The National Map’s Elevation Point Query Service (EPQS) returns elevations that are interpolated from the 3DEP dynamic elevation service. This is a web coverage service (WCS) that includes multiple resolutions of DEMs including 1 meter resolution lidar-based DEMs where available, and 1/3 arc-second seamless DEMs. The accuracy of the elevations returned by EPQS will vary according to the source data for each particular location. The overall accuracy of the elevation service currently has a RMSE (root mean square error) of 0.53 meters. These interpolated point elevations are not official and do not represent precisely measured ground surveyed values.
Elevations derived for a specific location using the Elevation Point Query Service might differ from monumented control elevation values that are published on USGS topographic maps or cited in National Geodetic Survey Marks and Datasheets. Differences between these elevations might exist for features such as mountain peaks or summits, and where the local relief is significant. For most purposes other than engineering, interpolated point elevation values are sufficiently accurate.
The Elevation Point Query Service, the Bulk Point Query Service, and the Spot Elevation widget within The National Map Viewer all query the same dataset and have the same accuracy.
Learn More: The accuracy and consistency of 3D Elevation Program data: A systematic analysis
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Elevations provided by your GPS receiver might disagree with elevations on a USGS map, but they could both be correct if they reference different vertical and/or horizontal datums. The default horizontal datum setting on most GPS receivers is the World Geodetic System of 1984 (WGS84) and the elevations are based on the NAD83 ellipsoid. USGS topographic maps published after 2009 are georeferenced...
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