VARIANCE OF ANGLES

[Giovanna Ranci]

I need a FORMULA to calculate VARIANCE when arguments are ANGLES (in degree).
For example, I need to calculate variance of aspect map in 3x3 neighborhood.
Aspect is expressed in degrees. It's clear that a cell with value 5 and a cell 
with value 355 are almost the same aspect so variance formula has to consider i
t.
I got already a formula with sin and cosine but there should be some mistakes i
n it
(I calculate variance using command r.mapcalc and r.neighbors but results are s
trange...definetely unreal. I'm sure error is in the formula not in my procedur
e in Grassland).
If somebody knows the rigth formula, I will be glad to receive it.
Thanks,

Giovanna Ranci Ortigosa
e-mail: ranci at quercia.elet.polimi.it