[R-sig-Geo] Step characteristics on raster maps

[Robert J. Hijmans]

On Thu, Sep 30, 2010 at 9:57 PM, Mathieu Basille
<basille at ase-research.org> wrote:
> Dear Robert,
>
> I just understood the interest of 'crosstab' with 'mask', this is pretty
> neat! Thanks for the suggestion.
>
> However, I can see some potential drawbacks with this approach: as my
> objective is to describe each step (each segment), I should first cut
> each 'SpatialLines' into a list of 'Lines', each 'Lines' being a segment
> (and not the whole trajectory). I apology if I'm totally wrong, I'm not
> that familiar with SpatialLines...
>
> But then, what about crossing/overlaping segments, i.e. several segments
> that all fall in the same pixel (which usually happens a lot)? With
> linesToRaster, only the last segment would be kept. So that I should run
> independently the approach for each 'Line' (each segment) just to avoid
> these crossings, and that would result in as many new rasters as the
> number of segments (and I'm talking about hundreds of thousands here,
> over large rasters).
>


You are right, of course. I have added a function "lineValues" (akin
to "polygonValues" that existed) to the raster package. This function
should not have this problem. It is in the development version
available from R-Forge, please try it if you can.

Robert

>
> Basically, to rephrase my problem, here is what I want to achieve. Given
> a set of points (say a SpatialPointsDataFrame, or SpatialLinesDataFrame,
> or ltraj), I'd like to be able to compute new variables that give for
> each point summaries or characteristics (e.g. proportion of each type of
> the raster if it is categorical, or mean if is continuous) of the
> segment from that point to the next (or previous, it doesn't matter).
>
> It seems to be a simple problem in terms of low-level functions
> (segments over raster), but a complex one in terms of data structure (an
> single object with a set of individual trajectories which are themselves
> sets of segments). And I have to admit that I have some troubles going
> from the former to the latter.
>
> Thanks again for your time,
> Mathieu.
>
>
> Le 2010-09-30 11:28, Robert J. Hijmans a écrit :
>> Perhaps you can do something like
>>
>> r is a Raster* object
>> line is a SpatialLines* object
>>
>> library(raster)
>> rr <- linesToRaster(line, r)
>> rm <- mask(r, rr)
>> crosstab(rm, rr)
>>
>> Robert
>>
>> On Thu, Sep 30, 2010 at 5:46 AM, Mathieu Basille
>> <basille at ase-research.org> wrote:
>>> Dear list members,
>>>
>>> I'm trying to compute characteristics along steps (i.e. segments between two
>>> points), based on underlying raster maps. The steps originally come from
>>> radiotracking data, converted to ltraj objects (adehabitat). The idea is to
>>> compute (for example) the habitat composition corresponding to each step
>>> instead of each points, as we are interesting in the movement path.
>>>
>>> I tried different solutions, as I would like to do it with R. I did not find
>>> any solution using adehabitat (or the development versions adehabitatMA/LS);
>>> 'join' only works for point characteristics, not steps. I also tried using
>>> sp and SpatialLinesDataFrame, but overlay does not seem to work with
>>> SpatialLines(DataFrame) and SpatialPixelsDataFrame:
>>>
>>> Error in function (classes, fdef, mtable)  :
>>>  unable to find an inherited method for function "overlay", for signature
>>> "SpatialPixelsDataFrame", "SpatialLinesDataFrame"
>>>
>>> I also investigated packages raster, trip, and rgeos, without success. Maybe
>>> the low level functions of rgeos could be used, but it seems a bit out of my
>>> skills (and time available) at the moment.
>>>
>>> Another solution might be to use spgrass6 in conjunction with GRASS, but I'm
>>> not familiar enough with GRASS to judge if it is a viable alternative...
>>>
>>> I'd welcome any hints/thoughts on this question.
>>> Sincerely,
>>> Mathieu Basille.
>>>
>>>
>>> --
>>>
>>> ~$ whoami
>>> Mathieu Basille, Post-Doc
>>>
>>> ~$ locate
>>> Laboratoire d'Écologie Comportementale et de Conservation de la Faune
>>> + Centre d'Étude de la Forêt
>>> Département de Biologie
>>> Université Laval, Québec
>>>
>>> ~$ info
>>> http://ase-research.org/basille
>>>
>>> ~$ fortune
>>> ``If you can't win by reason, go for volume.''
>>> Calvin, by Bill Watterson.
>>>
>>> _______________________________________________
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>>>
>
> --
>
> ~$ whoami
> Mathieu Basille, Post-Doc
>
> ~$ locate
> Laboratoire d'Écologie Comportementale et de Conservation de la Faune
> + Centre d'Étude de la Forêt
> Département de Biologie
> Université Laval, Québec
>
> ~$ info
> http://ase-research.org/basille
>
> ~$ fortune
> ``If you can't win by reason, go for volume.''
> Calvin, by Bill Watterson.
>