[R-sig-eco] Compute the distance between the trajectories of two animals with varying sampling intervals

[Mathieu Basille]

Hi André,

Yes, that's one way to do it. If you already have a single ltraj with all 
individuals (one individual per burst), you can simply give it the ltraj 
alone. With a single ltraj, the function looks for the closest point in 
other bursts. Check the example of the function.

Note that I do not guarantee that my function is efficient! This is R: 
there is no use of spatial indexes or bounding boxes, so that for each 
point, distances are computed to all other points in the temporal window. 
Depending on your data, I guess it could be fairly slow...

Again, if you're looking for performance and/or robustness, I would give a 
try to PostGIS here.
Mathieu.


Le 22/07/2015 15:31, André Zehnder a écrit :
> Thank you for all your answers!
>
> @ Krzysztof: Yes, I think an approximated method that uses only the nearest points to compute the distance will suffice. When I include a threshold that ensures that only temporal differences smaller than xy minutes are taken into consideration, reasonable results should be achievable.
>
> @ Sarah: I think you are referring to the simulation of a Brownian motion by the function simm.bb()  for an exact solution? I could not find a function that directly computes the distances. Or did I miss something?
>
> @ Jeremy: Thanks for the hint! A also stumbled upon the pathFrechet() / distFrechet() function of longitudinalData but rejected them because they seem not to include the temporal dimension. However, this seems to be only an erroneous information of inside-r. When I looked at the official reference manual I saw that distFrechet() considers the time as well.
>
> @ Mathieu: That is pretty much what I am intending to do for a first (and possibly also final) try, thanks! So I need to input the ltray-object of the first individual  as 'from' and the other one as 'to' in order to check the distances between their closest points, right? I also tried to program something like that but the result is shockingly time inefficient and might still contain some errors. So I will probably try out your function tomorrow :)
>
> Best regards
> Andre
>
>
>
> -----Ursprüngliche Nachricht-----
> Von: Mathieu Basille [mailto:basille.web at ase-research.org]
> Gesendet: Mittwoch, 22. Juli 2015 19:44
> An: André Zehnder; R-sig-eco
> Cc: Jeremy Chacon; Sarah Goslee
> Betreff: Re: [R-sig-eco] Compute the distance between the trajectories of two animals with varying sampling intervals
>
> As far as I can tell, there is no such function in adehabitatLT. However, I wrote a function 'closest' [1] in the package 'hab' [2], which specifically does that: find the closest relocations from ltraj objects. It can work with only one ltraj, in which case it will look in other bursts, or with two different ltraj, in which case it will in the other ltraj. Most importantly, you can define a temporal windows to restrict the search, which may solve your problem of irregular time (the ltraj do not have to be regular, or on the same time schedule). It's only Euclidean distance though. Lastly, it works at the location level, i.e. it's not considering lines (steps). If that's what you're looking for, I would probably suggest using PostGIS...
>
> Hope this helps,
> Mathieu.
>
>
> [1] https://github.com/basille/hab/blob/master/R/closest.r
>
> [2] https://github.com/basille/hab
>
>
> Le 22/07/2015 10:09, Jeremy Chacon a écrit :
>> I bet Sarah's right and adehabitatLT has something, but I also know
>> that the longitudinalData package can compute distances between two
>> trajectories including the frechet distance.
>>
>> http://www.inside-r.org/packages/cran/longitudinalData/docs/pathFreche
>> t
>>
>> However, it does not deal with the timing problem. You will have to
>> roll your own method of aligning the trajectories in time.
>>
>> On Wed, Jul 22, 2015 at 9:02 AM, Sarah Goslee <sarah.goslee at gmail.com>
>> wrote:
>>
>>> Try the adehabitatLT package.
>>>
>>> On Wed, Jul 22, 2015 at 9:41 AM, André Zehnder
>>> <andrezehnder at hotmail.com>
>>> wrote:
>>>> Hi list,
>>>>
>>>>
>>>>
>>>> I want to compute the distances between two trajectories that
>>>> consist of
>>> a
>>>> sequence of point measurements. The trajectories do not have the
>>>> same
>>> length
>>>> or sampling interval. So, while for animal A, there is a data point
>>>> at
>>> 15.00
>>>> p.m. and another at 15.38 p.m, animal B might have data points at
>>>> 14.56
>>> p.m.
>>>>
>>>> and 17.02 p.m.
>>>>
>>>>
>>>>
>>>> A possible approach would be to connect these point series to
>>> trajectories
>>>> and then calculate the distance between the two trajectories at the
>>> sampled
>>>> positions of one trajectory (take the sampled position of the second
>>>> trajectory that is temporally closest). Are there any packages in R
>>>> (or tools outside of R) that allow me to do this? The support of
>>>> other
>>> distances
>>>> than the Euclidean one (e.g. Fr chet) would be an advantage.
>>>>
>>>>
>>>>
>>>> Best regards,
>>>>
>>>> Andre
>>>>
>>>>
>>> --
>>> Sarah Goslee
>>> http://www.functionaldiversity.org
>>>
>>> _______________________________________________
>>> R-sig-ecology mailing list
>>> R-sig-ecology at r-project.org
>>> https://stat.ethz.ch/mailman/listinfo/r-sig-ecology
>>>
>>
>>
>>
>

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Mathieu Basille
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