If you're using brushes, scaling them with always affect the look they have ( mostly blurry them) so you have to draw a set with all the several sizes you want for them on your map adn don't scale them. By the way, when looking close up to those mountains, the noise effect you used doesn't look that good and kind of kill the outlining. If you can have access to a scanner, I would suggest to draw them by hand, scan them, import them in GIMP in turn them into brushes. If you don't, try to draw simple outlines without too much effects on them in GIMP ( possibly using some thin light grunge roud brushes. Shwarzkreuz did post some of his own in his DeRust taining maps WIP, maybe it will help you). Hope it helps.
Well, I drew them by hand and scanned them. Opened up gimp used the lasso tool and then made them into brushes. Maybe it is my low budget pencils that creates the problem?
probly you are using a photocopy as scanner what make a big diference. I had this problem with my scans too, trying to stel some machines in my job when i use the properly scan i get it right, when use the photocopy to get it, i have the same of your result.
I guess that you can changes the configuration of your scanner (if it is a photocopy like i do) i change the resolution and create a TIFF file, sometimes it clean all my image copied.
Pencil has a tendency to do that when scanned because the graphite sits on top of the paper and doesn't fill in the grain the way pen does, and the scanner heightens the difference. It can help if you set your scanner to greyscale, 600 resolution, and turn the brightness down. Then in GIMP you can adjust the levels or curves (values/lighting/whatever it's called in GIMP) to get a higher quality outcome and then when you're happy with the lighting, re-size the image to 50% (resolution 300, which is a standard print resolution). Doing all that should make the grain appear less pronounced, if it doesn't nix it altogether (depends on your editing).
I did this in Photoshop by doing a fuzzy color selection to grab the white, inverting the selection, creating a new layer and giving it a mask with the selection, filling the layer with black and adjusting the mask boundary and contrast until it looked mostly filled in (more like what a pen would do). I don't know how you'd do it in Gimp.