The Earth is a somewhat pear shaped ellipsoid, with a slightly larger southern hemisphere than northern hemisphere. No single mathematical formula works for all of the Earth's surface, so different regions are represented by different formulas. Position coordinates determined for one formula (datum) will usually not match the position values determined by another datum.

USGS topo maps specifies the reference datum in the information at the bottom of the map. Most GPS receivers allow the user to select the desired datum from a list of common datums. North American Datum 1927 (NAD 27) and Department of Defense World Geodetic System 1984 (WGS 84) are commonly used in North America. Most USGS 7 1/2 minute series topo maps for regions in the United States use NAD 27. When providing your position coordinates to someone from another agency (such as, a helicopter pilot), be sure to also include the datum as part of the information.

In 1958, a World Geodetic System (WGS) was designed by the DOD as the datum for intercontinental ballistic missiles, and subsequent revisions have become the de facto worldwide standard, with WGS-84 as the most current version.

WGS-84 Ellipsoid -- The reference ellipsoid used by NASA/GSFC is the Defense Mapping Agency (DMA) World Geodetic System 1984 (WGS-84) ellipsoid model. The ellipsoid is defined as:

Flattening (ellipticity): 1/298.257

Semimajor axis: 6378137 m

GPS receivers in the United States provide coordinates relative to the North American Datum (NAD), which was revised in 1983. NAD differs from WGS-84 by up to a meter in some areas. But many paper maps used as sources for computer map vectors were compiled at least 20 years ago using NAD-27 instead of NAD-83. Actual positional differences of 95 meters for identical coordinates have been noted between the two NAD reference systems.