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 <d...@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 > > d...@uw.edu > > > > [[alternative HTML version deleted]] _______________________________________________ R-sig-ecology mailing list R-sig-ecology@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-ecology
