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    Re: Double Altitudes: Prelude to Sumner's line?
    From: Jim Thompson
    Date: 2005 Feb 28, 09:35 -0400

    Thanks Fred.  I understand Sumner's method very well, and the connection to
    Saint Hilaire's 30 years later, but I don't understand all the factors that
    lead up to Sumner's discovery of his method.  The "Double Latitude" concept
    obviously had been floating around already in his time, and may have given
    him the basic tools that allowed him to develop his approach.  If that's
    what I think it is, then maybe the idea of using one sight to calculate two
    solutions based on two assumed latitudes (one from DR, and one theoretical)
    was not that novel, in his time.  What was novel was his application of the
    Jim Thompson
    Outgoing email scanned by Norton Antivirus
    > -----Original Message-----
    > From: Fred Hebard
    > Sumner didn't use St. Hilaire's method, assuming one location and then
    > finding a line of position based on azimuth and distance from that
    > position.  Rather, he assumed two latitudes and found the longitudes
    > corresponding to those latitudes.  Then fitted a line of position
    > through those two locations.
    > Having the assumed latitude plus the sight, he determined longitude.
    > Assuming a second latitude gave him a second longitude, with the same
    > sight data.
    > Fred

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