[R-sig-Geo] subsetting

[Edzer Pebesma]

Tomislav Hengl wrote:
> In GIS terms, getting a raster from one grid topology to (any) other grid topology (also subgrids)
> is referred to 'resampling' (I do not think that this is yet implemented in any R package; a
> combination of akima and spTransform will do the trick but it is not really compact).
I do think so: overlay in sp is your friend:

library(sp)
data(meuse.grid)
gridded(meuse.grid) = ~x+y
grd = spsample(meuse.grid, n = 100, type = "regular")
image(meuse.grid["dist"])
points(grd, pch = 3)

gridded(grd)= TRUE
overlay(meuse.grid, grd)
grddf = SpatialPixelsDataFrame(grd, as(meuse.grid, 
"data.frame")[overlay(meuse.grid, grd),])
image(grddf["dist"])
points(as(grd, "SpatialPoints"), pch = 3)

Note that there is only a single code line that does the resample. If 
anyone has suggestions for a cleaner or simpler interface, or a function 
or method resample, please let me know.

This of course does not do any interpolation; I consider that a 
different chapter (and package). I cannot see how spTransform could help 
here.When warping grids, you'd have to revert them first into polygons 
and spTransform them, to conserve the non-square grid cells resulting.
--
Edzer


>  A generic
> function to combine R+ILWIS to put a map to a different grid (e.g. to LatLon coordinates) would be:
>
> # write to ILWIS:
>   
>> writeGDAL(dem25m[1], "dem25m.mpr", "ILWIS")
>>     
> # create a new grid:
> ...
>   
>> gridparameters(geoarc)
>>     
> # resample the map (Bilinear) to the new geographic grid:
>   
>> shell(cmd=paste(ILWIS, " crgrf geoarc.grf ",geoarc at grid@cells.dim[[2]],"
>>     
> ",geoarc at grid@cells.dim[[1]]," -crdsys=LatlonWGS84
> -lowleft=(",geoarc at grid@cellcentre.offset[[1]],",",geoarc at grid@cellcentre.offset[[2]],")
> -pixsize=",geoarc at grid@cellsize[[1]],sep=""), wait=F)
>   
>> shell(cmd=paste(ILWIS, "dem25m_ll_c.mpr = MapResample(dem25m_c.mpr, geoarc, BiLinear)"), wait=F)
>> shell(cmd=paste(ILWIS, "open dem25m_ll_c.mpr -noask"), wait=F)
>>     
> see http://geomorphometry.org/R.asp 
>
> hth,
>
> Tom Hengl
> http://spatial-analyst.net 
>
>
>
>
> -----Original Message-----
> From: r-sig-geo-bounces at stat.math.ethz.ch [mailto:r-sig-geo-bounces at stat.math.ethz.ch] On Behalf Of
> Pieter Beck
> Sent: Thursday, October 23, 2008 9:15 PM
> To: r-sig-geo at stat.math.ethz.ch
> Subject: [R-sig-Geo] subsetting
>
> Dear all,
>
> I am a new user to the world of spatial applications of R and am wondering
> about this trivial question:
>
> What is the easiest way to subset a SpatialGridDataFrame, based on 4 corner
> coordinates?
>
>  
>
> The reason I ask is that I want to do an overlay of a polygon file on a
> large raster dataset, to extract summary statistics from it. Both files are
> extremely large, however, and just applying overlay causes memory trouble. I
> was hoping to loop through the polygons, subset the SpatialGridDataFrame
> using the bounding box surrounding a polygon and then create the summary
> statistics one polygon at-a-time. To do this, I'd need to subset the
> SpatialGridDataFrame based on the bounding box corner coordinates.
>
> Thanks in advance for the help,
>
>  
>
> Kind Regards,
>
> Pieter Beck
>
>  
>
>
> 	[[alternative HTML version deleted]]
>
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-- 
Edzer Pebesma
Institute for Geoinformatics (ifgi), University of Münster
Weseler Straße 253, 48151 Münster, Germany. Phone: +49 251
8333081, Fax: +49 251 8339763 http://ifgi.uni-muenster.de/
http://www.springer.com/978-0-387-78170-9 e.pebesma at wwu.de