[R] Variograms and large distances
Ruben Roa
RRoa at fisheries.gov.fk
Wed Nov 9 11:32:49 CET 2005
> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch [SMTP:r-help-bounces at stat.math.ethz.ch] On Behalf Of Julian Burgos
> Sent: Tuesday, November 08, 2005 10:41 PM
> To: r-help at stat.math.ethz.ch
> Subject: [R] Variograms and large distances
>
> 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?
>
> 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?
>
> Any help would be appreciated.
>
> Julian
>
>
Hi Julian,
I suggest you transform your coordinates to UTM. I use Eino Uikannen's
GeoConv program
http://www.kolumbus.fi/eino.uikkanen/geoconvgb/index.htm
but i believe there are packages and functions in R to carry out the
transformation. GeoConv is a DOS program that runs in batch
mode from the DOS console.
Ruben
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