Unknown CRS of Raster – How to Find Unknown Coordinate Reference System of a Raster in QGIS

coordinate systemqgisraster

I am working with an elevation raster from the Federal Agency for Cartography and Geodesy in Germany. Unfortunately, the CRS of the raster is unknown. Now I am trying to identify the correct CRS, so that I can polygonize the raster and merge it with my other shapefiles all of which are referenced in EPSG:31467.

This link leads to the description (in German) of the raster file and this link starts the download of the raster as .zip.

I have tried the following steps for this list of CRS. EPSG:3246, EPSG:31467, EPSG:25832 and ETRS89.

  1. Inspect the raster –> has unknown CRS
  2. Polygonize the raster using the polygonize tool
  3. Assign CRS using the assign projection tool
  4. Save the resulting shapefile as EPSG:31467 using Export: Save Features as
  5. Load the EPSG:31467 shapefile with other shapefiles with the same CRS

I did this for all CRS listed above and in all cases the resulting shapefiles with previously unknown CRS was not located where all the other EPSG:31467 shapefiles were.

I am working with QGIS 3.16.

Solution: The answers below by Babel and Zman3 are both right and the error cannot be reproduced for the raster file under the download link. It turns out that I was working with a previous version of the raster file that I downloaded a few months ago. The old version did not use EPSG:35832 and so reprojecting as such did not yield the correct placement relative to the basemap.

Best Answer

Try the following workflow. First, you have to assign the correct CRS to the raster before you do anything else - if the raster's CRS is not correctly defined, any further step will just reproduce this error.

  1. Add a basemap to check if the raster is in the right place (e.g. adding OpenStreetMap from XYZ tiles).

  2. Open the raster in QGIS (drag and drop the file to the QGIS main window)

  3. Your raster has an unknown CRS - set the raster's CRS to EPSG:25823 as proposed by @nmtoken - I tried it with your data - it worked.

  4. Use the polygonize tool and save the result, selecting any CRS you like in the export dialog (=reproject).

Always make sure your layers are in the correct place on Earth's surface. This is why you should always have a basemap in the background: than you immediately see if layers are way off. In this case or if the CRS of the layer is not recognized correctly, assigning the correct CRS is the only way to get things right.

Afterwards, proceed with whatever you want to do - including reprojecting. But never reproject a layer that has unknown or wrong CRS - repreojecting an error retains the error.

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