Peter, For my purposes (I.e., estimating exposure and drying potential in northern hemisphere temperate forests), I simply subtract 45 degrees from the measured aspect in degrees, convert to radians, and then take the cosine of the adjusted angle. If I want to make exposure positive, I then reverse the sign. In this way, southwest-facing slopes get the maximum value (1) and northeast-facing slopes get the lowest (-1). As others have mentioned, this approach gives equal weight to east-west and north-south variation in exposure, which may or may not be valid for a given situation.
In your case, it sounds like you want to assume the east-facing aspects are maximally exposed. In that case, I would just subtract 90 degrees from your degrees measurement, convert to radians, and then take the cosine, which I believe amounts to the same approach that Don suggested. East-facing slopes should end up with a value of 1 and west-facing slopes a value of -1 (due north and south will have values of 0). If you want to give north-facing aspects less exposure than south-facing aspects (I don't know whether you are in the northern or southern hemisphere), then you could subtract 135 degrees from your measurements, making southeast aspects the most exposed. Steve J. Stephen Brewer Professor Department of Biology PO Box 1848 University of Mississippi University, Mississippi 38677-1848 Brewer web page - http://home.olemiss.edu/~jbrewer/ FAX - 662-915-5144 Phone - 662-915-1077 On 10/15/13 11:59 AM, "Peter Nelson" <[email protected]> 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 >[email protected] >https://stat.ethz.ch/mailman/listinfo/r-sig-ecology _______________________________________________ R-sig-ecology mailing list [email protected] https://stat.ethz.ch/mailman/listinfo/r-sig-ecology
