At:
http://mapserver.sara.nl/downloads/gdal_tps.png
you can see a screenshot of the original image (right) and the
georeferenced one (left, rotated almost 90 degrees). The red markers
with the number 3 inside them show one of the control points: to the
right the scan pixel (5023/3421), to the left the targeted
georeferenced coordinate in EPSG:28992 (121527/487174). As you see, the
georeferenced point is not exactly on the border of parcel 7, as I
expected. Above the map you see the four coordinates I used as control
points; I added four extra points half way the original ones, so the
complete warp was done with 8 control points.
Adding or removing points doesn't make any difference, and the other
control points are shifted too, although slightly differently. When I
use "-order 1" with the same control points I get exactly the same
picture; however, with really weird control points the images for -tps
and -order 1 get distorted in different ways.
It's only 10 meters or so, but it is very visible and too much for my
purposes. How can I get a georeferenced image with the control points
exactly on their indicated location, no matter what happens in between?
Jan
On 05/01/10 00:21, Even Rouault wrote:
It should work... Perhaps you can share how to reproduce what you see
Le Saturday 01 May 2010 00:17:06 Jan Hartmann, vous avez écrit :
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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