How to crop a shapefile ?

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