[R-sig-Geo] increment and distance in correlog

[Edzer Pebesma]

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
>>
>> _______________________________________________
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>> R-sig-Geo at stat.math.ethz.ch
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> 
> 
> 
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-- 
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.pebesma at wwu.de