Thanks Don. I tried the transformation you suggested, but the results don't appear promising (0 deg doesn't = 360 deg, intervals vary):

0	6.12574E-17
10	-0.544021111
20	0.912945251
30	-0.988031624
40	0.74511316
50	-0.262374854
60	-0.304810621
70	0.773890682
80	-0.993888654
90	0.893996664

I tried 

east.exposure=cos(exposure*PI/180) 

This seems better (e.g., 0 degrees = 360 degrees) see below), but the absolute values of the intervals aren't consistent. No surprise, I suppose, but what to do?  

Thanks, Peter

0	1
10	0.984807753
20	0.939692621
30	0.866025404
40	0.766044443
50	0.64278761
60	0.5
70	0.342020143
80	0.173648178
90	6.12574E-17
100	-0.173648178
110	-0.342020143
120	-0.5
130	-0.64278761
140	-0.766044443
150	-0.866025404
160	-0.939692621
170	-0.984807753
180	-1
190	-0.984807753
200	-0.939692621
210	-0.866025404
220	-0.766044443
230	-0.64278761
240	-0.5
250	-0.342020143
260	-0.173648178
270	-1.83772E-16
280	0.173648178
290	0.342020143
300	0.5
310	0.64278761
320	0.766044443
330	0.866025404
340	0.939692621
350	0.984807753
360	1
On Oct 15, 2013, at 11:45 AM, Don McKenzie <dmck@u.washington.edu> wrote:

> 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
>> _______________________________________________
>> R-sig-ecology mailing list
>> R-sig-ecology@r-project.org
>> https://stat.ethz.ch/mailman/listinfo/r-sig-ecology
> 
> 
> 
> 
> Don McKenzie
> 
> Affiliate Professor
> School of Environmental and Forest Sciences
> University of Washington
> 
> dmck@uw.edu
> 
> 
> 
> 


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