Re: [R] rpart with circular data?

From: Dylan Beaudette <dylan.beaudette_at_gmail.com>
Date: Mon, 12 May 2008 07:30:19 -0700

On Mon, May 12, 2008 at 6:29 AM, Bálint Czúcz <czucz_at_botanika.hu> wrote:
> Hi Rainer,
>
> In a similar situation I used the two components of the normal vector
> of the surface ("northing" & "easting"). I.e. for a horizontal plane
> both are 0, for a vertical slope facing south northing=-1and
> easting=0, etc. This descartian decomposition of the slope vector
> avoids the problem of circularity present in the widespreadly used
> polar (aspect, slope) decomposition, and thus seems to suit ecological
> problems much better to me. However I have not looked into this much,
> I am also very interested in the opinion of others.
>
> Bálint
>
>
> On Mon, May 12, 2008 at 11:35 AM, Rainer M Krug <r.m.krug_at_gmail.com> wrote:
>> Hi
>>
>> I am planning to use a CART analysis with rpart() to analyse the
>> impact of, among others, slope, altitude and aspect on mortality
>> rates.
>> My question is:
>> Is there a p[roblem with using aspect as a predictor as it is circular?
>> And if it is a problem (which I suspect), is there a transformation I
>> could use to transform aspect?
>>
>> Thanks
>>
>> Rainer
>>
>> --
>> Rainer M. Krug, PhD (Conservation Ecology, SUN), MSc (Conservation
>> Biology, UCT), Dipl. Phys. (Germany)
>>
>> Plant Conservation Unit
>> Department of Botany
>> University of Cape Town
>> Rondebosch 7701
>> South Africa
>>
>> ______________________________________________
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>>
>
>

I would recommend going straight to the source-- modeling solar radiation. This gives a purely physical-based generalization of terrain-induced variation in microclimate. Keep an eye out for this paper, soon to be published in Soil. Sci, Soc, America:

D.E. Beaudette, and A.T. O'Geen. Quantifying the Aspect Effect. Soil Sci. Soc. Am. J. March 3, 2008. (In Review)

This paper uses a GRASS-based solution to the problem.

Cheers

Dylan



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