# Question regarding GPS units

From: Thomas Fly (tfly@alumni.caltech.edu)
Date: Sat Jun 12 2004 - 15:59:09 EDT

• Next message: Kevin Z Grey: "RE: Question regarding GPS units"

```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

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.

-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

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