[gdal-dev] gdal2tiles and EPSG:27700

[Javier Jimenez Shaw]

Hi

I have a GeoTIFF in EPSG:27700, "OSGB36 / British National Grid"
For consistency reasons, I want to run gdal2tiles with an equivalent
transformation than the one I will use later to transform points using a
PROJ pipeline. For consistency reasons I will use that pipeline along the
time to transform the points, so an update in EPSG/PROJ does not change the
results (I generate the gdal2tiles at project creation time, but run the
transformation on the marker points along the time).

Getting the PROJ pipeline is easy with something like
projinfo -s EPSG:27700 -t EPSG:3857 --spatial-test intersects -o PROJ
--single-line -q
(the option --spatial-test intersects is very important. Otherwise you get
a ballpark. I am not using grids. I can use the area of use of the GeoTIFF
as well) It produces
+proj=pipeline +step +inv +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717
+x_0=400000 +y_0=-100000 +ellps=airy +step +proj=push +v_3 +step +proj=cart
+ellps=airy +step +proj=helmert +x=446.448 +y=-125.157 +z=542.06 +rx=0.15
+ry=0.247 +rz=0.842 +s=-20.489 +convention=position_vector +step +inv
+proj=cart +ellps=WGS84 +step +proj=pop +v_3 +step +proj=webmerc +lat_0=0
+lon_0=0 +x_0=0 +y_0=0 +ellps=WGS84

However this pipeline does not work in gdal2tiles with the "-s" option,
complaining with these errors, and placing the data in the wrong place (not
very far from Null Island)
ERROR 1: PROJ: proj_crs_get_coordinate_system: Object is not a SingleCRS
ERROR 1: PROJ: proj_as_wkt: PROJBasedOperation can only be exported to WKT2
ERROR 1: PROJ: proj_crs_get_coordinate_system: Object is not a SingleCRS
ERROR 1: PROJ: proj_as_wkt: PROJBasedOperation can only be exported to WKT2

Running projinfo on EPSG:27700 gives me the ballpark one
projinfo EPSG:27700 -o PROJ --single-line
PROJ.4 string:
+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000
+ellps=airy +units=m +no_defs +type=crs

The difference using this pipeline (without Helmert parameters) is about
100 m near London. Perfectly noticeable ;)

(I think it does not include the Helmert transformation, because the area
of use of that transformation is a bit smaller than the geographic crs,
EPSG:4277)

Of course I could create the command manually, but I would like to have a
generic solution for multiple projects.

What can I do?

I'm using GDAL 3.0.4 (Ubuntu 20.04)

Thanks
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