[R-sig-Geo] Thanks to all: Distances in geoR package

[Tim Keitt]

Perhaps we could get it added into R core. The spherical-earth GC
distance function is only a few lines of code.

THK

On Fri, 2005-06-17 at 15:22 +0200, Roger Bivand wrote:
> On Fri, 17 Jun 2005, Paulo Justiniano Ribeiro Jr wrote:
> 
> > A few words more on this.
> > 
> > My feeling is that the best way to handle this is with coordinate
> > trasformation (for instance for UTM) and then backtransform
> > at the  end if required.
> > 
> 
> And a couple more ...
> 
> Which way to go depends on the relative scale of the phenomena of 
> interest. For up to hundreds of km, the distortions introduced by 
> projecting to UTM or other suitable projections are small. But at 
> continental scales, working with Great Circle distances for building 
> distance matrices is a much better choice, not depending on the origin of 
> the projection. 
> 
> I've had C code in spdep for some time to do GC (an if() on a flag passed
> through .Call to choose between the R built-in pythag() or the GC
> distance). I've put a copy in the spgwr package, but multiple copies means
> maintenance problems. The rdist.earth() function in fields is at the R
> level - the nice comment on the help page does hold: "Surprisingly this
> [is] all done efficiently in S".
> 
> I'd like to expose the GC function in the sp package, but have not yet
> looked at how contributed packages can share functions (and header files)
> at the C level - the header file is the real issue, I think. Would there
> be interest in doing this? The GC function as it is does not have an R
> wrapper, because in spdep, most of the distances are thrown away
> immediately as being beyond threshold.
> 
> Feedback, please?
> 
> Roger
> 
> > There were a few messages along these lines and I will take the
> > opportunity to mention that there is a package under development (aRT)
> > which intends to provide this and other functionalities typical from GIS
> > systems via Terralib
> > The package page is: http://www.est.ufpr.br/aRT
> > 
> > Still changing rapidly but already some functionalities are already
> > working.
> > There is a package vignette and a few exemples in the tests directory.
> > This package adheres the the "sp" classes andin newer versions also allows
> > "proj4".
> > 
> > Best
> > P.J.
> > 
> > 
> > 
> > On Fri, 17 Jun 2005, giovanna jona lasinio wrote:
> > 
> > > Dear all, thanks for the many suggestions, I'll try and let you know
> > > what worked best. Package fields works well in computing variogram's
> > > cloud with lon-lat coordinates (returns by default distances in miles)
> > > and usually I use a loess-built curve to get a nice-looking version of
> > > the variogram. However I wasn't able to use the "vgram" function's
> > > output in order to obtain a "variofit" estimated model. What would be
> > > extremely usefull, in my opinion, is to allow "variog" to accept a
> > > distance matrix as input, in this way no coordinate transformation would
> > > be needed. I'm not yet good enough at programming in R to find a good
> > > solution myself :-) but if I find a solution I'll let you know....
> > >
> > > Best
> > > Giovanna Jona Lasinio
> > >
> > > _______________________________________________
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> > >
> > >
> > 
> > Paulo Justiniano Ribeiro Jr
> > LEG (Laboratório de Estatística e Geoinformação)
> > Departamento de Estatística
> > Universidade Federal do Paraná
> > Caixa Postal 19.081
> > CEP 81.531-990
> > Curitiba, PR  -  Brasil
> > Tel: (+55) 41 3361 3573
> > Fax: (+55) 41 3361 3141
> > e-mail: paulojus at est.ufpr.br
> > http://www.est.ufpr.br/~paulojus
> > 
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> > 
> 
-- 
Tim Keitt
Section of Integrative Biology
http://www.keittlab.org/