[Europe] Geoid grids for Germany

[Thomas Corve]

Hello everyone,

I am the developer of an app which, among other things, receives WGS84 coordinates from a GNSS and which must place the user's position on a map.
This plan is expressed (for example) in CC49, and uses a Raf18 correction grid.

After downloading the grids with the projsync tool, the altimetric correction works (I didn't have the grids before).

Here is how I use proj, followed by proj_trans commands:
PJ* pj = proj_create_crs_to_crs(m_projCtx, "epsg:4326", "epsg:3949+5720", nullptr);

I am now trying to do the same for a project in Germany. For the planimetry I used the code epsg:4647.
To add the vertical correction, my clients seem to use "DHHN2016". After a search on espg.io I find the corresponding epsg code epsg:7837.

Unfortunately that doesn't work. After searching with projinfo, code 5720 gives a string Proj with the name of a grid file, but code 7837 does not:


projinfo epsg:5720
PROJ.4 string:
+geoidgrids=fr_ign_RAF18.tif +vunits=m +no_defs +type=crs


projinfo epsg:7837
PROJ.4 string:
+vunits=m +no_defs +type=crs

So I said to myself that I had to modify the Proj string to add the name of the Grid file. In the doc, there is mention of ini files, in which you can modify the string with the keyword +init. So I created a file with the following line:

+init=epsg:7837 +geoidgrids=monfichier.tiff


I haven't seen any effect. And projinfos didn't add this line in the Proj string.

If a good soul could give me some clues... Eventually we are looking for a subcontractor/teacher to help us on this part...

Sincerely, Thomas C.


Thomas Corvé

R&D Lead Engineer

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