Converting Aspect degree to westness using ArcGIS Desktop?

Question

I have extracted Aspect values in degree from DEM in ArcMap 10.2 full license. I want to investigate effect of Westness in my model so I was using conversion formula of Folded Aspect : ABS (180- ABS(Aspect-270)) But it seems like this works when we deal with radiation and sunlight. There is another formula to calculate southwestness of aspect yet it gives degree between 0-200 with ((cos(aspect-255)+1) * 100). (( And why 255?Shouldn't be 225?))

Do you have any idea of the formula for converting to "Westness" from 0-180?

My model is not water-related so that formula having catchment area is also not useful for me.

Answer 1

The general terrain attribute that you are describing is called Relative Aspect (RA) and in its general form, it can be calculated for any input azimuth (Az) as follows:

RA = Abs(aspect - Az)

if (RA > 180) { RA = 360 - RA }

where aspect is the terrain slope aspect calculated from the DEM. Here is an example of code that calculates the index directly from an input DEM. And I should point out that the equation and code above will output in the range 0-180.