Wilbur/GIMP Questions: Height Values and River Generation

[Coggleton]

Hello,

I am in the progress of generating rivers for a map using Wilbur, and had a couple of questions:

1. I use as an input a grayscale map with eight color blocks for elevation ranges as seen below, with the ocean/sea level as black (000000) and the maximum height range of 6000+ m as white (#ffffff); in Wilbur, what units and values could I expect these to equate to (e.g. if 00 equates to 0m/ft/"units", what would ff equate to)?
elevation sample.PNG

2. Is there any way to control where rivers start and end? While mentally I have a good idea of the general neighborhood of my larger rivers, my current method (that of using the above map and fractal noise to generate hills followed by erosions capped off with Incise Flow) can sometimes result in weird rivers that hug a mountain's edge as seen below- is there any way of telling Wilbur to start an Incise Flow-caused erosion pattern to start somewhere?
river hugging mountain.PNG

3. This may be more related to GIMP than Wilbur, but there is a stark contrast between the ocean and everything in the 0-1000m range (#2a2a2a). While I've been able to blur the other height categories for smooth gradation, this contrast results in my continent consisting entirely of cliffs. How can I blur these two ranges together without causing my landmass to overflow into the ocean? (I'd like the 2a region to become closer to 00 at the border, but I don't want the 00 at the border to become closer to 2a).

[waldronate]

1. #ffffff translates to 255 in grayscale (how the import is managed). You can scale that to whatever you like in Wilbur, however. Using Filter>>Mathematical>>Span with a Low of 0 and a High of 6000 will scale your map to the 0 to 6000 range you mentioned.

2. If you want rivers to follow a specific path, draw that path in there using a darker value than the underlying landscape. If you're using a coastal mask, make another map with rivers in black and use that mask for the first few iteration scales to force the rivers into the exact path that you want.

3. Make a sea mask that has land as pure white and ocean as pure black. Load that mask as a selection when doing operations like precipiton erosion to enforce your coastline. You can invert the coast mask and set the world to something like -20 to allow the edges of the terrain to erode below sea level (reload your original mask before doing your erosion). If you want, you can lower everything to get closer to sea level after your initial scaling.