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 ",[EMAIL PROTECTED]@cells.dim[[2]],"
",[EMAIL PROTECTED]@cells.dim[[1]]," -crdsys=LatlonWGS84
-lowleft=(",[EMAIL PROTECTED]@cellcentre.offset[[1]],",",[EMAIL
PROTECTED]@cellcentre.offset[[2]],")
-pixsize=",[EMAIL PROTECTED]@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: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of
Pieter Beck
Sent: Thursday, October 23, 2008 9:15 PM
To: r-sig-geo@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
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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 [EMAIL PROTECTED]
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