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    Index corr. for back sights, was: Octant as dipmeter
    From: Alexandre Eremenko
    Date: 2004 Nov 20, 11:33 -0500

    I propose the following procedure for Index correction
    for back sights on land. It seems simpler, and independent
    of the index correction for the fore sights.
    Choose two distant objects in opposite directions.
    They don's have to be too distant. Horizontal edges
    of roofs of the buildings
    will be OK. Or two assistants holding some poles.
    The difference in bearings should be 180d.
    Now measure the vertical angle between the directions
    on these two objects by a back sight.
    Then do the same with your quadrant upside down.
    One angle will be negative, another positive.
    Their algebraic sum, divided by 2 is your index correction
    for back sights.
    Remark. Taking a back sight with your quadrant upside down
    may be somewhat tricky: you have to incline your head
    to horizontal position. Not to obscure your back sight
    with your body. You may need another assistant for this
    (to hold your body from falling:-)
    On Sat, 20 Nov 2004, George Huxtable wrote:
    > I am especially interested in this matter because Lewis & Clark
    > On land, the following procedure seems plausible-

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