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I have downloaded a 100Km map from OS, but the problem is that it is far larger than I need and hence takes a long time to load every refresh in QGIS. Is there a way to cut down the size of the accompanying shape files to a more manageable size of, say, 10Km square?

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The data is VectorMap District vectors I guess? If you are happy about working from a command line or in a terminal you can clip a shapefile to a precise 10 km square using a command like this:

ogr2ogr -spat 460000 190000 470000 200000 -f "ESRI Shapefile" SU_Road_clipped.shp SU_Road.shp

where "460000 190000" forms the coordinate of the bottom left-hand corner of a rectangle and "470000 200000" forms the coordinate of the top right-hand corner of the same rectangle. The original shapefile will be clipped to a subset having the extents specified by the rectangle (the output will be a new shapefile, the original will not be harmed in any way).

Nick.

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I've downloaded ogr2ogr from FWTools (Is that the best place as it's given me 100s of new files!!!), tired it and it works, brilliant thanks. Better than the Sextante option – Peter Cornelissen Aug 27 '12 at 14:20
FWTools is great, but in fact if you have QGIS you already have GDAL/OGR installed :) N. – nhopton Aug 27 '12 at 15:15
I thought I had but could not work out how to use it!!! – Peter Cornelissen Aug 27 '12 at 19:14
I should have mentioned that I could not find an ogr2ogr executable on my PC which was why I installed FWTools – Peter Cornelissen Aug 27 '12 at 19:25
It should be in C:\Programs (x86)\Quantum GIS Lisboa\bin if you installed by standalone installer, else C:\Osgeo4w\bin. – Andre Joost Aug 28 '12 at 4:19
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Use the GRASS v.generalize tool, you can use it with the qgis/grass plugin or now also with the sextante plugin (the last option is easier as you don't need to get deep into the grass data model).

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most probably the op wants to clip/extract the 100 km² data to parts of 10Km² each.. – vinayan Aug 27 '12 at 10:22
That's right. I have not used Sextante or Grass as it was not needed for my purposes. Grass does not seem to fit the bill, but would Sextante? – Peter Cornelissen Aug 27 '12 at 11:00
OK, I have just installed Sextante and I think it will do the trick, so thanks Giovanni – Peter Cornelissen Aug 27 '12 at 11:05

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