[R-sig-eco] angular statistics

[Steve Brewer]

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" <pnelson at cfr-west.org> 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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