Hi Rainer,

A good transformation used for aspect in ecological analysis is the Beers (1966) transformation.  Here is the citation:

Aspect transformation in site productivity research. TW Beers, PE Dress, LC Wensel - Journal of Forestry, 1966

~Matthew

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Today's Topics:

   1. Re: [R] rpart with circular data? (Dylan Beaudette)


----------------------------------------------------------------------

Message: 1
Date: Mon, 12 May 2008 07:30:19 -0700
From: "Dylan Beaudette" 
Subject: Re: [R-sig-eco] [R] rpart with circular data?
To: " B?lint Cz?cz " 
Cc: r-help , r-sig-ecology@r-project.org, Rainer
 M Krug 
Message-ID:
 <3c5546140805120730m74b05d0ctac5cc46bc767f5ae@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1

On Mon, May 12, 2008 at 6:29 AM, B?lint Cz?cz  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.
>
> B?lint
>
>
> On Mon, May 12, 2008 at 11:35 AM, Rainer M Krug  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
>>
>>  ______________________________________________
>>  R-help@r-project.org mailing list
>>  https://stat.ethz.ch/mailman/listinfo/r-help
>>  PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>>  and provide commented, minimal, self-contained, reproducible code.
>>
>
>

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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