[Moved from R-help] Re: [R] confine a dataframe with a polygon
On Wed, 4 Apr 2007, temiz wrote:
Roger Bivand wrote:
On Tue, 3 Apr 2007, temiz wrote:
hello
how can I confine a dataframe with a known polygon that is part of whole
data ?
Could I suggest following up this question on the R-sig-geo list? It is not clear what you want to do - are the rows in the data frame points which may lie inside or outside a polygon? If so, the overlay() methods in the sp package may be suitable.
regards
thank you for your reply I have not been aware of that list (R-sig-geo list) My data is elevation point data in a text file. I want to create DEMs with different interpolation methods for comparision. Actually I created DEM in GRASS using spline tension. Then I want to create DEM in R with different techniques to compare. But my data seems so huge for GSTAT-kriging and FIELDS-krig. So I decided to select smaller interpolation area within whole data. How can I define a subregion in which I do interpolation ?
In R-GRASS, use system("g.region ...") to set the subregion before
readRAST6().
For interpolation, you have two data sets, the point data for which you
know z, and the newdata points, often a raster, to which you want to
interpolate. I guess that you want to define the prediction model based on
all the data, then predict for tiles of newdata locations for export with
writeRAST6(). They can be put together in GRASS with system("r.patch
..."), or a more detailed process taking care of possible overlaps.
I found locator and chull function that may serve this. I will regard your recommendations. PS : Thinking facing a memory problem, I refrained from GRASS-R interface.
Use system("g.region ...") to choose a small enough region.
If your observed data are also large volume (a big DEM, for example), you
will need to be careful how you do things, possibly either using the GDAL
GRASS plugin and rgdal, or r.out.gdal for say geotiff, and then subset the
amount of data read in in readGDAL. Users report success with very large
data sets and rgdal, you just need to do things in sensible subsets.
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