[R-sig-eco] Assign constant speed to animal trajectory - recommendations for function?

[Pinaud David]

Hi Kylie,
To my knowledge, there is no such a function.
Maybe you could calculate the distance between successive locations then 
recalculate the time at each location, assuming a constant speed :
ie t2 = t1 + dist(t12) / cstSpeed

Something like:

# tr is a trip object or a SpatialPointsDataFrame
dist <- trackDistance(coordinates(tr), longlat = T)
tr at data$newtime <- c(tr at data$time[1], tr at data$time[1] + 
cumsum(dist/CstSpeed))

HTH

David


Le 27/07/2012 12:29, Kylie Scales a écrit :
> I am currently conducting a 'First Passage Time' (fpt) animal movement
> analysis, in order to identify ARS zones from GPS tracks.
>
> The study species in question frequently rests on the water surface during
> darkness hours.  To avoid artificial inflation of fpt, I have removed these
> points and have used the variance in the log of fpt to identify scales of
> ARS behaviour within each of these track sub-sets. This has worked for
> smaller-scale ARS zones, but not larger scales, owing to the separation of
> each trip into daylight-only sections.
>
> I would like to assign a constant speed to the whole trajectory for each
> bird and then conduct a fpt analysis on the whole trajectory, to get an
> indication of larger-scale ARS.
>
> Could anyone recommend a function that could be used to rediscretize an
> animal trajectory to constant speed?
>
> I've tried redisltraj() from package adehabitatLT to rediscretize step
> length at a constant interval, and then setNA() followed by sett0() to
> create constant time interval, but this results in a lot of NAs in x/y,
> which then seems to constain the fpt analysis.
>
> Is there a better way to assign constant speed to the whole trajectory?
>
> Any advice much appreciated.
>
> Kylie Scales
> Postgraduate Researcher
> Plymouth Marine Laboratory
>
> 	[[alternative HTML version deleted]]
>
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>

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Dr. David PINAUD
Ingénieur de Recherche "Analyses spatiales"

Centre d'Etudes Biologiques de Chizé - CNRS UPR1934
79360 Villiers-en-Bois, France
poste 485
Tel: +33 (0)5.49.09.35.58
Fax: +33 (0)5.49.09.65.26
http://www.cebc.cnrs.fr/

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