[Taxacom] Impossible geocodes

mesibov at southcom.com.au mesibov at southcom.com.au
Fri Jan 30 16:02:39 CST 2015

Tom Schweich wrote:

"The centroid of the points was shifted to the northeast about 1.5-2.0 m.,
something I was unable to account for."

Plate movement? I'm guessing your control points were survey marks in the
local coordinate reference system, which moves with the plate. Here in
Australia, survey marks are databased with the GDA datum, which is more
than 1 metre different from WGS84 because the whole continent is moving NE
at about 6 cm a year.

"I went out on my bicycle and visited 7 control points 14 times each, and
learned that my GPS receiver was accurate (on that day at that place) to
between 4-5 meters"

Excellent test. Have you checked to see how close Google Earth gets to the
control point positions? On a well-georegistered image tile with a
reasonably high-resolution image, Google Earth can do better than a
handheld GPS.

Not much help here in Tasmania under a tree canopy, though. Where in hilly
terrain my GPS struggles to get an 'accuracy' declaration less than 20 m.

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