The Trigonometric Stage (1978)
[slope * cos(aspect)] or [slope * sin(aspect)]
An a priori assumption of a maximum in the NW quadrant (45 azimuth)
and a minimum in the SW quadrant can be replaced by an empirically
determined location of the optimum without repeated calculations of
the regression fit. In addition it is argued that expressions for
the effects of aspect should always be considered as terms involving
an interaction with slope (Stage, 1976)
For slopes from 0
bounded from -1 to 1. Greater than 100
out of the -1 to 1 range.
An alternative for slopes with values approaching infinity is
to take the square root of slope/100 to reduce the range of
values.By default this model test all values greater than 100
to 101