There is precedent in the ecological literature for using a cosine transformation IF you have reason to believe that your predictor varies continuously and symmetrically in its effects around a circle.  For example, if due east were the "most" exposure, and due west the least, with due north and south being roughly equal, you could create a new predictor called "east.exposure" with (most basically)

east.exposure = cos(exposure - PI/2)

Many more complicated extensions of this idea are possible, associated with nonlinear or asymmetrical gradients, but I will leave that to you or others on the list.

On Oct 15, 2013, at 9:59 AM, Peter Nelson wrote:

> I want to include the exposure (measured in degrees, for example, East-facing is 90) of various coastal sites in GLM and CCA analyses. Is there an appropriate transformation that I can apply to these measurements that will allow me to do this? I've found plenty of information on comparing headings, calculating means, etc, but nothing on how exposure might be used as a continuous independent variable. 
> 
> Treating exposure as a categorical variable (East, Southwest, etc) seems like a fallback option, but then there is just as much of a 'difference' between SE and E sites as there is between SE and NW sites!
> 
> Thanks, Pete
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Don McKenzie

Affiliate Professor
School of Environmental and Forest Sciences
University of Washington

dmck@uw.edu





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