I can still make little sense of the Wilbur file saving and opening formats.
The consequence is that I can't recover the height field that I saved and the (old) manual doesn't help.

So I did a systematic test with the formats that I generally use. It involved an island with a constant altitude of 255 m that I saved in format X and again opened in the same format X. It gave me the results :
- 8 bit BMP Surface : the saved 255 m become 1 m
- BMP Texture : the saved 255 m become 1 m
- PNG Surface : the saved 255 m become 65535 m
- PNG Texture : the saved 255 m become 65535 m
- when I open a file saved in format X specifying in the Open file tab another format Y, I obtain about anything (including an error message).

So when I save a height field, I recover all kind of numbers with the exception of the numbers I saved.
Can somebody (Waldronate ?) explain me how this system works ?
Also what is the difference between saving/opening in PNG Surface and PNG Texture ?

And most importantly how should I save a file so that when I open it later, I get back exactly the same height field I saved ?