Thanks Peter, good points. One further question: even if the
georeferenced map as a whole is distorted due to too few control points,
the control points *themselves* should always be projected to their
exact specified georeferenced location, right? At least that is wthat I
thought was the difference between rubber sheeting algorithms like thin
plate spline and polynomial rectifications, where some kind of error is
distributed between the control points.
Oh, and I would be very much interested to know more of your experiences
with the algorithms to be used for rectifying distorted historical maps.
Regards,
Jan
On 05/04/10 08:16, Peter J Halls wrote:
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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