[R-sig-Geo] Clipping grid using shapefile?
edzer.pebesma at uni-muenster.de
Mon Mar 1 20:35:39 CET 2010
please tell us what was unexpected after you overlayed vector and ppt01.
Christopher Swingley wrote:
> I'm quite new to the spatial functions in R (and to R as well), and am
> having trouble getting R to clip an ASCII grid using a shapefile raster.
> What I've got are ASCII grids that store climatological variables (one
> grid for each variable; temperature, precip, etc.). I also have a
> shapefile containing a series of watershed polygons. What I want to do
> is clip the ASCII grid using each polygon in the shapefile and calculate
> the mean and standard deviation of the climate data within each polygon.
> Reading the data in:
> > library(rgdal)
> > library(sp)
> > vector <- readOGR("shapefiles", "watersheds")
> > ppt01 <- readGDAL("ppt01.asc")
> > proj4string(ppt01) <- CRS("+proj=aea +lat_1=55 +lat_2=65 +lat_0=50
> +lon_0=-154 +x_0=0 +y_0=0 +ellps=GRS80 +datum=NAD83 +units=m +no_defs
> I think overlay() is the function I want, but I'm not getting the
> results I expect. After the clipping, I'd like to be able to do:
> > means <- mean(ppt01_clip, na.rm = TRUE)
> > sds < -sd(ppt01_clip, na.rm = TRUE)
> And get a table of watershed IDs with their means, and a table of
> watershed IDs with their standard deviations.
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