Calculates the Roberts and Cooper (1989) Solar-radiation
Aspect Index
Roberts and Cooper (1989) rotates (transforms) the circular aspect to assign a
value of zero to land oriented in a north-northeast direction, (typically the
coolest and wettest orientation), and a value of one on the hotter, dryer
south-southwesterly slopes. The result is a continuous variable between 0 - 1.
The metric is defined as: trasp = ( 1 - cos((pi/180)(a-30) ) / 2
where; a = aspect in degrees