[R] Variograms and large distances

[Roger Bivand]

On Tue, 8 Nov 2005, Julian Burgos wrote:

> Hello R list,
> I need to compute empirical variograms using data from a large 
> geographic area (~10^6 km2).  Although I could not find a specific 
> reference, I assume that both geoR and gstat calculate distances among 
> data points assuming points are on a flat surface (using the Pythagorean 
> Theorem).  Because the location of my data is large and located near the 
> pole, assuming that latitude and longitude are coordinates on a flat 
> surface would introduce a -possibly large- bias in the empirical 
> variogram estimate.  My questions are the following:
> 
> a)  Does geoR and gstat assume that points are on a flat surface?

Yes.

> 
> b) If I first calculate the distances among points using the Haversine 
> formula, it is possible to calculate the variogram with a matrix of 
> distances among points (where n is the number of observations) and a 
> vector of observation values?
> 

I don't think so.

I suggest that you at least consider the fields package, partly because 
its disciplinary heritage makes large area coverage more natural, and 
because it has rdist.earth() and exp.earth.cov() functions. Whether it can 
do everything you need straightaway is another question, though.

> Any help would be appreciated.
> 
> Julian
> 
> Julian M. Burgos
> 
> Fisheries Acoustics Research Lab
> School of Aquatic and Fishery Science
> University of Washington
> 
> 1122 NE Boat Street
> Seattle, WA  98105 
> 
> Phone: 206-221-6864
> 
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-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no