[R-sig-Geo] Crop a raster using a shapefile

[Mauricio Zambrano-Bigiarini]

On 01/06/12 20:49, Thiago Veloso wrote:
>    Dear all,
>
>    When cropping a raster using a shapefile, through 'crop' function from "raster" package, only the extent is taken into account:
>
>> r<-raster("lai2011361.Lai_1km.tif")
>> extent(r)
>
> class       : Extent
> xmin        : -104.4326
> xmax        : -29.99736
> ymin        : -40.00064
> ymax        : 10
>
>> br<-readShapePoly("/mnt/disco3/MODIS/shapes/brazil.shp")
>
>> extent(br)
>
> class       : Extent
> xmin        : -73.83943
> xmax        : -34.8581
> ymin        : -33.77086
> ymax        : 5.38289
>
>> c<-crop(r,br)
>
>> extent(c)
> class       : Extent
> xmin        : -73.83772
> xmax        : -34.85554
> ymin        : -33.76852
> ymax        : 5.38428
>
>    However, the cropped raster is rectangular. Is there any way to keep also the "shape" of a shapefile (and not only its extent) when performing this kind of crop operation?
>

You may use a combination of crop and mask:

------------ START ---------------
library(raster)

# Reading the shapefile
myshp <- readOGR("myshapefile.shp", layer=basename(myshapefile) )

# Getting the spatial extent of the shapefile
e <- extent(myshp)

# Reading the raster you want to crop
myraster <- raster(myraster.filename)

# Cropping the raster to the shapefile spatial extent
myraster.crop <- crop(raster, e, snap="out")

# Dummy raster with a spatial extension equal to the cropped raster,
# but full of NA values
crop          <- setValues(myraster.crop, NA)

#  Rasterize the catchment boundaries, with NA outside the catchment 
boundaries
myshp.r <- rasterize(myshp, crop)

# Putting NA values in all the raster cells outside the shapefile boundaries
myraster.masked <- mask(x=myraster.crop, mask=myshp.r)

------------ END ---------------

IHTH,

Mauricio

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Water Resources Unit
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>    Thanks in advance,
>    Thiago.
>
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