What do I need to know to get my GPS into correct operation?
We generally emphasize the following four points:
a) Select a coordinate system (also called a "position format") you feel comfortable using. We suggest using Universal Transverse Mercator (UTM) or latitude/longitude. Most users find UTM coordinates easier to locate on USGS 7.5-minute quadrangle maps. The coordinate system displayed by your GPS reflects the selection you made on your unit's Setup page.
b) If working with a reference document (topographic map or coordinates you got from a book, website, or other third party), your GPS datum must agree with the reference document's horizontal datum. Most GPS units default to a datum called the World Geodetic System of 1984 (WGS84). However, about 95% of the published USGS maps are referenced to the North American Datum of 1927 (NAD27); the remaining 5% are set to the North American Datum of 1983 (NAD83), which is virtually identical to WGS84. Check the map information at the lower left corner of the USGS 7.5-minute quadrangle to determine the map's datum. If your published map uses the NAD27 datum and your GPS setup menu offers variations of NAD27, select NAD27 CONUS. All new US Topo maps released over the Internet (not printed maps) use the NAD83 horizontal datum.
c) Be aware that the vertical heights displayed by your recreational GPS receiver may not agree well with USGS map elevations. This is because the earth blocks the satellites below the horizon and because maps and GPS use different points of reference for zero elevation. GPS heights are based on an ellipsoid (a mathematical representation of the earth's shape), while USGS map elevations are based on a vertical datum tied to the geoid (what we commonly call "mean sea level"). GPS elevations can disagree with map elevations by +/-400 feet. Use these values with caution when navigating. GPS units do not replace basic map and compass skills.
d) Select which "North" your GPS receiver will use as the zero degree reference. The default in your receiver is probably true north, but you can also select magnetic north or grid north (the north-south lines of the UTM grid). There is no "correct" selection - it's a matter of personal preference.