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    Re: Sextant Positions versus Map Datums?
    From: Jared Sherman
    Date: 2002 Jan 17, 11:18 AM

    Interestingly enough I have gotten a response on this from the ask{at}usgs.gov folks.
    
    They directed me to a datum conversion program (CORPSCAN) which is publicly 
    available but mainly limited to NAD27 and NAD84, NAD84 arguably being 
    "substantially identical" etcetera to the later US domestic datums.
    
    There is also a more advanced tool available from NIMA but that can only be 
    downloaded to recipients having .MIL and .GOV email addresses--if someone 
    here has one of those I'll be glad to give them a URL if they can make it 
    available afterwards.
    
    Anyway...using CORPSCAN in the area of N40d 35.703m, W073d 29.596m and 
    inputting that as NAD84, that position shifts by some 38 meters when 
    translated to NAD27. (38 meters found by taking the offsets produced by the 
    program and calculating the length of the diagonal.)
    
    And that ignores the question of whether land maps (topos) may be using 
    various state co-ordinate systems...I have no idea about the topos, have only 
    used them on a limited "we're here" basis in the past when hiking.
    
    I don't think anyone would argue about 38 meters when doing sextant 
    navigation, just note that there is that much difference simply from a datum 
    shift--so there *must* be some difference between astronomical positions and 
    positions taken from various map datums showing landmarks. Hopefully less 
    with the newer datums.
    

       
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