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    Re: Equation for dip?
    From: James R. Van Zandt
    Date: 2006 Sep 30, 23:01 -0400

    Geoffrey Kolbe  wrote:
    >   I grabbed an old Nautical Almanac, opened it randomly and lo and behold,
    >   d = 0.0293*h^0.5 where d is in degrees and h is height of eye above
    >   the horizon (sea level) in metres.
    If you work out the geometry, you find
        d = sqrt(2*h/Re)
    where Re is the radius of the earth.
    In radar analyses, it's usual to use the "four-thirds earth model", i.e.
        d = sqrt(2*h/(1.33*Re))
    which corrects for refraction.
    See for example: http://twister.ou.edu/papers/Gao_Xue_Brewster_AAS2006.pdf
    The NA formula above works out to
        d = sqrt(2*h/(1.2*Re))
    which is a "six-fifths earth model", i.e. less refraction.  I wonder
    whether that's because of the different wavelength, or different
    typical heights (so different temperature and pressure profiles), or
                   - Jim Van Zandt
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