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Archiver > TMG > 2011-02 > 1297752788

From: Dennis Lee Bieber <>
Subject: Re: [TMG] Giving locations by lat/long, GPS, etc.
Date: Mon, 14 Feb 2011 22:53:08 -0800

On or about 2/14/2011 10:07 AM a carrier pigeon from Darrell A.
Martin delivered:

>For example, I start with the coordinates, precise but not fanatically
>so. As discussed, each reading on a commercial GPS is off by an
>unpredictable, small, amount. The difference between two such readings
>could be up to 25 meters (e.g. different receivers and/or different
>days). So, I *add* to that, something like "17.4 meters east-northeast
>of the trunk of the large oak tree, 8.3 meters due south of the south
>side of the DUTTON obelisk." The GPS gets me close. The landmarks, a
>small compass, and a cloth tape measure nail it down. If the combination
>doesn't find it, either I have gone blind or it "jist ain't there".

Pity TMG is not rigged for UTM coordinates...

UTM coordinates ARE in meters, and if you aren't taking too
long (meaning the Navstar satellite configuration changes
noticeable), taking the difference of the UTM easting and northing
for the landmark item and the subject item would give you the direct
meter measure...

Accuracy of GPS is much better now than when Selective
Availability was turned on. And in the future, when the next
generation of satellites take over -- new GPS receivers should be
even more precise. The Navstar system currently uses two frequencies
for military (and the precise positioning system which has, uhm, I
think, 1000 "chips" where the Coarse/Acquisition signal has one
"chip"). Two frequencies lets military GPS correct for ionospheric
propagation delays. The next generation puts the C/A signal on both
frequencies (the military gets even more frequencies, and us poor
civilians still just get the slow speed C/A signal)

If one's GPS shows active satellites in a tetrahedron (on
overhead, three on the horizon ~120 degrees apart) one should have
the most accurate position report. If the active birds are almost in
a line, one likely has good accuracy in the direction of the line,
but poorer accuracy perpendicular to it.

And you do want at least four satellites in the computation
-- with four satellites, the GPS unit can compute: time (you didn't
think the quartz clock in the GPS unit is that accurate, did you --
Navstar birds have Cesium and/or Rubidium "atomic" clocks, corrected
for relativity effects); latitude, longitude, and altitude (the least
accurate term).

Also -- one needs to note what datum system was used. The
datum covers the "shape" of the earth and where "0, 0" is located. I
tend to leave mine in WGS-84 (World Geodetic Survey 1984). Many USGS
maps still use NAD-27 [?] (North American Datum 1927) though there is
a NAD-72 [or 74?].

And, as I recall, one minute of longitude, at the equator,
equals on nautical mile.

Lee Bieber


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