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    Re: Spherical earth model vs. ellipsoid
    From: Jim Manzari
    Date: 1999 Mar 16, 7:02 AM

    Henning Umland wrote:
    > When I made my experiments, I was aware that the spirit level can be
    > deflected by local anomalies of the earth's surface like ,e.g., ore
    > deposits, mountains, lava plugs, etc.
    My guess, then, is that if one selects an ellipsoid or geoid, applies
    the correct altitude above the selected ellipsoid or geoid, and
    applies the correct gravity model for the geographic position, your
    method should provide sub-100 meter accuracy anywhere on land after
    correction for chart datum and charting errors.
    If higher levels of accuracy are desired, there appear to be a lot of
    subtle difficulties to overcome (see paper next below).  There are a
    number of satellites in orbit (TOPEX, ERS-2, etc) devoted to the
    measurement of sea-level heights and the shape of the ellipsoid.
    You may find the following paper to be interesting (plus references
    As you will see these people are working in the realm of 5cm accuracy.
    There are a few other papers in the above symposium that may interest
    you.  Unfortunately, I wasn't able to find an actual description of
    the gravity models used, especially any surface level models.  If you
    find a useful surface gravity model please let me know.
    Also see the following for TOPEX maps, etc...
    You may already know all this, so please disregard as necessary.
    Best regards,
    Jim Manzari
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