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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
mark.

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:

http://www.ngs.gov

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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