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    Re: Fix by Lunar Distances... for missiles in 1950
    From: Richard M Pisko
    Date: 2006 Dec 04, 23:32 -0700

    On Sun, 03 Dec 2006 19:26:32 -0700, Bill  wrote:
    
    > 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?
    >
    I would suspect a lookup table, but I don't know.  A place like  
    etrex@yahoogroups.com would be one place to find out; or might be able to  
    direct you to an answer.
    
    Or, from :
    =======quote=======
         Most GPS receivers have internal data and an algorithm to compute the  
    declination after the position is established. For example, the Garmin  
    GPS-45 displays the value on the Navigation Setup page, Heading: "Auto  
    Mag," and uses it to supply magnetic bearings. However; this data cannot  
    be updated from satellite transmission, therefore it is subject to become  
    outdated.
    
    When asked about this issue for the 12XL, Garmin responded:
    
    "The magnetic variation model used within our products is a series of  
    tables derived from the NOAA IGRF '90 model, which accounts for the  
    movement of the magnetic poles over time."
    =======end quote=======
    
    > 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?
    >
    If you have a map of some sort, so you don't end up on the wrong side of a  
    ravine, it's more being able to find the container without calling undue  
    attention to yourself.  But mostly, I think it is an excuse to get some  
    exercise for your body and mind, at your own pace.
    
    The first two part I tried needed first to find a small, dogbone shaped  
    rock.  Now this seemed to be somewhere on a ridge, and the readings were  
    different depending on which direction you approached the peak.  Turns out  
    I climbed the wrong side first (200 feet), got a better bearing from the  
    top, slid down into a little hollow where the readings were terrible, and  
    finally got a match on my cheap little eTrex.  There, just off the deer  
    path, was that one little rock.  On the underside was written another set  
    of coordinates about two hundred yards in the next valley to the south.   
    This time I wandered in a criss/cross pattern until I thought I had a fix,  
    which was fairly easy to find once within five feet or so.  This was a  
    slump area of very hummocky ground, had lots of little bushes and deer  
    beds; but still close to a walking trail.  It was a beautiful area close  
    to home that I never would have found on my own.
    
    I think it best to have a small compass with you, so you can read the  
    distance (say a good thirty yards) and bearing to the cache on your GPS;  
    and mentally plot a small house sized elliptical area on the ground in the  
    right direction.  Continue on an easy path for another fifty yards and  
    check again.  Turn a right angle at a convenient location and make a third  
    check.  A good place to start looking *without* the GPS is in the centre  
    of the overlap area of those three house sized ellipses.  If you have tree  
    cover or hills blocking the southern sky (as I did on that two part I  
    mentioned) the position readings can even stop updating; so you have to  
    stand away further.
    
    -- 
    Richard . . .
    Using Opera's e-mail client since Dialog, "the Dog", died.
    
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