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    Re: Sextant Positions versus Map Datums?
    From: Jared Sherman
    Date: 2002 Jan 15, 10:27 PM

    Yes, I found a USGS topo and a nuatical chart both on the Maptech mapserver, 
    and "the same" place--a landmark on the maps--was located 1.1 miles apart 
    when I plotted the position shown on the topo map against the position shown 
    on the chart. The topo may have been NAD27 because it was older, the nautical 
    chart should be NAD84 or something similarly recent.
    
    I don't know the datums used on the two electronic maps because Maptech 
    "tiles" the entire US seamlessly, stripping off all collar information from 
    the online mapserver. It is optionally available on their CD products, but I 
    wasn't going to drop $200 on the spot to verify that.
    
    Some years ago I also noticed a similar displacement in lower NY harbor, where 
    a position in the Verrazano Narrows under the bridge jumped to well inland on 
    Staten Island. I've tried to be more aware of map datums since then, even if 
    I don't use the ones in the Pacific atolls.
    

       
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