I am trying to merge two height maps in QGIS.

I have DEMs of the lake of constance in germany. It is a result of a goverment project and the data is available in a asc format. I want to 3dprint a landscape relief model. Also i need to split that model into pieces (200x200mm) to print it in blocks for a total length of 1000cm.

There is no problem to import the data in Qgis, also it is no problem to split the height maps into pieces for 3d printing. The fun begins when i try to merge the two DEMs into one. The DEM of the surrounding area ends where the lake begins.

Is it possible to "insert" the lake into the image of the surrounding area and recolor it?

The deepest point in the new DEM should be the darkest in the whole new DEM. At the end i need a single height map. --> Target: DEM of the whole area wir a dried of lake.

Lake plus surrounding area Shows the lake in the middle plus the surrounding area.

surrounding area without lake

Shows only the surounding area

enter image description here

  • 2
    We call them "heightmaps" or "DEM"s (digital elevation models). Is the map of the lake showing the depth of the lake? Lakes in a heightmap are usually flat surfaces at the lake's water level. You want to show the landscape as if it was dried up? What are the two images?
    – Spacedman
    Commented May 11, 2018 at 15:34
  • Yes it shows the depth of the lake an also yes i want to show the whole relief ,later in the print, with the dried up lake. Commented May 11, 2018 at 15:38
  • the first image shows the surrounding area with the lake in the middle. the second image shows only the surrounding area. This image ends in the middle at the starting level of the lake. in this area the map is just flat. Commented May 11, 2018 at 15:40

2 Answers 2


I found the SAGA module "Mosaic raster layers" very useful to merge a lake bathymetry with a topographic elevation model.

Processing > Toolbox > Mosaic raster layers

You have to choose the right classifying option in "Overlapping Areas" to use the bathymetry values where both rasters overlap.

Both rasters must have the same SRS, same pixel size and be referred both to the same reference surface .


I think that you mean "elevation map" by "high map", right?

You probably want to combine the two sets of elevation (height) data into one by subtracting the lake depth (negative height) from the smaller set of elevation data from the larger set, where the lake is treated as flat but the surrounding mountains are included?

If I understand you correctly, then you want to use the Raster Calculator (or Rastererrechner) to subtract the lake DEM (digital elevation model) from the larger DEM.

  • Hi :D Yes that is what i want. It is really hard to find the right and specific words when you are not an expert on this kind of programs. I try my best. I try this function now. :) Commented May 11, 2018 at 16:05
  • You did pretty darn well! Aside from a few technical words, your English was extremely good.
    – Zack
    Commented May 11, 2018 at 16:14
  • Thanks! do you think this is the right operation: "LC_All_20151210_ETRS89_3m_ac_int@1" - "output_srtm@1" (LC_ALL is the lake and Output_srtm is the surrounding area) Commented May 11, 2018 at 16:16
  • no success... after the operation i have now a new layer in my panel but it shows up nothing. When i open the layer it says "nan" by black and white..... Commented May 11, 2018 at 16:23
  • 1
    ok, i try to get both sets in one format first. Thank you :) Commented May 11, 2018 at 16:54

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