[Qgis-user] Using a NTv2 grid file for transforming between different CRSs.

[Thomas Wunder]

Hello,
I've created a new project (CRS=EPSG:25832 ETRS89/UTM32N)  using QGIS 3.10.8 
(on Debian 11, PROJ version 7.1.0) which contains several multi polygon layers 
(land parcels). Those layers were imported from shape files containing either 
EPSG:25832 (as is the project) or legacy EPSG:31468 (DHDN/3° Gauss-Krueger 
Zone 4) data.  QGIS gave me a couple of transformation options via Project -> 
Properties -> CRS -> Datum Transformations but none of those is accurate 
enough for my purposes. I need sub centimeter accuracy.

Therefore I'd like to go for an NTv2 grid based approach using a gsb file. I've 
put the grid file to /usr/share/proj/kanu_ntv2_oberfranken.gsb and added 
several rows to /usr/share/qgis/resources/srs.db like so (don't remember all 
the variations):

INSERT INTO tbl_datum_transform VALUES (99999, 1000997, 4314, 4326, 9615, 
'kanu_ntv2_mittelfranken.gsb', NULL, NULL, NULL, NULL, NULL, NULL, 'Für ALKIS 
Bayern, amtliche Gitterdatei des LDBV', 'Für Genauigkeitsanforderungen im cm 
Bereich', 1, 0, NULL);

or

INSERT INTO tbl_datum_transform VALUES (99999, 1000998, 4314, 4258, 9615, 
'kanu_ntv2_mittelfranken.gsb', NULL, NULL, NULL, NULL, NULL, NULL, 'Für ALKIS 
Bayern, amtliche Gitterdatei des LDBV', 'Für Genauigkeitsanforderungen im cm 
Bereich', 1, 0, 3339);

No matter what I add there QGIS never lists a new transformation option in 
Project -> Properties -> CRS -> Datum Transformations -> '+' when selecting 
Source = EPSG:31468 and Destination = EPSG:25832. 

I only get
+proj=pipeline +step +inv +proj=tmerc +lat_0=0 +lon_0=12 +k=1 +x_0=4500000 
+y_0=0 +ellps=bessel +step +proj=hgridshift +grids=de_adv_BETA2007.tif +step 
+proj=utm +zone=32 +ellps=GRS80

+proj=pipeline +step +inv +proj=tmerc +lat_0=0 +lon_0=12 +k=1 +x_0=4500000 
+y_0=0 +ellps=bessel +step +proj=push +v_3 +step +proj=cart +ellps=bessel 
+step +proj=helmert +x=598.1 +y=73.7 +z=418.2 +rx=0.202 +ry=0.045 +rz=-2.455 
+s=6.7 +convention=position_vector +step +inv +proj=cart +ellps=GRS80 +step 
+proj=pop +v_3 +step +proj=utm +zone=32 +ellps=GRS80

+proj=pipeline +step +inv +proj=tmerc +lat_0=0 +lon_0=12 +k=1 +x_0=4500000 
+y_0=0 +ellps=bessel +step +proj=push +v_3 +step +proj=cart +ellps=bessel 
+step +proj=helmert +x=597.1 +y=71.4 +z=412.1 +rx=0.894 +ry=0.068 +rz=-1.563 
+s=7.58 +convention=position_vector +step +inv +proj=cart +ellps=GRS80 +step 
+proj=pop +v_3 +step +proj=utm +zone=32 +ellps=GRS80

+proj=pipeline +step +inv +proj=tmerc +lat_0=0 +lon_0=12 +k=1 +x_0=4500000 
+y_0=0 +ellps=bessel +step +proj=push +v_3 +step +proj=cart +ellps=bessel 
+step +proj=helmert +x=584.8 +y=67 +z=400.3 +rx=0.105 +ry=0.013 +rz=-2.378 
+s=10.29 +convention=position_vector +step +inv +proj=cart +ellps=GRS80 +step 
+proj=pop +v_3 +step +proj=utm +zone=32 +ellps=GRS80

+proj=pipeline +step +inv +proj=tmerc +lat_0=0 +lon_0=12 +k=1 +x_0=4500000 
+y_0=0 +ellps=bessel +step +proj=push +v_3 +step +proj=cart +ellps=bessel 
+step +proj=helmert +x=590.5 +y=69.5 +z=411.6 +rx=-0.796 +ry=-0.052 +rz=-3.601 
+s=8.3 +convention=position_vector +step +inv +proj=cart +ellps=GRS80 +step 
+proj=pop +v_3 +step +proj=utm +zone=32 +ellps=GRS80

I would expect there to be something like +nadgrids=kanu_ntv2_oberfranken.gsb.

What could be going wrong here?
Am I missing something? 

Thank you in advance and best regards!
 Thomas