Jens, in your code snippet below, the number 6370 rings a bell. According to http://en.wikipedia.org/wiki/Earth_radius it is close to the average earth radius. In km. Given that there's just one such number, it seems the code assumes a sphere is close enough to approximate the earth.
On 10/06/2010 12:37 PM, Jens Floeter wrote: > Hello again, > > okay, i found the ncf package and the correlog function. > > # correlog(x, y, z, w = NULL, increment, resamp = 100, latlon = FALSE, na.rm > = > FALSE, quiet = FALSE) > > my short question is: > > *if latlon = T and the positions are "degree, decimal-minutes" then increment > has the unit of "km" and distance bins are in "km" as well - is that correct > ?* > > I found it in the sourcecode : > > #then generating geographic distances > if(latlon){ > #these are geographic distances from lat-lon coordinates > dmat <- matrix(0, nrow = n, ncol = n) > for(i in 1:(n-1)) { > for(j in (i+1):n) { > dmat[j, i] <- gcdist(x[i], y[i], x[j], y[j]) > dmat[i, j] <- dmat[j, i] > > with gcdist = > > r <- 360/(2 * pi) > lon1 <- x1 / r > lat1 <- y1 / r > lon2 <- x2 / r > lat2 <- y2 / r > dlon <- lon2 - lon1 > dlat <- lat2 - lat1 > a <- (sin(dlat/2))^2 + cos(lat1) * cos(lat2) * (sin(dlon/2))^2 > c <- 2 * atan2( sqrt(a), sqrt(1-a) ) > return(6370 * c) > } > > but i cannot the answer from this....does gcdist deliver km units ? > > Cheers! > jens > > > > > > Am 05.10.2010 12:57, schrieb Jens Floeter: >> Dear list members, >> >> i need a hint - i am stuck on my search for a function. >> >> I would like to calculate cross-correlograms between two variables (predator >> and prey densities) and use this as an index of spatial overlap. >> >> I used the spdep pakcage and use e.g. >> >> sp.correlogram(xy.data.nb, prey, order = 5 , method = "I", zero.policy=TRUE, >> style = "W") >> sp.correlogram(xy.data.nb, predator, order = 5 , method = "I", >> zero.policy=TRUE, style = "W") >> >> to calculate the (auto-) correlograms for predator and prey individually. >> >> But how do i combine them to calculate the cross-correlogram ? >> >> Any hint would be highly appreciated... >> >> Best wishes ! >> jens >> >> _______________________________________________ >> R-sig-Geo mailing list >> R-sig-Geo@stat.math.ethz.ch >> https://stat.ethz.ch/mailman/listinfo/r-sig-geo > > > > _______________________________________________ > R-sig-Geo mailing list > R-sig-Geo@stat.math.ethz.ch > https://stat.ethz.ch/mailman/listinfo/r-sig-geo -- Edzer Pebesma Institute for Geoinformatics (ifgi), University of Münster Weseler Straße 253, 48151 Münster, Germany. Phone: +49 251 8333081, Fax: +49 251 8339763 http://ifgi.uni-muenster.de http://www.52north.org/geostatistics e.pebe...@wwu.de _______________________________________________ R-sig-Geo mailing list R-sig-Geo@stat.math.ethz.ch https://stat.ethz.ch/mailman/listinfo/r-sig-geo