Follow up since it's too late to edit.

Gimp works fine, Photoshop doesn't (at least not w/o plugins) in regards to opening .pgm files. The version of PaintShopPro I used earlier was 6, but I'd presume the following versions still maintained the ability to open .pgm files.

And another thing you can do is use the heightmap as a reference to draw a black and white separator that can be used with a multi-channel mixer. Thus when making a road going through the mountains, you can make a separate b/w image with where the road is white and everything else black. (Or vice versa, doesn't matter as long as the contrast is there.) Then have one channel in the multi-channel mixer be a terrain terrain texture and the other the road texture and the b/w image of the road path as the blender.

And you can repeat using the heightmap as a reference as to painting where certain foilage or other objects appear, and then use the grayscales saved off of that as controls for the surface replicator.

I might have to test this out in a render, unless somebody beats me to it.

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Barbequed Pixels?

Your friendly neighborhood Wings3D nut.
Also feel free to browse my freebies at ShareCG.
There might be something worth downloading.