Apologies if the title does not explain it well...
I use https://heightmap.skydark.pl/beta/ for the heightmap data on most of my maps, but sometimes it returns a result where the eastern 1/4 of the map is of a different contrast (I think it might be due to being queried from a different map tile on whatever source it uses).
Sometimes I can get the heightmap to render and download correctly after a few tries, but other times (usually on the most interesting terrains), the map will not download correctly, and it is not usable.
My question is, can the differing contrast scales that the heightmap is drawn from be rectified easily in a program like GIMP? I'm not sure exactly what operation to do after selecting the area to be adjusted. Maybe it could be something like contrast or levels, but I don't know how to get it to match the height scaling of the rest of the map.