[R-sig-eco] adehabitatHR: Problem with BRB and sample size (number of fixes)

[Dagmar]

Dear all,

I want to compare the homerange size between animals based on GPS data 
but it seems not to work.

Here is what I did:

I calculated the homeranges using the Biased random bridges from 
adehabitat HR.

Animal_ltraj <- as.ltraj(animalxy, animaltime, id=TierID)
D_Animal <- BRB.D(Animal_ltraj, Tmax=21600, Lmin=36)

Animal_Grid <- ascgen(allanimals_xy, cellsize=32)  # I chose 32 because 
it was the smallest cellsize chosen automatically. I thought to compare 
homerange size between animals I must use the same grid for all of them.

Animal_BRB <- BRB(Animal_ltraj, D= 
D_Animal,type=c("UD"),Tmax=21600,Lmin=36, hmin=100,grid=Animalgrid)
kernel.area(Animal_BRB, unout=c("km2"))

My problem:

I have very different sample sizes (i.e. number of fixes): Most animals 
were located hourly but two animals were located about each 5 min. This 
results in sample sizes of about 150 fixes per animals and some animals 
do have almost 10.000 fixes for the same periods of time.

Because I want to compare the resulting home range sizes I tested if the 
UD size is related to the number of fixes and unfortunatly (!!) they are.

That means that I cannot compare homerange sizes between my animals

My question:

- did I do something wrong?

- is that problem known and is there a way to solve it?

The only way that I find is to reduce the number of fixes of my 
frequently located animals artificially / randomly to start with the 
equal number of fixes for all animals. This would be a shame though!

Help would be very, very, very much appreciated!!!