[alma-config] topography mask

[Simon Radford]

Bryan,

All masks are "cost" masks. In a "gradient" mask, the cost just equals
(some linear multiple of) the gradient. If the gradient is the dominant
parameter and the cost scales linearly with gradient, that's ok. 

For a "binary" mask, you must define the threshold. Say you pick a 15%
gradient. Then a pixel with a 14% gradient is ok, but with a 16%
gradient is not. But I would guess the cost of construction on those two
pixels is very similar. So a binary mask assumes a (very) non-linear
cost scaling.

Leaving aside cliffs, etc., the road gradient is probably a larger
concern than the local gradient at the antenna station. But the road
gradient may be determined by conditions some distance from the station.

Simon