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    Re: Sumner's Line (Navigation question)
    From: George Huxtable
    Date: 2006 Feb 15, 11:56 -0000

    Gary LaPook wrote-
    | Capt. Sumner had to work the time sight twice, doing the math two times
    | which was difficult in the days before inspection tables and computers.
    | Marc De St. Hilaire's contribution was his method that allowed you to
    | lay down Sumner's LOP while only doing one computation. Most still cling
    | to this method today in spite of the development of compact computers.
    | This still means you have to determine an AP, plot it on the chart,
    | measure the ZN and measure off the intercept.
    | A simpler method is to use Sumner's original system but with a computer.
    | A long time ago I programed my handheld calculator to compute the
    | latitude of a LOP given a longitude or the longitude given a latitude.
    | It automatically computed two such points, one east and one west of the
    | dead reckoned position (or one north and one south) so all I had to do
    | was plot these two points on the chart and draw a straight line between
    | them with a straight edge, no measuring azimuth or intercept, an easier
    | way to lay down the LOP and less chance for error. People should
    | consider using this new (old) method today.
    Response from George-
    What a good idea! It can hardly be called original, of course, dating back to Sumner's day. I can't
    see any snags, as long as the pair of latitudes (or longitudes) have been suitably chosen to span
    within a chart or plotting sheet.
    With a span of 1 degree, between whole degrees, the error in taking the position line to be a
    straight line instead of a gentle curve, on a chart, is insignificant, except near the poles where
    we don't expect to be sailing. At other directions, presumably you could do the job, for both
    bodies, with just one pair of chosen coordinates.
    Perhaps, if one body is observed close to due South, and the other due West, you would have to
    define two chosen longitudes for one body, and two chosen latitudes for the other, to plot the
    intersection. But that wouldn't be a big problem.
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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