> I have a question concerning the ADEhabitatLT package in R. We are
> analyzing GPS data of beavers sampled at an 15 min interval from approx.
> 7 pm - 7 am for 7 nights. The aim of our study is to look at changes in
> movement behavior after capture (we tag them with GPS units and release
> them afterwards).
>
> We used the ltraj function to create an ltraj object with bursts for
> every individual. We then divided the bursts into sub-bursts for every
> night with cutltraj by using time lags over 8 hrs (when beavers are
> inside the lodge during daytime) for splitting up. However, we get a
> warning message saying that
>
> Warning message:
>
> In cutltraj(Beav, "foo(dt)", nextr = TRUE) :
>
> At least 3 relocations are needed for a burst
>
> 9 relocations have been deleted
>
> For our purpose, the first night after capture is really important. And
> just for this night, we sometimes only got 2 relocations since the
> animals went into their lodge (no GPS signal), meaning that the function
> just deletes the first night for some of our animals. We thus would like
> to include these bursts for analysis and wonder whether there is a
> possibility to create a burst with only 2 relocations?
Some of the statistics used to describe the trajectory require at least
two steps, i.e. 3 relocations (e.g. the relative angle, or turning
angles, between successive steps). However, if you insist, you can edit
the function cutltraj, e.g. using the command:
cultraj<-edit(cutltraj)
and delete the following commands at the end of the function:
resb <- res[rrr >= 3]
if (length(res) != length(resb))
warning(paste("At least 3 relocations are needed for a burst\n",
sum(rrr[rrr < 3]), "relocations have been deleted"))
This will avoid this unwanted behaviour. However, I cannot guarantee
that the other functions in adehabitatLT will work with a 2-relocations
ltraj (Two relocations do not define a trajectory; you need at least
three). You are on your own if you decide to do that!
HTH,
Clément Calenge
--
Clément CALENGE
Cellule d'appui à l'analyse de données
Direction des Etudes et de la Recherche
Office national de la chasse et de la faune sauvage
Saint Benoist - 78610 Auffargis
tel. (33) 01.30.46.54.14
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