The Basics of GPS Coordinates

GPS coordinates identify the location the satellites have triangulated for you

A GPS navigation unit in a car

 Scott Meis/Flickr/CC 2.0

Most of us never need to use numerical GPS coordinates to take advantage of the many location-based services that are available to us. We simply input an address, or click through from an internet search, or automatically geotag photos, and our electronic devices take care of the rest. But dedicated outdoors-people, geocachers, pilots, sailors, and more do often need to use and understand numerical GPS coordinates.

The global GPS system actually doesn't have a coordinates system of its own. It uses "geographic coordinates" systems that already existed before GPS, including.

Latitude and Longitude

Latitude and Longitude
Djexplo / Wikimedia Commons

GPS coordinates are most commonly expressed as latitude and longitude. This system divides the earth into latitude lines, which indicate how far north or south of the equator a location is, and longitude lines, which indicate how far east or west of the prime meridian a location is.

In this system, the equator is at 0 degrees latitude, with the poles being at 90 degrees north and south. The prime meridian is at 0 degrees longitude, extending east and west.

Under this system, an exact location on the earth's surface 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 may also be expressed in a numbers-only format, per: 40.748440, -73.984559. With the first number indicating latitude, and the second number representing longitude (the minus sign indicates "west"). Being numeric-only, the second means of notation is the most commonly used for inputting positions into GPS devices.

Universal Transverse Mercator

GPS devices may also be set to show position in Universal Transverse Mercator. UTM was designed for paper maps, helping to remove the effects of the 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.

Related to UTM are the Military Grid Reference System and the United States National Grid. These systems are commonly used by military personnel, federal agencies, and law-enforcement and search-and-rescue teams.

Datums

No map is complete with a datum, which indicates the year and type of the calcuation of the center of the earth. Because maps are two-dimensional representations of a three-dimensional space, the datum affixes a specific point as the "center" for all subsequent work. Different maps use different datums, so mixing the two yields small, but non-trivial, errors in geolocation and distance tracking.

In the United States, three datums are commonly used. NAD 27 CONUS is a 1927-era datum that's most often encountered on old-style maps from the U.S. Geological Survey. Newer USGS maps use NAD 83, the North American Datum of 1983. However, by default, most GPS systems default to WGS 84, the World Geodetic System of 1984. When in doubt, use WGS 84.

Obtaining Coordinates

Most handheld GPS devices will provide you with a location from simple menu selection as well.

In Google Maps, simply left-click on your selected spot on the map, and the GPS coordinates appear in the drop-down box at the top left of the screen. You will see the numeric latitude and longitude for the location.

Apple's Maps app does not provide a way to obtain GPS coordinates. However, a full-featured outdoor GPS hiking app for iOS or iPadOS provides you with coordinates with a high degree of precision.