VARIANCE OF ANGLES

[Philip Verhagen]

> From owner-grassu at isis.cecer.army.mil Sat Dec 14 01:19 eur 1996
> X-Authentication-Warning: isis.cecer.army.mil: listserv set sender to grassu using -f
> From: Giovanna Ranci <ranci at quercia.elet.polimi.it>
> Subject: VARIANCE OF ANGLES
> To: grassu at cecer.army.mil
> Date: Fri, 13 Dec 96 19:35:28 MET
> Mailer: Elm [revision: 70.85]
> 
> 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 it.
> I got already a formula with sin and cosine but there should be some mistakes in it
> (I calculate variance using command r.mapcalc and r.neighbors but results are strange...definetely unreal. I'm sure error is in the formula not in my procedure 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
> 

It's a long time since I had trigonometry at school, but isn't it possible
to recalculate the aspect map in radians and calculate the variance from
that?

Philip Verhagen

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