GIMP :: Filter Levels Of A Picture Depending On Its Color

I'm in the process of making a command map for the free video game Wolfenstein Enemy Territory.

The games allows to generate a TGA file called Tracemap, which is used by the game to detect which places are outside or inside, for precipitations and artillery. It is commonly used as a base to make command maps, such as:

Other examples:

[URL].......

To obtain a comparable result, I have decomposed the making in 4 steps (4 layers):
- The terrain (background);
- The buildings (in shade of grey);
- The roads (a tool drawing white lines bordered with black and doing the junctions automatically would be neat, if it ever exists...?);
- The "foldings" (that's the Reverso translation, I think it can be guessed from the pic).

So, I exported the terrain in a separated map, compiled it, generated a Tracemap, and I have it in GIMP now. I would like GIMP to detect and do some lines between the different levels like Paint Shop Pro seems to do:

[URL]........

This way, I can draw the height lines accurately, and then color (I already made the palette with required colors); Does this exist in GIMP? Tried few filters but never came close to something like in the Warchest forums.

If it isn't possible I will do it manually ^^'