[R-sig-Geo] How to crop a shapefile ?

[Roger Bivand]

On Thu, 17 Dec 2009, Dan Putler wrote:

> Hi all,
>
> For this particular problem, Dylan Beaudette is probably exactly right.
> However, I ran into problems using similar tools in OpenJump when we
> wanted to create a shapefile of a region that we could postal address
> geocode against (the way the boundary line segments were clipped was a
> problem for the geocoder), which lead me to write tools in R to do it
> (outlined in my earlier post). That code does not clip lines, rather it
> enable the extraction of the original complete road segments. Since most
> of the ways in OSM don't have the attribute information needed for
> geocoding, the issue I was addressing won't be relevant here.
>
> Besides GRASS (which can be a bit overwhelming at first), and OpenJump
> (mentioned above), the Clip tool within the fTools plug-in for QGIS
> should also work. Given the size of the OSM road network files, a
> machine with a fair amount of memory will probably be needed to handle
> the problem.

It also looks as though the clipping capabilities of ogr2ogr will increase 
when GDAL 1.7.0 arrives - the beta is already available:

http://www.gdal.org/ogr2ogr.html
http://download.osgeo.org/gdal/gdal-1.7.0b1.tar.gz

In addition, something like this might be a candidate for rgeos (on 
R-Forge) but would involve C coding in the GEOS C API.

Roger

>
> Dan
>
> On Thu, 2009-12-17 at 10:27 -0800, Dylan Beaudette wrote:
>>> On Thu, 2009-12-17 at 11:07 +0100, Baptiste Coulmont wrote:
>>>> Hi,
>>>>
>>>> I'm working with R and various shapefiles created by the OpenStreetMap
>>>> project. Those shapefiles (the roads in particular) are quite big.
>>>>
>>>> How can I crop (or "trim", or "prune" those objects -- once in a
>>>> SpatialLineDataFrame -- if I need to work not on a national level but
>>>> on a municipal level ?
>>>>
>>>> Is it possible to create a subset of a smaller zone ?
>>>> Coordinates:
>>>>          min       max
>>>> r1  2.560776  6.403847
>>>> r2 49.493613 51.505419
>>>>
>>>> Thansk,
>>>> B. Coulmont
>>
>> This type of work is most efficiently done in a standard GIS. I would suggest
>> installing GRASS GIS or something like that. This is a standard vector
>> operation, and should be very fast. For this GRASS approach, see the
>> v.overlay command.
>>
>> Cheers,
>> Dylan
>>
>>
>>
>

-- 
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