Another Wilbur Question

[waldronate]

What would this look like from an overhead perspective? The side shots look an awful lot like the sort of step that happens by clipping inside or outside a mask. When I tried to use that expression on a surface, I got everything above 128 showing as Not a Number (-1#IND) and everything below 128 as the original value.

I'm still unclear what you're really trying to get here. One way to make a crater with an arbitrary rim is to:

Draw a selection to represent the rim shape. I used a squarish one so that it would look vaguely like the one in Arizona.
Fill that selection with a mound (Filter>>Fill>>Mound) where the Maximum is less than the minimum. I used 10 for minimum and 0 for maximum.
Apply an exponent to make the crater more bowl-shaped. Filter>>Mathematical>>Exponent with the exponent set to 4 will work.
Invert the selection to work on the outside (Select>>Inverse)
Fill with a mound (Filter>>Fill>>Mound) where the Maximum is less than the minimum. The minimum should be the same as the original fill. I used 10 for minimum and 5 for maximum.
A small exponent here might work as well. Filter>>Mathematical>>Exponent with the exponent set to 1.5 is a good starting point.
Deselect (Select>>Deselect).
Apply an overall bit of noise to make things rougher. Filter>>Noise>>Fractal Noise with an amplitude of 3 and Operation of Add.