Custom map projections do not align in newer versions of qgis

[kerkovits]

I use a custom map projection to georeference old topographic maps. The Proj4 string is: +proj=sterea +lat_0=47.4860018439082 +lon_0=19.0491441390302 +k=1 +x_0=500000 +y_0=500000 +ellps=bessel +towgs84=595.75,121.09,515.50,8.2270,-1.5193,5.5971,-2.6729 +units=m +vunits=m +no_defs

This screenshot was created using QGIS 3.4, and we can see that the old map aligns well with contemporary data.

Opening the same project in QGIS 3.10, the old map does not align.

Possibly related to #32795

Example data:

c.zip

[roya0045]

You are possibly in proj6, you will probably have to change your custom projection.

[gioman]

you will probably have to change your custom projection

is this that the user must be warned they have to do (when they move to proj6 based build of qgis)?

[kerkovits]

The old and new QGIS (3.4.13 and 3.10.0) uses the same version Proj6 (6.2.1). The error is likely to be in QGIS, not in proj.

[gioman]

The old and new QGIS (3.4.13 and 3.10.0) uses the same version Proj6 (6.2.1). The error is likely to be in QGIS, not in proj.

this is an important bit of info, definitely seems a QGIS issue.

[roya0045]

The old and new QGIS (3.4.13 and 3.10.0) uses the same version Proj6 (6.2.1). The error is likely to be in QGIS, not in proj.

this is an important bit of info, definitely seems a QGIS issue.

But 3.4.13 is the latest LTR, its not old, and I think the switch to proj6 was a few weeks ago, unless he used a custom build.

[kerkovits]

The old and new QGIS (3.4.13 and 3.10.0) uses the same version Proj6 (6.2.1). The error is likely to be in QGIS, not in proj.

this is an important bit of info, definitely seems a QGIS issue.

But 3.4.13 is the latest LTR, its not old, and I think the switch to proj6 was a few weeks ago, unless he used a custom build.

I Use the qgis-full and qgis-ltr-full packages from OSGEO4W64.

[kerkovits]

I did some extra investigation in QGIS 3.10.1: On the screenshot it is visible that qgis does not apply any datum transformation and ignores not only the towgs84 parameter but also the datum shift of the destination crs. projinfo however does report the transformation using correct parameters.

[gioman]

@kerkovits isn't this duplicate of #33323 ?

[kerkovits]

I confirm that the fix works in the nightly build. Thank you.