[gdal-dev] Thin Plate Spline

[Peter J Halls]

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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-- 
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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
This message has the status of a private and personal communication
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