[gdal-dev] Transforming from EPSG:29902 to EPSG:4326 at the limit of the area of use.

[Javier Jimenez Shaw]

Assuming that it does not happen in the main area of Ireland...I guess it
is because it is outside of the area of usage of the datum transformation
https://epsg.org/transformation_1641/TM65-to-WGS-84-2.html
Probably (this is just a guess) gdalwarp is not taking it into
consideration, making a ballpark transformation between TM65 and WGS84,
while ogr2ogr doesn't.

On Wed, 8 Nov 2023 at 13:35, Sam Hall via gdal-dev <gdal-dev at lists.osgeo.org>
wrote:

> I have some data that covers the whole of the Republic of Ireland,
> including the Blasket Islands off the West Coast, situated around 24439,
> 95540 in EPSG:29902. These islands straddle the western boundary of the
> area of use of the coordinate system -10.56° W.
>
> When transforming a raster dataset from EPSG:29902 to EPSG:4326 using
> gdalwarp at gdal >= 3.5.2, there seems to be a roughly 50 m shift in the
> data at the area of use boundary, while if I use ogr2ogr on a vector copy
> of this data, I don't see this shift.
>
> I have a reproducible example of this at
> https://gitlab.com/Sam.Hall1/29902_gdalwarp_vs_ogr2ogr.git. In this
> example I created a rectangular polygon with interpolated points and saved
> it to a shapefile, then used gdal_rasterize to create a GeoTiff. gdalwarp
> gives the desired result using a gdal 3.5.1 docker image. The full commands
> are in the docker-compose.yml in the repository but the parameter's I'm
> using are:
>
> gdalwarp -t_srs EPSG:4326 -tr 0.000045 0.000045 -tap -overwrite input.tif
> output.tif
> ogr2ogr -t_srs EPSG:4326 output.shp input.shp
>
> I've also seen similar results when transforming from EPSG:27700 to
> EPSG:4326 in Jersey and Guernsey in the English Channel but I'd have to use
> a much earlier version of gdal to reproduce the desired result.
>
> Is there a gridshift file or any additional parameters that I could use to
> get the output to look more continuous across the area of use line?
>
> Sam
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