[R-sig-Geo] subsetting

[Dylan Beaudette]

On Fri, Oct 24, 2008 at 8:24 AM, Edzer Pebesma
<edzer.pebesma at uni-muenster.de> wrote:
> 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).

Unless there are some compelling reasons not listed, all of these
operations can be done in GRASS as well. There are no memory
constraints, and several resampling approaching built-in. Once you
have your resampled data, it is a simple matter to read in a subset
(defined by the current 'region') from GRASS into R.

I would imagine other open source GIS apps have similar functionality.
R is an excellent, multi-purpose tool-- but some operations (i.e.
resampling large grids / polygon overlays) are handily accomplished by
brute force algorithms found software like GRASS.

Cheers,

Dylan


>
> 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
>
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