If you use the -tps flag of gdalwarp, it will strictly honour your GCPs.
Cf https://gdal.org/programs/gdalwarp.html#cmdoption-gdalwarp-tps and
https://gdal.org/api/gdal_alg.html#_CPPv424GDALCreateTPSTransformeriPK8GDAL_GCPi
"The thin plate spline transformer produces exact transformation at all
control points and smoothly varying transformations between control
points with greatest influence from local control points. It is suitable
for for many applications not well modeled by polynomial transformations."
Le 29/11/2023 à 16:05, Joe Lovick via gdal-dev a écrit :
Could someone explain how GCP's placed into a VRT file work, i was
expecting a linear/bi-linear, or polynomial fit between the points for
the simplest EPSG:4326 projection.
however that is not what i see, adjusting a single point, effects the
projection of all corner points.
for example,
a vrt file that starts...
<VRTDataset rasterXSize="5328.0" rasterYSize="2160.0">
<GCPList Projection="EPSG:4326">
<GCP Id="1" Info="a" Pixel="0.0" Line="2160.0" X="-93.6369420815179"
Y="41.9822570038769" />
<GCP Id="2" Info="b" Pixel="5328.0" Line="2160.0"
X="-93.636924110907" Y="41.9822308438819" />
<GCP Id="3" Info="c" Pixel="5328.0" Line="0.0" X="-93.6369153669777"
Y="41.9822348674463" />
<GCP Id="4" Info="d" Pixel="0.0" Line="0.0" X="-93.636927955436"
Y="41.9822605751492" />
</GCPList>
.....
if i adjust the 0,0 point to be further east, when i look at the
resulting remaped image, all corner image points have shifted, rather
than just the single corner location for 0,0
what am i missing here?
many thanks
Joe
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