Most of us never need to use numerical GPS coordinates to take advantage of the many location services available to us. We simply enter an address, or go through an internet search, or automatically assign photos, and our electronic devices take care of the rest. But dedicated outdoorsmen, geocachers, pilots, sailors, and more often need to use and understand numerical GPS coordinates.
Global GPS system on does not really have its own coordinate system. It uses «geographic coordinates» systems that already existed before GPS, including.
Latitude and longitude
GPS coordinates are most commonly expressed as latitude and longitude. This system divides the Earth into lines of latitude, which indicate how far north or south of the equator the location is, and lines of longitude, which indicate how far east or west of the prime meridian the location is.
In this system, the equator is at 0 degrees latitude and the poles are 90 degrees north and south. The prime meridian is at 0 degrees longitude, extending east and west.
With this system, the exact location on the surface of the Earth can be expressed as a set of numbers. The latitude and longitude of the Empire State Building, for example, is expressed as N40°44.9064′, W073°59.0735′. The location can also be expressed in a number-only format, for: 40.748440, -73.984559. The first number indicates latitude and the second number indicates longitude (the minus sign indicates «west»). Being only numeric, the second notation is most commonly used to enter positions into GPS devices.
Universal Transverse Mercator
GPS devices can also be configured to display position in Universal Transverse Mercator. UTM was developed for paper maps, helping to eliminate the effects of distortion created by the curvature of the Earth. UTM divides the globe into a grid of many zones. UTM is less commonly used than latitude and longitude and is best for those who need to work with paper maps.
Associated with UTM are the Military Network Help System and the US National Grid. These systems are commonly used by military personnel, federal agencies, and law enforcement and search and rescue teams.
Not a single map is filled with a datum, which indicates the year and type of calculation of the center of the Earth. Since maps are two-dimensional representations of three-dimensional space, the datum attaches a particular point as the «center» for all subsequent work. Different maps use different datums, so mixing the two produces small, but non-trivial, errors in geolocation and distance tracking.
Three datums are commonly used in the United States. NAD 27 CONUS is the 1927 datum most commonly found on USGS old-style maps. Newer USGS cards use N.A.D. 83 , North American Datum 1983. However, by default, most GPS systems default to WGS 84 , World Geodetic System 1984. If in doubt, use WGS 84.