[R-sig-eco] question about argument GRID in KernelUD function (package: adehabitatHR)

[Diego Pavon]

Dear all

I am trying to estimate Kernels using the KernelUD in package adehabitatHR
and I have some question regarding the argument GRID. I first run the
function:

>Accgen.kernel<-kernelUD(Accgen_Sxy.laea, h = "href", grid = 100,
>                                          same4all = FALSE,
>                                          kern = c("bivnorm", "epa"),
extent = 1,
>                                          boundary = NULL)


And then I estimate the 50, 75 and 95% kernels, the centroids and the area
of these polygons.

>ver_Accgen <- getverticeshr(Accgen.kernel, 95, unout= "km2")  ## 95 %
>coordinates(ver_Accgen)                                                             ##
centroid
>as.data.frame(ver_Accgen)
## area of the kernel


So, first question: Which value of GRID is the most appropriate? Is there a
rule of thumb to specify this value?

second, why does the area of the kernel change when I change the value of
GRID from 60 to 100? Shouldn't it be always the same? And if it shouldn't,
which one should I take as "good"?

Third, I have noticed that, for some species, when I try to calculate the
50% kernel, I get an error message ("out of bounds") which is dealt with by
increasing the GRID value. So, if I want to compare results (e.g. area or
centroid's location) between species, should I use the same grid value for
all of them?


Thank you so much for your time and help!

Best

Diego





-- 
*Diego Pavón Jordán*

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

*Helsinki. Finland*



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