------- Original Message --------
Subject:        RE: Google Earth miss-matches
Date:   Mon, 26 May 2008 02:27:14 +0000
From:   Brendan Whyte <[log in to unmask]>
To:     Maps, Air Photo & Geospatial Systems Forum <[log in to unmask]>




The mismatches are partly a resolution issue: while you can zoom in to
car level on the image, the boundary polygons may have been digitised
from 1:1 million maps or worse, and not from the images. All the usual
problems of the accuracy of lines on a map appear when you then try to
overlay those lines on an image.
Certainly the India-Bangladesh boundary is very badly drawn.

In some cases the boundaries may be old or inaccurately known.
PointAsia, the Thai version of Google Earth has terrible polygons for
Thai administrative units. They appear to have been digitised by hand
off outdated or small scale maps (or both). But for units below the
province, it is difficult to find anyone ho knows where the actual
boundary should be, and local govt units themselves have very inaccurate
maps, with many areas claimed by two neighbouring subdistricts, or left
apparently unclaimed/unadministered! And once you go to down to village
level it's even worse.

Thirdly, there is! an issue of orthorectification. When I save two side
by side screen shots (e.g. an east and a west screenshot of a linear
east-west feature), and trace the feature in a drawing package on one
image, when I try to line up the other image underneath it, there is
parallax or distortion of some sort along the edges. This distortion of
the images may or may not distort overlaid boundary polygons etc in the
same way.

And then there's the datum questions too...

Dr Brendan Whyte
Ubon Ratchathani University
THAILAND


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