An alternative might be to consider cyclic splines in mgcv? this would
enable you to fit a gam(m) with two 'ends' (1deg and 359 deg) that match
up, but might allow more flexibility than combinations of sine/cosine?
something along the lines of  gam(response ~ s(exposure, bs='cc'),
family=...)
I've used it successfully to look at seasonal effects through the year. I
imagine it would apply to circular data too, but happy to be told
otherwise....
good luck!
cheers
rob


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Dr Rob Robinson, Principal Ecologist
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====== "How can anyone be enlightened, when truth is so poorly lit" =======



On 16 October 2013 12:47, David Warton <david.warton@unsw.edu.au> wrote:

> Hi Peter and Don,
> unless you have an actual reason for only caring about the East-West axis,
> rather than all directions, you should include both cos and sin:
> cos(exposure*PI/180) and sin(exposure*PI/180)
> as Jeffrey originally suggested.
> [or more generally, cos(exposure*2*PI/T) and sin(exposure*2*PI/T) where T
> is the period of your circular variable]
>
> Geometrically, this maps the variable onto the unit circle (or in this
> case perhaps we should call it the unit compass?).
> All the best
> David
>
> ______________________________________________________
>
> David Warton
> Associate Professor and Australian Research Council Future Fellow
> School of Mathematics and Statistics and the Evolution & Ecology Research
> Centre
> The University of New South Wales NSW 2052 AUSTRALIA
> phone (61)(2) 9385-7031
> fax (61)(2) 9385-7123
>
> http://www.eco-stats.unsw.edu.au
>
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