Converting coordinates to actual distances

Hello,

I've been searching for a method of converting Lat/Lon decimal
coordinates into actual distances between points, and taking into
account the curvature of the earth.  Is there such a package in R?  I've
looked at the GeoR package, but this does not seem to contain what I am
looking for.  Ideally the output would be a triangular matrix of
distances.  

Using C code in the sp package (which will be exposed at the R level 
shortly) and sp > 0.8 and maptools > 0.5:
library(maptools)
Loading required package: foreign
Loading required package: sp
xx <- readShapePoly(system.file("shapes/sids.shp", package="maptools")[1])
ll <- getSpPPolygonsLabptSlots(xx)
# ll is a matrix of long-lat centroids of North Carolina county polygons
str(ll)
num [1:100, 1:2] -81.5 -81.1 -79.3 -79.8 -78.7 ...
plot(ll)
x <- as.double(ll[,1])
y <- as.double(ll[,2])
n <- as.integer(length(x))
dists <- vector(mode="double", length=n)
lonlat <- as.integer(1)
res <- matrix(as.double(NA), 100, 100)
for (i in 1:100) res[i,] <- .C("sp_dists", x, y, x[i], y[i], n, dists, 
+ lonlat)[[6]]

gives a full matrix measured in kilometers for the WGS-84 ellipsoid. 
Accessing the C function like this puts the responsibility for checking 
the argument modes on the user.

If this seems scary, rdist.earth() in the fields package has an R version 
of this. But maybe you need the actual functions to use great circle 
distance instead of Euclidean, rather than just to generate a distance 
matrix?

Hope this helps,

Roger Bivand
Thanks in advance, 
Paul Brewin

Paul E Brewin (PhD)
Center for Research in Biological Systems
University of California San Diego
9500 Gilman Drive MC 0505
La Jolla CA, 92093-0505
USA

Ph: 858-822-0871
Fax: 858-822-3631

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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

Converting coordinates to actual distances

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