Evidently, the normal operating mode of most GPS units gives NOT the elevation above Mean Sea Level- which presumably is the elevation given on USGS topological maps: http://www.topozone.com - but instead gives the elevation above the WGS84 ellipsoid; though many units supposedly provide a MSL option. MSL presumably correlates closely with the Geoid (surface of constant gravitational potential) that it also serves to define. The difference between the Geoid and the WGS84 ellipsoid is given by the Earth Gravity Model, EGM96; typically, the Geoid is about 30 meters below the WGS84 ellipsoid in North America: Color-coded world Geoid map http://earth-info.nga.mil/GandG/images/ww15mgh2.gif NGA EGM96 online Geoid Calculator http://earth-info.nga.mil/GandG/wgsegm/intpt.html (That website is apparently hosted by an Apple II, and may take an hour or two to load.) I'm curious as to what elevations others are reading with their GPS units. I have a 3 year-old DeLorme GPS (not quite as fast or accurate as their snazzy new USB unit, apparently), which seems to average about 250 meters for my location, while USGS topo maps indicate about 237 meters (presumably above MSL). The GTOPO30 Digital Elevation Model gives 213 meters (above the Geoid / MSL, supposedly) as an approximate elevation, representing roughly the 1 square kilometer area around me, and the online NGA Geoid calculator indicates that the Geoid is about 33 meters below WGS84 here. So something's not adding up: -33 meters (Geoid elevation above WGS84, from EGM96) + 213 meters (my elevation above the Geoid, from GTOPO30) != 250 meters (my elevation above WGS84, from my GPS) ... possibly my GPS is reporting in MSL mode, rather than WGS84 mode, in which case there's a 37 meter discrepancy between it and GTOPO30; otherwise, the discrepancy is 70 meters. I'm curious as to what elevations others may be reading with their GPS units, compared to USGS or other topo maps. Incidentally, the GLOBE project is largely parallel to GTOPO30, and provides a means of reading DEM data in small (i.e., localized) chunks: http://flighttrack.sourceforge.net/dems.html http://flighttrack.sourceforge.net/screenshots.html ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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