Jan,
the number of points required will depend upon the nature of the deformation
in the originals. You need sufficient control points to ensure that the
algorithm can sort things out, so relatively more points are required in the
deformed areas. However, although I've not used the GDAL Thin Plate Spline,
with many of these algorithms it is necessary to also have an even spread of
control points, or new distortion can be introduced in the previously
undistorted areas.
I would regard five control points as the minimum and suitable only for
originals that were completely flat when scanned. Five points should be enough
(just) to deal with orientation errors on the scanner and systematic scanner
errors. If the original had been folded, for example, even had it been ironed
before scanning, additional points would be needed either side of the former
folds, in order to sort out the compression they introduce. The worst case I've
ever had to deal with was a late 19th century map with no known projection
details that had been segmented and mounted on linen - with gaps between each
segment. In that case, I ended up transforming each segment and then stitching
the segments together - it worked remarkably well.
Hope that helps,
best wishes,
Peter
Jan Hartmann wrote:
Hi,
I'm rubber-sheeting old, deformed maps using the thin plate spline
option of gdalwarp (-tps). My impression was that the control points
would be transformed to exactly their georeferenced locations, but in
the georeferenced image the control poimts don't quite align with the
gcp-coordinates I specified. I'm only using five points per map, so
could this be the problem? If so, how many points do I need to get the
control points on their exact location? Or are there other ways to
achieve this?
Jan
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Peter J Halls, GIS Advisor, University of York
Telephone: 01904 433806 Fax: 01904 433740
Snail mail: Computing Service, University of York, Heslington, York YO10 5DD
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