[R-sig-Geo] how to calculate centroid (or centre of gravity) of a population (count data)

[Diego Pavon]

Dear all

I am working with count data and I want to assess whether the centre of
gravity of the population (centroid or mean latitude?) has change over
time, indicating some redistribution or shift ongoing. To simplify, let's
say that I have ca. 2000 sites censused in two consecutive years (same
sites censused both years - all sites) and the abundance (count) of the
species registered.

I first thought about doing a kernelUD (package adehabitatHR) but
apparently this only takes into account the location of the sites to
calculate the kernel and then the centroids. Thus, since I have the exact
same sites in both years, the centroids for year 1 and year 2 are the same.
In my case, what I would like to do is to calculate that centroid but
taking into account the counts, because a site that had 3 individuals in
both years can't have the same weight than a site that hosted 3000
individuals when calculating the centroids.

So, what I would like to have is the centroid (or centre of gravity) of the
counts not of the sites surveyed (which is what adehabitatHR does,a s far
as I understood).

Do you have any suggestions which package other than adehabitatXX to use
for this purpose? Or if this can be done with adehabitat?

Thank you very much for your help.

Diego


















-- 
*Diego Pavón Jordán*

*Finnish Museum of Natural History*
*PO BOX 17 *

*Helsinki. Finland*



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