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

[Nyall Dawson]

On Mon, 27 Jul 2020 at 19:08, Thomas Wunder
<foren.thomas at thomas-wunder.de> wrote:
>
> 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):
>
> What could be going wrong here?
> Am I missing something?

srs.db transforms are not used at all in builds based on proj 6.0 or
later. Rather proj's internal transformation database and logic are
used.

In order to use your grid you need to get it included in the proj
library. See https://github.com/OSGeo/PROJ-data/blob/master/CONTRIBUTING.md

Nyall