Drawing height maps by hand

[passengerpigeon]

Dear community,
For a future game, I want to create a custom height map because I am not satisfied with the default world generation. However, I don't want to use a real world region, I want to create a fictional landscape. Is there a way to draw .ppm heightmap files by hand instead of importing them? I know the "Island Nations" map created for Simutrans Experimental uses a fictional heightmap; how was that created?
Thank you,
Passengerpigeon.

[An_dz]

You just randomly paint with the grey colours needed with the pencil tool (most drawing software call it that).

Just follow the tutorials on how to do a heightmap from real terrain but ignoring the entire part where you obtain the real terrain image.