[gdal-dev] tps - gdalwarp vs ogr2ogr

[Stijn Tallir]

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) ...

(sorry for the double e-mail, I didn't respond correctly the first time)

Stijn

Op wo 16 aug 2023 om 10:22 schreef Rahkonen Jukka <
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> *Puolesta *Stijn
> Tallir
> *Lähetetty:* keskiviikko 16. elokuuta 2023 10.29
> *Vastaanottaja:* 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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