[gdal-dev] tps - gdalwarp vs ogr2ogr

[Stijn Tallir]

Hi Jukka,

I finally found the time to produce a test set.

You can download it here:
https://drive.google.com/file/d/1Y08Q-tIm5dxyKFKNdVqilvAO3H7FFFbx/view?usp=sharing

I started from an unreferenced raster file (raster2tps.tif) with gcp's
(gcp4tps.gcp) and transformed it with tps (tpsraster.tif).
Then polygonized the unreferenced raster file (vector2tps.shp) and
transformed the result with  the same gcp's (gcp4tps.gcp) and with tps
(tpsvector.shp).

The vector2tps.shp polygons are "flipped" because of the different Y-origin
for rasters and vectors but this way both datasets can use the exact same
gcp's.

When you lay the tpsvector-result on top of the tpsraster-result (in QGis
for instance) you'll see the differences in how both are transformed.

Kind regards,

Stijn

Op wo 16 aug 2023 om 13:16 schreef Stijn Tallir <stijn+gdal-dev at strict.be>:

> Yes, I checked them visually for both raster and vector.
>
> I compared the results also visually. The rasters are transformed in a way
> that the end ponts of the gcp's align exactly with the result so that is
> why I referred to it as "right". The vector data result is in the
> neighbourhood of the end points (sometimes a rather significant distance).
>
> The result is different from order 1-3 transformations so I presume the
> tps option isn't ignored.
>
> Stijn
>
> Op wo 16 aug 2023 om 11:52 schreef Rahkonen Jukka <
> jukka.rahkonen at maanmittauslaitos.fi>:
>
>> 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>:
>>
>> 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
>>
>>
>>
>>
>> --
>>
>> Stijn Tallir - StrICT BV
>>
>>
>>
>> Wijnveld 8
>>
>> 9112 Sinaai-Waas
>>
>>
>>
>> GSM: 0486 750220
>>
>>
>>
>> E-mail: Info at strict.be
>>
>> Web: www.strict.be
>>
>>
>>
>> BTW: BE 0567.559.668
>>
>>
>>
>>
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