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    Geodetic Survey Web site
    From: Paul Hirose
    Date: 1997 Dec 14, 8:20 AM

    All the GPS surveying rigs I've seen in GPS World magazine appear to
    be of the relative positioning type.  A base station sits on a point
    of known coordinates, and a "rover" visits the points of interest.  At
    each point the rover can simply collect data to be processed into a
    accurate coordinates after the fact.  Or it can display a position in
    real time, based on corrections obtained through a radio link to the
    base station.  The latter arrangement seems to be what they were using
    on that Pearl Harbor job someone described.
    The techniques used in these receivers go well beyond the differential
    GPS familar to boaters.  There is sophisticated counting of phases
    between the two stations.
    I'm skeptical about this SA eliminator program.  The developer says it
    has .0001 minute accuracy?  Maybe it does, but I'll bet this is an
    extreme figure, based on occupying the site for an impractical length
    of time.  I'd like to see his test results.  For a good test I think
    you'd have to set up the receiver and laptop at a geodetic survey
    There is now a Web site where you can obtain the coordinates of any
    horizontal control station in the National Geodetic Survey database.
    These stations are those little brass disks embedded in cement, used
    by surveyors.  (Not to be confused with bench marks, also brass disks,
    but used for vertical rather than horizontal control.)  On the US
    Geological Survey 1:24000 maps, they're shown as tiny triangles with a
    dot in the center.
    The address is:
    Click on "products & services", then "data sheet", then "retrieve data
    sheet by control point name".  (I'm typing this from memory, but
    should be close enough to get you there.)
    Select the state and type in the name of the point.  Try the state of
    Kansas, station "Meades Ranch".  This is the central point of the 1927
    North American Datum, although there's nothing on the data sheet to
    indicate there's anything special about this station.
    By the way, this station is only a few miles from Bob Dole's home town
    of Russell.
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