[AniMov] kernel size?

[Jennie Sheldon]

How does one specify the value for the smoothing parameter ( e.g.
consistently for all animals across all ranges) in metres?

J.W. Sheldon

J. W. Sheldon
_________________________________ 
Yellowstone Ecological Research Center
2048 Analysis Drive
Bozeman, MT  59718
406-582-0447
sheldon at yellowstoneresearch.org

-----Original Message-----
From: animov-bounces at faunalia.com [mailto:animov-bounces at faunalia.com] On
Behalf Of Clément Calenge
Sent: Tuesday, May 09, 2006 2:29 AM
To: cavallini at faunalia.it; Animal Movement
Subject: Re: [AniMov] kernel size?

Hi Paolo,

>We did some kernel analyses (h=href); everything seems to run fine, but 
>the resulting ranges seem to me a bit exaggerated (see 
>http://www.faunalia.it/download/kernel_9.png). Of course, the right 
>thing to do is to compare R results with those of other programs, but 
>did someone already tested this?
>  
>

Actually, "h=href" corresponds to a smoothing parameter estimated under the
hypothesis that the underlying utilisation distribution is normally
distributed (UD unimodal, i.e. one center of activity, and symmetrical). 
This is not the case in your example. When the UD is not bivariate normal,
the use of href greatly overestimates the UD (see Silverman 1986, Worton
1995 J. Wild. Manage.).

I never compared the estimates of adehabitat with other softwares, when
smoothing values are estimated with the ad hoc method. But before developing
adehabitat, I was using the software RANGES V, which also returned greatly
overestimated home-ranges with "multi-center" home ranges. So that in my
opinion, the reference method would return similar results whatever the
software (though not exactly identical, because the different softwares do
not use exactly the same algorithms - not the same size of the grid, not the
same way to compute the limits of the HR from the UD).
I agree that it would be interesting to perform such a comparison...

One alternative, when several centers of activity are present, is to use the
LSCV method, but, again, the results would differ among softwares (and more
dramatically). For example, I used the dataset "puechabon" of the package
adehabitat (all animals pooled), and I estimated the UD with different home
range estimation programs. The smoothing values (in
metres) returned by these programs are:

83 m        adehabitat
310 m      Arcview - animal movement analysis (AAMA)
44 m        ranges V
802 m      Calhome

And two other programs use one smoothing parameter for x and one for y:

59 m for X and 161 m for Y    The home ranger
131 m for X and 364 m for Y   kernelHR

As you can see, there is much variation in the estimation. The main cause of
variation is that the different softwares do not use the same algorithms to
smooth the UD. In addition, the algorithm often fail to minimise the
smoothing error, so that bad results are returned by the function  (this is
a property of the method, see the help page).

Finally the last method (which I prefer, personnally), is to specify a value
for the smoothing parameter (the same for all animals), based on some visual
exploration of the data.
HTH,

Clem.

--
Clément CALENGE
LBBE - UMR CNRS 5558 - Université
Claude Bernard Lyon 1 - FRANCE
tel. (+33) 04.72.43.27.57
fax. (+33) 04.72.43.13.88 


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