[gdal-dev] tps - gdalwarp vs ogr2ogr

[Rahkonen Jukka]

Hi,

Did you check the ground control points? What is your reference when you say that one result is right, and another wrong? Have you used some other software for comparison? Or do you only know that the results are different?

-Jukka-

Lähettäjä: Stijn Tallir <stijn at strict.be>
Lähetetty: keskiviikko 16. elokuuta 2023 12.27
Vastaanottaja: Rahkonen Jukka <jukka.rahkonen at maanmittauslaitos.fi>
Kopio: gdal-dev at lists.osgeo.org
Aihe: Re: [gdal-dev] tps - gdalwarp vs ogr2ogr

Hi Jukka,

I thought of the density as an option for the "error" as you suggested and I made a point-file with a point for every pixel in my original image and used this as a source for the ogr2ogr transformation. So you could say the desnity for both sources raster and vector) are then alike.

The results were still the same (and wrong) ...

Stijn


Op wo 16 aug 2023 om 10:22 schreef Rahkonen Jukka <jukka.rahkonen at maanmittauslaitos.fi<mailto:jukka.rahkonen at maanmittauslaitos.fi>>:
Hi,

Without test data it is very hard to say much. I believe that the promise of tps is that the ground control points stay where they are set. The intermediate points follow the least tension surfaces and I do not know how exactly those spline algorithms are defined. Raster data is full of points to warp but probably in the vector data the transformation is done vertex by vertex. I would first check if the GCPs are in the same place in both outputs. Then I would make a test by densifying the vector data before georeferencing to have much more vertices and see if it has any effect on the result.

-Jukka Rahkonen-

Lähettäjä: gdal-dev <gdal-dev-bounces at lists.osgeo.org<mailto:gdal-dev-bounces at lists.osgeo.org>> Puolesta Stijn Tallir
Lähetetty: keskiviikko 16. elokuuta 2023 10.29
Vastaanottaja: gdal-dev at lists.osgeo.org<mailto:gdal-dev at lists.osgeo.org>
Aihe: [gdal-dev] tps - gdalwarp vs ogr2ogr

Hi,

According to the documentation gdal and ogr use the same algorithm for the tps-transformation but I don't seem to get the same results using the same set of gcp's for images and vectors.

I have images that are unreferenced and vector data digitised on these images (in pixel coordinates).

The images are then georeferenced with +100 gcp's and warped with gdalwarp using the "tps" option.

When I use the same gcp's (with adjusted y-origin to lower left corner) to georeference the vector data with ogr2ogr and the "tps" option I get different results. The vector-result is similar to the image-result but never exactly the same and differences can be substantial.

Any thoughts?

Stijn


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