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    Re: Spherical earth model vs. ellipsoid
    From: Mike Wescott
    Date: 1999 Mar 12, 12:03 PM

    We have, I think, a misconception being promulgated in this thread, or
    at least some difficulty in the definitions in being used (or maybe
    it's just me).
    See the attached diagram.
    The measured or actual latitude is with reference to the vertical, as
    GRAVITY would have it. This corresponds to a line normal
    (perpendicular) to the plane of the horizon when at sea. This is the
    Latitude that we measure with a sextant or theodolite. The geocentric
    ("spherical model" ?) latitude is not easily measured. As the diagram
    shows, if the Earth isn't a perfect sphere, the two do not coincide.
    But neither is the Earth a perfect ellipsoid. And the measured
    latitude will be affected by regional variations in the shape of the
    Earth and by regional variations in gravity.
    The standard ellipsoids are attempts to model the actual earth with a
    standard, smooth shape. Each standard ellipsoid is an attempt to
    specify an ellipsoid that will minimize some given set of discrepancies
    between the model and actual measurements. That minimization is either
    for the whole earth (i.e. WGS84 ellipsoid) or just for some region
    (e.g. the country standards).

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