I mostly make heightmaps for various games but most of the Digital Elevation Maps I get are equirectangular which I can't really work with to draw in lakes and rivers for semi-accurate placement of stuff in game.

I have a map I put together of Central and Western USA but its equirectangular and i need it in mercator so I can draw in the lakes and rivers to place towns properly, as well as make sure theres navigable rivers for transport sims and city builders.

I have tried G.Projector but it tries putting maps on a "globe" of sorts which means you get a large black image with only a small area where your map is if its not the entire world. Even if you where to make a heightmap of the entire world, 15000x7500px resolution isn't enough detail for large countries like USA or Africa.

Is there another program out there I can use to re-project heightmaps or perhaps someone could tell me how I could double or quadruple the area's coordinates to produce a proper mercator map (the coordinates I have is 50N at the top, 25N at the bottom, -126W on the left and -86 on the right). I've tried doubling the area by modifying the coordinates but the image doesnt come out right (It looks too vertically stretched).

Any help would be appreciated!

- Nekomaster