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    Re: Fix by Lunar Distances... for missiles in 1950
    From: Bill B
    Date: 2006 Dec 03, 21:26 -0500

    Bill wrote:
    
    >> Ah, the stigma attached to practicing tradition navigation today.  My
    >> GPS/Blue Chart hugging sailing buddies now want to go on a geocache hunt.
    >> They insist I use a sextant!  Could be a long day!
    
    Richard replied:
    
    > Use the sextant sideways on three well placed reference objects, use the
    > angles between them to plant your own cache, and see if *they* can find it
    > with GPS.  :-)
    
    Excellent point.
    
    Coming out of Chicago last summer I was playing.  From about 3 miles off
    shore I started taking a series of measurements (with a bit of lab-assistant
    help). Using Sears and John H we would record our current GPS position, GPS
    distances, GPS bearings, hockey-puck compass bearings, and angle between
    structures measured with the sextant.
    
    The GPS was set to read out true bearings, and the compass readings were
    adjusted to true using variation from a government site.
    
    As the bases of the structures sank below the horizon and the angular
    separation became extremely small, we recorded our current GPS position, GPS
    distance, GPS bearing, hockey-puck compass bearings, and angular height of
    the Sears Tower above the horizon measured with the sextant.
    
    I plotted all the possible combinations.  The GPS won in the near-shore
    tests, but not by much.  Often the sextant/compass and GPS points were
    superimposed.
    
    About the 15 mile range, the sextant (Sears angle above horizon) & hockey
    puck compass started winning at nailing the GPS position.  While distances
    were comparable, the GPS bearing (my Garmin 76 unit reads out in whole
    degrees) was a tad off the hockey-puck compass bearing.  Distance calculate
    by a modified Bowditch formula with a refraction value *suggested* by Frank
    and one I derived were all but spot on.
    
    So a good day for traditional navigation.
    
    It does raise a least two questions.
    
    1. When a GPS unit converts between true and magnetic or vice versa, does it
    use up-to-date variation values downloaded form a satellite or is it using a
    lookup table imbedded in its firmware?
    
    2. Is a geocache hunt really an exercise in navigation (except for being
    able to find your way through a road-map maze to a known point) or is it
    more being clever enough to find a toy soldier etc. hidden in a hollowed-out
    fence post when you are <20 feet from it?
    
    Bill
    
    
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