Working with multiple layers in different CRSs using QGIS

Question

I am trying to delineate a river basin located in Turkey. Turkey has divided in different zones as CRS. Half of my river basin locates in one zone (EPSG:2206), and the other half locates in a different zone (EPSG:2207). I am working with 7 different DEMs as raster layers. I am reprojecting my layers in first place by using Raster > Projections > Warp (Reproject) tool. I applied this step for 7 DEMs separately according to their CRSs. So I obtained 7 adjacent DEMs with different reprojections.

Secondly, I set the CRS of my project from the right bottom corner of the window. However, I am not sure whether if I have to use EPSG:2206 or EPSG:2207 as project projection. I tried with both of them. And each time I tried to merge 7 DEMs (reprejocted) by using Raster > Miscellaneous > Build Virtual Raster tool, I obtained odd results. I am merging 7 DEMs together because I will apply hydrological terrain analysis at the end, to obtain drainage area information.

I am sure that I am applying reprojection methods wrong, but I don't know either how to apply it correctly. I also tried to continue my study without performing any reprojection and by continuing with a global CRS (WGS84). However, that didn't work also.

Does anyone know where I am making mistakes and how should I proceed?

I am using QGIS 3.18 Zürich version.

Answer 1

If you have a DEM, that exists in zone 2207, and you set its projection as 2206, you will get odd results. Likewise, even if you set the zones of 7 raster files correctly, merging them into 1 single file will also produce strange results.

I would recommend the following

• Set your individual DEM's projection system to something that covers all of Turkey. A more global coordinate system. (EPSG:5637 looks like it covers all of Turkey).
• Then merge them together, wit the output file having the more global coord system. (EPSG:5637)

Your DEM will automatically reproject onto the map canvas when you load it into your project, so set your project properties to be something appropriate for the remainder of your work (eg: If you are going to work exclusively in EPSG:2207, then you could set your project to 2207. Your DEM will reproject onto the map canvas automatically.

If you have already tried using a global coord system and are still etting problems, do the following.

• Set your project to global coord system (WGS84)
• Load each individual file into the project. (remember QGIS will reproject on the fly).
• If they load into the right spot, then you can check each individual files coord system under Properties > Information. (just remember, because QGIS is reprojecting on the fly, just cause its in the right spot, it doesn't mean it has the same projection system as your project!).
• If an individual file is NOT appearing in the right spot, then its coord system is set incorrectly. you can change this by going to Layer CRS > Set Layer CRS. (unfortunately from here it will be trial an error, i can't tell you what CRS your data is in).

see how you go with the above.