Yes, there is a round issues when you use "extract or mask", http://gis.stackexchange.com/questions/112274/is-this-a-known-issue-in-gaps-between-masked-raster-and-spatial-polygon-in-r-wit.
I use "disaggregate" to improve(?) raster resolution. This is an example yourraster.dis <- disaggregate("yourraster" , fact = 2) Nahm Lee n...@valleywater.org Santa Clara Valley Water District -----Original Message----- From: R-sig-Geo [mailto:r-sig-geo-boun...@r-project.org] On Behalf Of Frede Aakmann Tøgersen Sent: Wednesday, December 03, 2014 10:37 PM To: Simone Ruzza; r-sig-geo@r-project.org Subject: Re: [R-sig-Geo] overlay polygon with raster Hi Simone If I understand you correctly, then have a look at the man page of raster(). Here is the relevant sections: ## S4 method for signature 'Raster,SpatialPolygons' extract(x, y, fun=NULL, na.rm=FALSE, weights=FALSE, cellnumbers=FALSE, small=FALSE, df=FALSE, layer, nl, factors=FALSE, sp=FALSE, ...) Where x: Raster* object y: points represented by a two-column matrix or data.frame, or 'SpatialPoints*'; 'SpatialPolygons*'; 'SpatialLines'; 'Extent'; or a numeric vector representing cell numbers and weights: logical. If 'TRUE', the function returns, for each polygon, a matrix with the cell values and the approximate fraction of each cell that is covered by the polygon(rounded to 1/100). The weights can be used for averaging; see examples. This option can be useful (but slow) if the polygons are small relative to the cells size of the Raster* object. I just googled for ' R area of raster cells within polygon' and got this hit with a reproducible small example: http://stackoverflow.com/questions/17766989/extract-data-from-raster-with-small-polygons-rounded-weights-too-small See that page for a solution to some rounding errors in the case of the polygon being very small compared to raster cell size. Yours sincerely / Med venlig hilsen Frede Aakmann Tøgersen Specialist, M.Sc., Ph.D. Plant Performance & Modeling Technology & Service Solutions T +45 9730 5135 M +45 2547 6050 fr...@vestas.com http://www.vestas.com Company reg. name: Vestas Wind Systems A/S This e-mail is subject to our e-mail disclaimer statement. Please refer to www.vestas.com/legal/notice If you have received this e-mail in error please contact the sender. > -----Original Message----- > From: R-sig-Geo [mailto:r-sig-geo-boun...@r-project.org] On Behalf Of > Simone Ruzza > Sent: 4. december 2014 01:15 > To: r-sig-geo@r-project.org > Subject: [R-sig-Geo] overlay polygon with raster > > Dear R-sig-geo users, > > apologies for the simplicity of my question, but I have been > struggling with this for a bit I have not managed to find a solution. > Basically, I would like to overlay a polygon onto raster grid and > extract some statistics for the polygon in question i.e. the area of > every raster cell covered by the polygon. Here is a figure that might > make it clearer > <https://dl.dropboxusercontent.com/u/33966347/example.JPG> . I have > been fiddling around with the extract function from the raster > package, which works to some extent, as I am able to retrieve > information on the categories of each of the raster cells covered by > the polygon. However, as I said, I am interested in the area covered > by the polygon within each raster cell. I know that perhaps I should > try and convert my polygon into another raster and then overlay the > two rasters, but this perhaps would mean that the polygon should be > converted into a raster at a high resolution? If this was case, it is > not computationally feasible for me as some polygons I have cover a > huge area and it would take forever to overlay all those rasters. Any > help, suggestions would be highly appreciated! > > thanks in advance! > > Simone > > _______________________________________________ > R-sig-Geo mailing list > R-sig-Geo@r-project.org > https://stat.ethz.ch/mailman/listinfo/r-sig-geo _______________________________________________ R-sig-Geo mailing list R-sig-Geo@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-geo _______________________________________________ R-sig-Geo mailing list R-sig-Geo@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-geo