[gdal-dev] How do GCP's in a VRT file work?

[Even Rouault]

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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