Have you ever wondered to yourself, Why don’t the elevations on your maps agree with those provided by my GPS system? Which ones are correct?
This answer comes via the USGS…
The USGS publishes approximately 57,000 different topographic maps covering the United States. Topographic maps show contour lines (elevation and landforms), hydrography (rivers, lakes, marshes), transportation (roads, trails, railroads, airports), vegetation, boundaries, survey markers, urban areas, buildings, and a variety of other features. These maps are drawn according to the National Map Accuracy Standard and are most commonly published at 1:24,000; 1:100,000; 1:250,000; and 1:500,000-scale, although many othermap scales exist.
The inherent accuracy limitations for USGS topo maps is typically about +- half a contour interval, although it can be worse than this in steep or heavily vegetated areas. This is much better than delivered by most GPS handhelds, whose elevations can be in error by several 10s of meters.
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.
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 representaion 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.
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