[R-sig-Geo] Re: Using adehabitat and splancs to achieve point-in-polygon analysis on raster?

[Roger Bivand]

On Fri, 25 Mar 2005, Clément Calenge wrote:

> 
> >
> > Hope you had a good time in the field! Is it spring in France?
> 
> Yes, though a little cloudy. And it is certainly colder than in South 
> Africa!

Probably about the same in Bergen, quite warm for the time of year, snow 
still on the hills around town.

> 
> > Thanks for the very helpful reply! I will try it right now! I need to
> > present at a conference in early April and would like to get these
> > results before that!
> > Just three quick questions before you go to enjoy the Easter weekend:
> >
> > - how do I get the values out of 'jj' (## jj contains the values of
> > elevation inside the home range of the animals) to calculate mean,
> > variance, etc, across all the points in 'jj'? I assume 'jj' is of class
> > 'kasc'. In my case it might be 'asc' as I will have only a single home
> > range at the time (one animal).
> 
> Just a small correction to my previous e-mail. If you creates jj with:
> 
> jj<-lapply(foo, function(x) x*el)
> 
> jj is not a map of class "kasc". To convert jj to the class "kasc", copy the
> attributes of the map "foo" to jj with getkascattr:
> 
> jj<-getkascattr(foo, jj)
> image(jj)
> 
> And now, jj is map of class "kasc", i.e. a data.frame with each column
> corresponding to a map. All the pixels located inside the home ranges
> are coded by numeric values in the data frame, and pixels located outside
> the home ranges are coded by NA. So you can get the pixels values with
> something like:
> 
> oo<-lapply(jj, function(x) x[!is.na(x)])
> 
> oo is a list with one component per animal. Each component corresponds to a
> vector of pixels inside the animals home range. It is quite easy after 
> that to get
> values such as average, mean, median, etc.:
> 
> lapply(oo, summary)
> lapply(oo, mean)
> lapply(oo, sd)
> etc.
> 
> > - can I get the points out of the 'kasc' object in a way that I can
> > still run spatial statistics on them? I would like to calculate the
> > nugget and range for the NDVI values within the home range.
> 
> 
> I never performed such spatial statistics with home ranges but this
> could certainly be done... IMHO, the best way to achieve is also to get
> the coordinates of the kept pixels when computing the vector oo:
> 
> xyc<-getXYcoords(jj)
> ## xyc contains the coordinates of the pixels of the maps
> ## it is a list with two components, the vectors of coordinates
> ## of the pixels of the maps
> ## compute the x values
> xc <- rep(xyc$x, times=length(xyc$y))
> yc <- rep(xyc$y, each=length(xyc$x))
> xyc<-data.frame(x=xc,y=yc)
> 
> lixyc<-lapply(1:ncol(jj), function(x) xyc[!is.na(jj[,x]),])
> 
> ## So that each point in oo is associated to its coordinates
> ## in lixyc
> ## for example
> plot(lixyc[[1]], col=grey(oo[[1]]/300), pch=15, cex=4, asp=1)
> plot(lixyc[[2]], col=grey(oo[[2]]/300), pch=15, cex=4, asp=1)
> plot(lixyc[[3]], col=grey(oo[[3]]/300), pch=15, cex=4, asp=1)
> 
> Then you have to compute a variogram. I never used functions
> allowing such analyses, but it seems that the package nlme has a function
> Variogram...
> 
> > - do you know how to convert a polygon shapefile into the same class as
> > produced by the command kernelUD? Alternatively I could import a raster
> > version of the polygon shape file, convert it to the correct format, and
> > repeat the spatial join method you are suggesting for the home ranges!
> 
> I'm sorry, I do not have any easy solution to this problem. The function 
> kernelUD
> returns a *raster* map of class "asc", a central class in adehabitat. I 
> don't know how
> to rasterize a shapefile in R (the library shapefiles or Maptools, which 
> deal with
> shapefiles do not allow such operations). Maybe the simpler way would be to
> rasterize the shapefile with a GIS...

This is something we've been looking at for the sp package, how to make an 
object that represents the raster cells falling into one or more rings. It 
could be a grid with NA outside the vector MASK, or a cell object with 
just the (regularly spaced) points inside the MASK. We are not there yet, 
and point-in-polygon is as close as we've got yet (that is, generate the 
full raster using expand.grid() and test each point in turn. This is 
needed, but isn't finished yet. Writing good functions to suit adehabitat 
package objects is going to be important, so the work flow you, Sander, 
have described, is very useful. It would be nice to be able to say that 
this is ready now, but it isn't yet ...

At least one of the things sp is trying to do is provide shared object 
structures so that writing functions from/to different polygon 
representations can be done from -> sp and sp -> to, so that various 
packages should be able to "talk to" each other with fewer barriers.

Best wishes,

Roger


> Hope this helps,
> 
> 
> Clem.
> 
> >
> > Thanks again,
> >
> > Sander.
> >
> >
> > Clément Calenge wrote:
> >
> >> Dear Sander,
> >>
> >> Sorry for my late answer, I was in the field.
> >>
> >>> I send this email to you directly, because it concerns the link 
> >>> between Adehabitat and Splancs.
> >>>
> >>> I have two sources of polygons:
> >>> - polygon shape files with areas of interest;
> >>> - polygons calculated using the home range methods in Adehabitat.
> >>> For the moment, each polygon source will only contain a single polygon.
> >>> I have one source of points: a grid representing NDVI values. I read 
> >>> the grid using the import.asc function in Adehabitat.
> >>> I would like to use both polygon sources to select the points in the 
> >>> NDVI raster which are inside the relevant polygons. Then I want to 
> >>> run several statistics (mean, standard deviation, etc) on the NDVI 
> >>> values of these points.
> >>
> >>
> >> The solution depends on what you call "polygon". The function 
> >> mcp.rast() is intended to "rasterize"
> >> a polygon on a raster grid of class "asc" or "kasc". The polygon is 
> >> here stored as a data frame with
> >> two columns (the X and Y coordinates of the polygon). The function 
> >> hr.rast() can be used if you have
> >> an object of class "area", i.e. a data.frame with three columns (the 
> >> X and Y coordinates of the polygon,
> >> and a third column defining the ID of the polygon).
> >>
> >> Now, it seems that  you work with more complex type of polygons, e.g. 
> >> kernel estimations of home ranges
> >> (where each animal has an home range, and each home range can be made 
> >> of several polygons). In such cases,
> >> the "Spatial join" operation can be more complex. I detail below an 
> >> example of "spatial join" with a kernel
> >> estimation of wild boar home ranges. Note, that in this case, you do 
> >> not have to compute the contour of
> >> the estimation. Just copy and paste the lines below on R:
> >>
> >> ## loads and display the data
> >> library(adehabitat)
> >> data(puechabon)
> >>
> >> ## keep only the elevation
> >> el<-getkasc(puechabon$kasc,1)
> >> image(el)
> >>
> >> ## estimate a kernel home-range
> >> kud<-kernelUD(puechabon$locs[,c("X","Y")], puechabon$locs$Name, grid=el)
> >> ## kud contains a density
> >> ## note that the grid containing the values of the elevation is used 
> >> for the estimation
> >>
> >> vud<-getvolumeUD(kud)
> >> image(vud)
> >> ## vud contains the limits of the home ranges estimated at different 
> >> levels
> >>
> >> toto<-lapply(vud, function(x) x$UD)
> >> ## toto is the list of maps of Utilization Distribution
> >> tutu<-lapply(toto, function(x) {x[x<95]<-1; x[x>95]<-NA; return(x)})
> >> ## tutu is a list of raster maps containing 1 if the pixel is inside
> >> ## the 95% home range and NA otherwise
> >>
> >> foo<-as.kasc(tutu)
> >> image(foo)
> >> ## foo is a kasc object
> >>
> >> jj=lapply(foo, function(x) x*el)
> >> image(jj)
> >> ## jj contains the values of elevation inside the home range of the 
> >> animals
> >>
> >> The last method that is available for home-range estimation of 
> >> animals home ranges
> >> is the Nearest Neighbor Convex hull method of Getz and Wilmers 
> >> (2004). The function
> >> NNCH.rast() can be used to "rasterize" the home range estimation in 
> >> the same way, and then
> >> you can multiply the resulting maps with your NDVI in the same way.
> >> Hope this helps,
> >>
> >>
> >> Clément.
> >>
> >
> 
> 
> 

-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Breiviksveien 40, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 93 93
e-mail: Roger.Bivand at nhh.no