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?
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:
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).
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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).