[birkskyum]

When EPSG only defines a 7-parameter Helmert transformation from WGS 84 to a datum, the bound CRS export writes its forward parameters as +towgs84, so the shift goes the wrong way. projinfo EPSG:6991 --boundcrs-to-wgs84 -o PROJ gives a string that PROJ itself places ~156 m from its own EPSG:4326 to EPSG:6991 result. proj_coordoperation_get_towgs84_values() returns the same forward values for every such reversed transformation, 17 of them among the operations to WGS 84.

A 7-parameter inverse stays an InverseTransformation, which keeps the parameter values of the forward one. This PR makes getTOWGS84Parameters() use the approximate inverse there, as _exportToWKT() already does.

• AI (Copilot or something similar) supported my development of this PR. See our policy about AI tool use. Use of AI tools must be indicated. (Claude Opus 5.5)
• Tests added
• Added clear title that can be used to generate release notes

[kbevers]

projinfo EPSG:6991 --boundcrs-to-wgs84 -o PROJ gives a string that PROJ itself places ~156 m from its own EPSG:4326 to EPSG:6991 result

Can you show this using PROJ command line tools? It would be easier for me to understand if expressed with an example using the standard tools

[birkskyum]

@kbevers Sure. With PROJ master, at a point in Jerusalem:

$ projinfo -s EPSG:4326 -t EPSG:6991 --summary
Candidate operations found: 1
unknown id, WGS 84 to IG05/12 Intermediate CRS + Israeli TM, 0.1 m, Israel - onshore; Palestine onshore.

$ projinfo EPSG:6991 -o PROJ -q
+proj=tmerc +lat_0=31.7343936111111 +lon_0=35.2045169444444 +k=1.0000067 +x_0=219529.584 +y_0=626907.39 +ellps=GRS80 +towgs84=-23.772,-17.49,-17.859,0.3132,1.85274,-1.67299,5.4262 +units=m +no_defs +type=crs

$ echo 31.7784 35.2066 | cs2cs -d 3 EPSG:4326 EPSG:6991
219660.851	631746.029 0.000

$ echo 31.7784 35.2066 | cs2cs -d 3 EPSG:4326 +to +proj=tmerc +lat_0=31.7343936111111 +lon_0=35.2045169444444 +k=1.0000067 +x_0=219529.584 +y_0=626907.39 +ellps=GRS80 +towgs84=-23.772,-17.49,-17.859,0.3132,1.85274,-1.67299,5.4262 +units=m +no_defs
219792.932	631827.902 0.000

The exported string lands 155 m from PROJ's own result. EPSG:9189 is defined from WGS 84 to IG05/12, so +towgs84 needs its inverse, but the export writes the forward parameters. --boundcrs-to-wgs84 gives the same string.

With this PR, projinfo EPSG:6991 -o PROJ gives +towgs84=23.772,17.49,17.859,-0.3132,-1.85274,1.67299,-5.4262, and the same cs2cs call returns 219660.851 631746.029, identical to EPSG:4326 to EPSG:6991.