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    Re: Q: how to calculate refraction at higher altitudes on land?
    From: Michael Wescott
    Date: 2002 Feb 28, 14:55 -0500

    > I am specifically trying to determine the elevation of my house.
    > The topographic maps of my area are decades old.  They do not show
    > our streets or houses, and in fact the area has been graded to some
    > extent as well, so even finding my exact location via GPS and then
    > looking on the map only gives a ballpark figure.
    > Since SA has been turned off, the GPS gives a range of elevations
    > from 580 feet to 650 feet.  The Garmin GPS 3 tends to wander over
    > this range.  I am trying to get a better value.
    From what I have read, GPS height info has about twice the inaccuracy
    as horizontal info. There is also the problem that heights, say on a
    USGS map, are often referenced to Mean Sea Level and not a particular
    ellipsoid. See http://www.ngs.noaa.gov/GEOID/GEOID99/ for more detail.
            Mike Wescott

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