[tlocoh-info] auto.a() function suggests much too high a-values

[André Zehnder]

Hi all,

 

I am performing a home range analysis with GPS data of some leopards and
lions. The input data has a highly variable point density and result in
quite large areas (roughly a magnitude of 500 to 1’000 km2 for the 95%
isopleth). In agreement with the tutorial, I begin with selecting the value
for the temporal parameter s and then select suitable k values. As an
orientation for that I use the rule of thumb ( k = sqrt(numberOfPoints)) and
the plots. When a k-value has been chosen, the tutorial recommends to use
the auto.a() function (lxy.nn.add(toni.lxy, s=0.003, a=auto.a(nnn=15,
ptp=0.98))).

 

However, the recommended a-value is massively too high and results in a
oversmoothed home range that lacks any details. The higher the s-value, the
more severe is this issue. While the result of the suggested a-value still
shows a few weak spatial details for s=0, almost circular home ranges result
for all isopleths with s>0. I checked whether this issue occurs only for one
dataset, but it is the same for all 5 datasets I have checked. I attached
two images that present the result when using the recommended a-value
(auto_) and one that presents a manually selected a-value (manually_). For
example, for s=0.005, I would rather take an a-value between 150’000 and
190’000 than the recommended value of 1’150’000. The auto.a() function
should thereby include at least k points for 90% of all hulls.

 

Therefore the question: Has anyone experienced the same issue or is it even
a known technical problem with the package? My datasets contain 5’000 to
30’000 fixes, have some gaps and includes sometimes different sampling
intervals. May the auto.a() function have severe problems due to that? The
choice of an a-value is rather subjective and not really intuitive,
especially when s>0. But when the auto.a() function can’t be used to get an
approximate reference, what other measures are available to be able to
justify your choice of the a-value?

 

PS: I use T-LoCoH version 1.34.00 with RStudio 0.98.1103.

 

Best regards,

André Zehnder

 

 

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