[R-sig-Geo] Clip a raster

[Robert Hijmans]

Etienne,

I added a function "clickBbox" to the raster package for increased interactivity

#install.packages("raster", repos="http://R-Forge.R-project.org")
require(raster)

# read a raster file from disk
filename <- system.file("pictures/Rlogo.jpg", package="rgdal")
r <- rasterFromFile(filename, TRUE)
plot(r)

# If you want, for example, the mean of the values of the clipped area,
# use the below click on the image (map) twice to create a rectangle

mean( crop(r, clickBbox())[] )


Best, Robert


On Wed, Feb 4, 2009 at 7:38 PM, Roger Bivand <Roger.Bivand at nhh.no> wrote:
> On Tue, 3 Feb 2009, Etienne Bellemare Racine wrote:
>
>> Is it possible to clip a raster after having it read with readGDAL ?
>> Instead of using region.dim() and offset(), I would like to clip it "on
>> the fly" to visualize statistics associated with objects of different
>> size. The clip is square -specified with upper left and lower right
>> corners- and fit with the pixels size. If you have an interactive way,
>> I'm even more interested.
>
> For an interactive method for a SpatialGrid* object, try:
>
> library(sp)
> data(meuse.grid)
> coordinates(meuse.grid) <- c("x", "y")
> gridded(meuse.grid) <- TRUE
> fullgrid(meuse.grid) <- TRUE
> bbox(meuse.grid)
> image(meuse.grid, "dist", breaks=seq(0,1,0.1), col=heat.colors(10))
> # interactive choice of rectangle of interest, ll and ur
> pts <- locator(2)
> pts
> points(pts)
> grd <- getGridTopology(meuse.grid)
> gi <- getGridIndex(coordinates(pts), grd)
> cd <- slot(grd, "cells.dim")
> ys <- sort(ceiling(gi / cd[1]))
> xs <- sort(gi %% cd[1])
> meuse_sub <- meuse.grid[ys[1]:ys[2], xs[1]:xs[2]]
> bbox(meuse_sub)
> image(meuse_sub, "dist", breaks=seq(0,1,0.1), col=grey.colors(10),
>  add=TRUE)
>
> What you call clipping is the "[" operator here.
>
> Hope this helps,
>
> Roger
>
>>
>> Etienne
>>
>>
>>        [[alternative HTML version deleted]]
>>
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>>
>
> --
> Roger Bivand
> Economic Geography Section, Department of Economics, Norwegian School of
> Economics and Business Administration, Helleveien 30, N-5045 Bergen,
> Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
> e-mail: Roger.Bivand at nhh.no
>
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