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    Re: Corrections for refraction in deserts
    From: Geoffrey Kolbe
    Date: 2006 Mar 6, 19:47 +0000

    Many thanks to Doug for his reply, and for sharing his experience of CN in
    desert conditions.
    The principal maps I will be using are the Russian 1:500,000 series based
    on the Pulkova 1942 reference datum. I also have an American Air
    (1:250,000) map of the Gebel Uweinat area (22 degrees North, 25 degrees
    East) based on the WGS 84 datum. At the minute level there appears to be
    good agreement between the lats and longs. GPS's usually work to the WGS 84
    system, so at the level of accuracy to which I expect to be working (one
    minute) I don't anticipate any problems in agreeing where we are on the map!
    Geoffrey Kolbe
    >I spend a good deal of time in the deserts of the American Southwest in the
    >last 2 years. During all seasons. I do play around, when the whim strikes
    >me, using CN to find a position when hiking/traveling cross country.
    >I use a Palm OS type NA and reduction program where one may input the
    >temperature of the time when the cut is taken. The accuracy of my positions
    >are relative. Sometimes not so close and sometimes very close to the
    >corresponding gps pos. I do this to lighten up the weight of my kit. PDAs
    >are smaller and lighter than a paper NA. I carry a wrist and stop watch, a
    >small pad of notepaper and pencil, an Astra IIIB sextant for the most part,
    >and a home made glass AH.
    >Now, in all fairness, I use neither the paper NA nor reduce any of the data
    >by hand. I just allow the software to do all the reductions.
    >So basically what I do is note the DR pos, the time when the sight is shot
    >and of what body (usually the Sun), and input that into the program to
    >obtain an LOP.
    >Another interesting wrinkle I use that may contribute to the accuracy or
    >inaccuracy of the positions obtained is using software to again convert the
    >Lat. - Long. Coordinates obtained into a UTM or MGRS coordinate because the
    >terrestrial coordinates of the projections I use are in that format. I have
    >found that when traveling cross country using this coordinate system instead
    >of lat-long I will have more "accurate" position represented on the 1:24,000
    >or 1:62,500 scale projections I use.
    >Good luck on your trip to the Libyan deserts.
    Border Barrels Ltd, Newcastleton, TD9 0SN, UK
    Tel: +44 (0)13873 76253   Fax: +44 (0)13873 76214

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