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    Re: Double Altitudes: Prelude to Sumner's line?
    From: Fred Hebard
    Date: 2005 Feb 28, 09:15 -0500

    After I posted, I realized I had not answered your question.  I can
    only speculate that Sumner was the first to draw a line of position
    through the two points.  But I don't know the meaning of the term,
    double latitudes.
    On Feb 28, 2005, at 8:35 AM, Jim Thompson wrote:
    > 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
    > procedure.
    > Jim Thompson
    > jim2@jimthompson.net
    > www.jimthompson.net
    > --------------------
    > 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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