Hi Diego,
A perhaps dumb yet straightforward way of doing it is to replicate each
point the number of times of its corresponding count, this will get you the
right unbiased centroid, essentially a weighted average as Marcelino
suggests, assuming that the weight you use is proportional to the count.
It's like saying that each bird contributes once to the centroid location,
instead of each point contributing once.
Note that just by using your suggested kernelUD you don't really get the
centroid of the points (I think, never used it myself actually), I suspect
you simply get a kernel estimate of the bivariate distribution of the
points in space. Going from that to an actual centroid is possible but non
necessarily straightforward.
The answer to your specific question is much simpler than that: in 2D,
simple calculate the means of the X and Y coordinates, multiply each
coordinate by a weight, the count - that would be an alternative to what I
suggested above - and there is your centroid.
Note that this gets you the right mean centroid, but not necessarily the
right variance for that centroid estimate. Also, you might want to think
about if that is the right weight. But those are all questions beyond your
original question ;)
cheers
Tiago
?s 09:36 de 20/04/2016, Marcelino de la Cruz escreveu:
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