RE: Question regarding GPS units

From: Kevin Z Grey (
Date: Sat Jun 12 2004 - 18:53:15 EDT

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    I have a Delorme Earthmate and always thought that the elevation was
    wildly off (if I remember correctly, by about 200 feet when at sea level
    in Manhattan.  If I have time today, I'll pull it out and see what the
    exact number it spits out.
    Its too bad that that Flight Track software is MacOS only!  I think I
    could have used that to map out hikes...
    -----Original Message-----
    From: Thomas Fly [] 
    Sent: Saturday, June 12, 2004 3:59 PM
    To: SeeSat-L
    Subject: Question regarding GPS units
    Evidently, the normal operating mode of most GPS units gives NOT the
    above Mean Sea Level- which presumably is the elevation given on USGS
    topological maps: - but instead gives the elevation above the
    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
    the Geoid and the WGS84 ellipsoid is given by the Earth Gravity Model,
    typically, the Geoid is about 30 meters below the WGS84 ellipsoid in
    Color-coded world Geoid map
    NGA EGM96 online Geoid Calculator
    (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
    while USGS topo maps indicate about 237 meters (presumably above MSL).
    The GTOPO30 Digital Elevation Model gives 213 meters (above the Geoid /
    supposedly) as an approximate elevation, representing roughly the 1
    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
    above the Geoid, from GTOPO30) != 250 meters (my elevation above WGS84,
    from my
    ... possibly my GPS is reporting in MSL mode, rather than WGS84 mode, in
    case there's a 37 meter discrepancy between it and GTOPO30; otherwise,
    discrepancy is 70 meters.
    I'm curious as to what elevations others may be reading with their GPS
    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:
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