received PRN signal. This indicates the beginning of a data bit of the navigation message (see section 1.4.2.5 of ^[17]). This enables the receiver to begin reading the 20 millisecond bits of the navigation message. Each subframe of the navigation frame begins with a Telemetry Word which enables the receiver to detect the beginning of a subframe and determine the receiver clock time at which the navigation subframe begins. Also each subframe of the navigation frame is identified by bits in the handover word (HOW) thereby enabling the receiver to determine which subframe (see section 1.4.2.6 of ^[17] and section 2.5.4 of "Essentials of Satellite Navigation Compendium"). There can be a delay of up to 30 seconds before the first estimate of position because of the need to read the ephemeris data before computing the intersections of sphere surfaces.

After a subframe has been read and interpreted, the time the next subframe was sent can be calculated through the use of the clock correction data and the HOW. The receiver knows the receiver clock time of when the beginning of the next subframe was received from detection of the Telemetry Word thereby enabling computation of the transit time and thus the pseudorange. The receiver is potentially capable of getting a new pseudorange measurement at the beginning of each subframe or every 6 seconds.

Then the orbital position data, or ephemeris, from the navigation message is used to calculate precisely where the satellite was at the start of the message. A more sensitive receiver will potentially acquire the ephemeris data more quickly than a less sensitive receiver, especially in a noisy environment.^[42]

This process is repeated for each satellite to which the receiver is listening.

Atmospheric effects

Inconsistencies of atmospheric conditions affect the speed of the GPS signals as they pass through the Earth's atmosphere, especially the ionosphere. Correcting these errors is a significant challenge to improving GPS position accuracy. These effects are smallest when the satellite is directly overhead and become greater for satellites nearer the horizon since the path through the atmosphere is longer (see airmass). Once the receiver's approximate location is known, a mathematical model can be used to estimate and compensate for these errors.

Ionospheric delay of a microwave signal depends on its frequency. This phenomenon is known as dispersion and can be calculated from measurements of delays for two or more frequency bands, allowing delays at other frequencies to be estimated.^[53] Some military and expensive survey-grade civilian receivers calculate atmospheric dispersion from the different delays in the L1 and L2 frequencies, and apply a more precise correction. This can be done in civilian receivers without decrypting the P(Y) signal carried on L2, by tracking the carrier wave instead of the modulated code. To facilitate this on lower cost receivers, a new civilian code signal on L2, called L2C, was added to the Block IIR-M satellites, which was first launched in 2005. It allows a direct comparison of the L1 and L2 signals using the coded signal instead of the carrier wave. (see Atmospheric Effects in "Sources of Errors in GPS")

The effects of the ionosphere generally change slowly, and can be averaged over time. Those for any particular geographical area can be easily calculated by comparing the GPS-measured position to a known surveyed location. This correction is also valid for other receivers in the same general location. Several systems send this information over radio or other links to allow L1-only receivers to make ionospheric corrections. The ionospheric data are transmitted via satellite in Satellite Based Augmentation Systems (SBAS) such as WAAS (available in North America and Hawaii), EGNOS (Europe and Asia) or MSAS (Japan), which transmits it on the GPS frequency using a special pseudo-random noise sequence (PRN), so only one receiver and antenna are required.

Humidity also causes a variable delay, resulting in errors similar to ionospheric delay, but occurring in the troposphere. This effect both is more localized and changes more quickly than ionospheric effects, and is not frequency dependent. These traits make precise measurement and compensation of humidity errors more difficult than ionospheric effects.^{[citation needed]}

Changes in receiver altitude also change the amount of delay, due to the signal passing through less of the atmosphere at higher elevations. Since the GPS receiver computes its approximate altitude, this error is relatively simple to correct, either by applying a function regression or correlating margin of atmospheric error to ambient pressure using a barometric altimeter.^{[citation needed]}

Multipath effects

GPS signals can also be affected by multipath issues, where the radio signals reflect off surrounding terrain; buildings, canyon walls, hard ground, etc. These delayed signals can cause inaccuracy. A variety of techniques, most notably narrow correlator spacing, have been developed to mitigate multipath errors. For long delay multipath, the receiver itself can recognize the wayward signal and discard it. To address shorter delay multipath from the signal reflecting off the ground, specialized antennas (e.g., a choke ring antenna) may be used to reduce the signal power as received by the antenna. Short delay reflections are harder to filter out because they interfere with the true signal, causing effects almost indistinguishable from routine fluctuations in atmospheric delay.

Multipath effects are much less severe in moving vehicles. When the GPS antenna is moving, the false solutions using reflected signals quickly fail to converge and only the direct signals result in stable solutions.

Ephemeris and clock errors

While the ephemeris data is transmitted every 30 seconds, the information itself may be up to two hours old. If a fast time to first fix (TTFF) is needed, it is possible to upload a valid ephemeris to a receiver, and in addition to setting the time, a position fix can be obtained in under ten seconds. It is feasible to put such ephemeris data on the web so it can be loaded into mobile GPS devices.^[54] See also Assisted GPS.

The satellite's atomic clocks experience noise and clock drift errors. The navigation message contains corrections for these errors and estimates of the accuracy of the atomic clock. However, they are based on observations and may not indicate the clock's current state.

These problems tend to be very small, but may add up to a few meters (tens of feet) of inaccuracy.^[55]

For very precise positioning (e.g., in geodesy), these effects can be eliminated by differential GPS: the simultaneous use of two or more receivers at several survey points. In the 1990s when receivers were quite expensive, some methods of quasi-differential GPS were developed, using only one receiver but reoccupation of measuring points. At the TU Vienna the method was named qGPS and adequate software of post processing was developed.

Selective availability

GPS includes a (currently disabled) feature called Selective Availability (SA) that adds intentional, time varying errors of up to 100 meters (328 ft) to the publicly available navigation signals. This was intended to deny an enemy the use of civilian GPS receivers for precision weapon guidance.

SA errors are actually pseudorandom, generated by a cryptographic algorithm from a classified seed key available only to authorized users (the U.S. military, its allies and a few other users, mostly government) with a special military GPS receiver. Mere possession of the receiver is insufficient; it still needs the tightly controlled daily key.

Before it was turned off on May 1, 2000, typical SA errors were 10 meters (32 ft) horizontally and 30 meters (98 ft) vertically. Because SA affects every GPS receiver in a given area almost equally, a fixed station with an accurately known position can measure the SA error values and transmit them to the local GPS receivers so they may correct their position fixes. This is called Differential GPS or DGPS. DGPS also corrects for several other important sources of GPS errors, particularly ionospheric delay, so it continues to be widely used even though SA has been turned off. The ineffectiveness of SA in the face of widely available DGPS was a common argument for turning off SA, and this was finally done by order of President Clinton in 2000.

Another restriction on GPS, antispoofing, remains on. This encrypts the P-code so that it cannot be mimicked by an enemy transmitter sending false information. Few civilian receivers have ever used the P-code, and the accuracy attainable with the public C/A code is so much better than originally expected (especially with DGPS) that the antispoof policy has relatively little effect on most civilian users. Turning off antispoof would primarily benefit surveyors and some scientists who need extremely precise positions for experiments such as tracking the motion of a tectonic plate.

DGPS services are widely available from both commercial and government sources. The latter include WAAS and the U.S. Coast Guard's network of LF marine navigation beacons. The accuracy of the corrections depends on the distance between the user and the DGPS receiver. As the distance increases, the errors at the two sites will not correlate as well, resulting in less precise differential corrections.

During the 1990-91 Gulf War, the shortage of military GPS units caused many troops and their families to buy readily available civilian units. This significantly impeded the U.S. military's own battlefield use of GPS, so the military made the decision to turn off SA for the duration of the war.

In the 1990s, the FAA started pressuring the military to turn off SA permanently. This would save the FAA millions of dollars every year in maintenance of their own radio navigation systems. The amount of error added was "set to zero"^[57] at midnight on May 1, 2000 following an announcement by U.S. President Bill Clinton, allowing users access to the error-free L1 signal. Per the directive, the induced error of SA was changed to add no error to the public signals (C/A code). Clinton's executive order required SA to be set to zero by 2006; it happened in 2000 once the U.S. military developed a new system that provides the ability to deny GPS (and other navigation services) to hostile forces in a specific area of crisis without affecting the rest of the world or its own military systems.^[57]

Selective Availability is still a system capability of GPS, and could, in theory, be reintroduced at any time. In practice, in view of the hazards and costs this would induce for U.S. and foreign shipping, it is unlikely to be reintroduced, and various government agencies, including the FAA,^[58] have stated that it is not intended to be reintroduced.

One interesting side effect of the Selective Availability hardware is the capability to add corrections to the outgoing signal of the GPS cesium and rubidium atomic clocks to an accuracy of approximately 2 × 10⁻¹³ This represented a significant improvement over the raw accuracy of the clocks.

On 19 September 2007, the United States Department of Defense announced that future GPS III satellites will not be capable of implementing SA,^[59] eventually making the policy permanent.^[60]

Special and general relativity

According to the theory of relativity, due to their constant movement and height relative to the Earth-centered, non-rotating approximately inertial reference frame, the clocks on the satellites are affected by their speed. Special relativity predicts that atomic clocks moving at GPS orbital speeds will tick more slowly than stationary ground clocks by about 7.2 μs per day.

For the GPS satellites, general relativity predicts that the atomic clocks at GPS orbital altitudes will tick more rapidly, by about 45.9 μs per day, because they have a higher gravitational potential than atomic clocks on Earth's surface.

When combined, the discrepancy is about 38 microseconds per day; a difference of 4.465 parts in 10¹⁰.^[62] To account for this discrepancy, the frequency standard on board each satellite is given a rate offset prior to launch, making it run slightly slower than the desired frequency on Earth; specifically, at 10.22999999543 MHz instead of 10.23 MHz.^[63] Since the atomic clocks on board the GPS satellites are precisely tuned, it makes the system a practical engineering application of the scientific theory of relativity in a real-world environment. Placing atomic clocks on artificial satellites to test Einstein's general theory was proposed by Friedwardt Winterberg in 1955.^[64]

Sagnac distortion

GPS observation processing must also compensate for the Sagnac effect. The GPS time scale is defined in an inertial system but observations are processed in an Earth-centered, Earth-fixed (co-rotating) system, a system in which simultaneity is not uniquely defined. A Lorentz transformation is thus applied to convert from the inertial system to the ECEF system. The resulting signal run time correction has opposite algebraic signs for satellites in the Eastern and Western celestial hemispheres. Ignoring this effect will produce an east-west error on the order of hundreds of nanoseconds, or tens of meters in position.^[65]

Natural sources of interference

Since GPS signals at terrestrial receivers tend to be relatively weak, natural radio signals or scattering of the GPS signals can desensitize the receiver, making acquiring and tracking the satellite signals difficult or impossible.

Space weather degrades GPS operation in two ways, direct interference by solar radio burst noise in the same frequency band^[66] or by scattering of the GPS radio signal in ionospheric irregularities referred to as scintillation.^[67] Both forms of degradation follow the 11 year solar cycle and are a maximum at sunspot maximum although they can occur at anytime. Solar radio bursts are associated with solar flares and their impact can affect reception over the half of the Earth facing the sun. Scintillation occurs most frequently at tropical latitudes where it is a night time phenomenon. It occurs less frequently at high latitudes or mid-latitudes where magnetic storms can lead to scintillation.^[68] In addition to producing scintillation, magnetic storms can produce strong ionospheric gradients that degrade the accuracy of SBAS systems.^[69]

Artificial sources of interference

In automotive GPS receivers, metallic features in windshields,^[70] such as defrosters, or car window tinting films^[71] can act as a Faraday cage, degrading reception just inside the car.

Man-made EMI (electromagnetic interference) can also disrupt, or jam, GPS signals. In one well documented case, the entire harbor of Moss Landing, California was unable to receive GPS signals due to unintentional jamming caused by malfunctioning TV antenna preamplifiers.^[72]^[73] Intentional jamming is also possible. Generally, stronger signals can interfere with GPS receivers when they are within radio range, or line of sight. In 2002, a detailed description of how to build a short range GPS L1 C/A jammer was published in the online magazine Phrack.^[74]

The U.S. government believes that such jammers were used occasionally during the 2001 war in Afghanistan and the U.S. military claimed to destroy six GPS jammers during the Iraq War, including one that was destroyed ironically with a GPS-guided bomb.^[75] Such a jammer is relatively easy to detect and locate, making it an attractive target for anti-radiation missiles. The

Geocaching provides hobby for modern-day treasure hunters

Fri, 13 Nov 2009 06:06:15 +0100

By Alexandra Hruz

It only took an hour of searching in the dark at a Confederate fort in Alexandria, La. while looking for a hidden geocache for Walt Adams to realize his obsession with treasure hunting. On hands and knees, Adams felt around in the dirt, hoping to get his hands on the treasure he sought. And while Adams did eventually give up for the night, he wasn't deterred by his loss. He woke up early, went to the site, found the cache and managed to catch his morning flight back to Idaho as well. Combined with a penchant for technology, Adams found the best of both worlds in geocaching, a modern day hobby that uses GPS technology to find hidden caches. And while it might not be the treasure hunting of pirate novels, it does inspire connotations of modern-day Magellans. Geocachers can travel around the country, or even around the world, to sate their need for discovery by using a handheld GPS device and a strong trailblazing energy. Here are a few tips and suggestions from experienced geocachers on how to jump start this treasure-hunting hobby.

What is Geocaching?

Geocaching as a hobby came about in 2000 after satellites that were originally used for military purposes were made civilian-friendly, allowing people to use GPS devices to track items to within 10 feet, according to Tom Dunigan, former adjunct associate professor in Computer Science at the University of Tennessee. The combination of Internet technology and the change in accuracy of the GPS unit allowed geocaching to develop as a serious hobby, Dunigan said. Today, there are between 2 and 3 million active geocachers worldwide, and more than 900,000 hidden caches.

"It's based on latitude and longitude, so it's not like the traditional map," Dunigan said. "It's up to the individual to figure out how to get there, and once you get there, often the cache is hidden in some fashion."

Dunigan explained that the caches are typically hidden in old Army ammo boxes or Tupperware containers, and they sometimes contain trinkets that past explorers left for others to find.

According to Dunigan, a log is often placed with the cache for people to sign after discovering it. Following a find, geocachers can create an online account on a geocaching Web site to create a virtual log of their finds.

Essentials for Geocaching

While a strong sense of direction and a taste for adventure can certainly be helpful while searching for hidden goods, geocaching does require a handheld GPS device, as well as access to the Internet. For those who are unsure about how to pick out a proper GPS device, Geocaching.com provides an informative online tutorial about how to buy the right equipment.

Having access to a geocache Web site is crucial, and dozens of sites are available for those ready to download coordinates onto their GPS before heading out to search. Most sites offer free membership, and there are geocache sites dedicated to specific countries around the world for the globe-trotting geocacher.

"You need the Internet not only to search for local caches, but also so you can log in your visits and create your own geocaches," Dunigan said.

For tracking more difficult caches, Dunigan also suggested that a topographical map be brought along on the hunt, as GPS signal can sometimes be weak in certain areas.

Tales from a trailblazer

While geocaching might not result in treasure chests filled with gold, hidden caches still provide plenty of excitement and anticipation for those searching for them. Not only is this hide-and-seek hobby perfect for those who are interested in technology, it can also satisfy the desire to explore new places.

Walt Adams, an Idaho resident who has geocached around the globe, is always able to sate his need for travel and his geocaching hobby simultaneously.

"I have found 1,115 geocaches to date and not too many of them have gotten away," Adams said. "I have seen a lot of country and enjoyed a lot of local history."

Through his geocache journeys, Adams has discovered hidden caches in air raid bunkers and church cemeteries, as well as ones 20 feet underground in England and near the USS Arizona memorial in Pearl Harbor, Hawaii.

Tips from the experts

For those who are less than sure about their technological skills or don't have a clue how to find their first cache, experienced GPS users can prove to be the best source of knowledge on the subject.

"The best thing would be to find someone else who is already doing it to get an idea if it's really worth investing in," Dunigan said. Dunigan, who provides latitudes and longitudes for Tennessee landforms, said that an interest in historical places and technology is what drew him to the hobby.

Through his travels around the world, Adams has also amassed some helpful advice for those who might be interested in geocaching.

"Lots of geocaches are hidden in holes or under bridges, or in hollow logs. Snakes, spiders and sharp objects can be there too," Adams said. "I almost always wear a pair of leather gloves when I reach in to feel around."

Adams also emphasized that cachers should carry extra batteries and a flashlight. He also hinted that new cachers should allow for more time than they actually think it will take.

For some, geocaching can bring back memories of earning scouting patches in orienteering. But for others, it provides an excellent opportunity to enjoy the outdoors while becoming more versed in the ways of technology.

Geocaching GPS Treasure Hunt News Source

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Awesome! This sound like fun for the adventurous gadget technocrat! Great job Alexandra!