Maybe I'm being naive, or missing the point, or something. But I would
presume that your data are something like:
x_1, x_2, ..., x_n # x-coordinates of the locations --- say, stored as x
y_1, y_2, ..., y_n # y-coordinates of the locations --- say, stored as y
k_1, k_2, ..., k_n # counts at the given locations --- say, stored as k
This is interpreted as meaning that (x_i,y_i) is the centroid of the
i-th region, in which count k_i was obtained.
If so, can you not just calculate:
xc <- sum(k*x)/sum(k)
yc <- sum(k*y)/sum(k)
???
What am I not understanding?
cheers,
Rolf Turner
--
Technical Editor ANZJS
Department of Statistics
University of Auckland
Phone: +64-9-373-7599 ext. 88276
On 20/04/16 18:45, Diego Pavon wrote:
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.
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