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.


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.


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