I have found a possibility to get directly heightmaps.
Place the grid about the terrain.
Expand the Grid to 30km (red)
Save the height map (green)
You will get a zip-file with heightmaps and a readme file. With the merged heightmap I got the best results. In the readme file you will find the resolution (here 30m):
fjord Height Map (ASTER 30m).png
ASTER is the most recent public survey of elevation on Earth. It has high
coverage and high (~30m) resolution. However, the instrument can get
confused by high concentraions of clouds and mountains, creating gaps in
the data that need to be repaired by hand.
Furthermore, you will find the min and max height. This three values you will need for the import into LT3D.
The original elevation models for this area contained elevations ranging from
0 through 1843 meters.
Open L3DT and with File->Import->Heightfield import it.
4) Choose the file with the merged heigthmap. ==> Press button Next.
5) Set the following value: “Horizontal scale = 30m”.
6) Activate “Specify new vertical range” and “Wrap edges”. Set the following values:
Minimum alt.: 0
Maximum alt.: 1843
==> Press OK Button
It can occur spikes in the terrain. Out this reason create a desing map from the imported heightmap and recalulate it.
7) Right click at the heightfield ==> Operations->Design map->Generate from HF…
8) Set the design map size. For more details I choose 16.
9) Press the buttons Next and OK.
10) Set the design map parameters. I set only the value “Default climate”. ==> Press OK and Yes.
11) Press the calc. button in the toolbar, choose the heightfield and recalculate it with the button OK.