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I want to integrate the NTv2_SN.gsb grid in QGIS. I have a dataset in CRS EPSG:31469 and want to transform into CRS EPSG:25833. Therefore I want to integrate the NTv2_SN.gsb into QGIS. But I have some problems.

  • I put the file NTv2_SN.gsb in the folder C:\Program Files\QGIS 3.4\share\proj
  • start in QGIS the DB Manager and connect to database C:\Program Files\QGIS 3.4\apps\qgis-ltr\resources\srs.db
  • then I update the table tbl_srs with the following SQL statement:
UPDATE tbl_srs
SET parameters ='+proj=tmerc +lat_0=0 +lon_0=9 +k=1 +x_0=3500000 +y_0=0 +ellps=bessel +datum=potsdam +nadgrids=NTv2_SN.gsb +units=m +no_defs'
WHERE auth_id = 31469
  • after that I add in the table tbl_datum_transform the values with this SQL statement:
  INSERT INTO tbl_datum_transform
  VALUES (99999, 100099, 31469, 25833, 9615, 'NTv2_SN.gsb', NULL, NULL, NULL, NULL, NULL, NULL, 'amtliche Gitterdatei von Sachsen', 'Für Genauigkeitsanforderungen im cm Bereich', 1, 0, NULL)
  • the SQL statements work and the values are updated

  • the I go the settings => options => CRS => ask for datum transformation if several are available

  • but if I want to transform a dataset with the CRS EPSG:31469 to CRS EPSG:25833 I can not see my own transformation

example

Where is my fault? Or does anyone know a good tutorial for the grid integration in QGIS?

3
  • Which proj version is used in your qgis installation? You can see this in the help menu -> about
    – Thomas B
    Dec 14, 2019 at 9:23
  • 1
    Similar to this? gis.stackexchange.com/questions/298011/…
    – Ralph Tee
    Dec 15, 2019 at 5:00
  • @joe_gerner: did you finaly solve the problem? i read somewhere, it's a bug in QGIS 3.10.2
    – Bar Tiger
    Feb 10, 2020 at 8:19

1 Answer 1

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+25

The datum transformation in QGIS srs.db must be registered between geodetic CRSs. Then you can assign a default datum transformation between projection CRSs in QGIS.

Projection EPSG:31469 CRS (DHDN / 3-degree Gauss-Kruger zone 5) is based in Geodetic EPSG:4314 CRS (DHDN).

All transformations must be defined to WGS84, proj uses it as an intermediate step. You can register a new datum transformation between EPSG:4314 (DHDN) and EPSG:4326 (WGS84):

INSERT INTO tbl_datum_transform 
VALUES (
  99999, 
  100099, 
  4314, 
  4326, 
  9615, 
  'NTv2_SN.gsb', NULL, NULL, NULL, NULL, NULL, NULL, 
  'amtliche Gitterdatei von Sachsen', 
  'Für Genauigkeitsanforderungen im cm Bereich', 
  1, 
  0, 
  NULL)

Save and close the database. Then, without restarting needed, every time you select EPSG:31469 or another geodetic DHDN based projection, you will see all combinations between available source and destination datum transformations.

1


Notes:

  • I don't know if registering the datum transformation in QGIS is enough, since the grid is not registered in PROJ's proj.db database yet. Validate the transformation with known coordinates points.

  • The change of the definition of EPSG:31469 CRS in QGIS is not a good practice, and I would think it has no effect, since the EPSG definitions are stored in PROJ's proj.db database. In QGIS database you can modify what QGIS will show, but not what proj will do, for registered projections. Creating a custom projection seems to me a better practice.

4
  • Hi, thanks for your response. But which parameters I need for the transformation from EPSG 31469 to EPSG 25833?
    – joe_gerner
    Dec 18, 2019 at 14:21
  • @joe_gerner, I found the ASCII version of the grid, so I can read it now. I am studying the NTv2 standard to understand it, but seems to me that it is not using WSG84 as pivot. So you can register the grid bteween CRSs 4314 and 4258. Also, I found that ETRS89 have not differences with WGS84, soy you can select the 0,0,0 row of the ETRS89 based projection. I need to translate some documentation from German to understand both datums used there. I will update my answer soon with new information. Dec 19, 2019 at 11:13
  • @GabrielDeLuca, WGS84 and ETRS89 have a difference (it was the same in 1989), but the movement of the Europen continent is more then 2 cm/year and ETRS89 moves together with the continent. Nowaday the difference is near to 1 metre.
    – Zoltan
    May 8, 2020 at 9:37
  • Thanks for the clarification, @Zoltan. This answer I think was incomplete at the time and is now obsolete, indeed the SN grid is already incorporated in PROJ. May 8, 2020 at 13:42

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