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I've downloaded the 350MB coastline shape file from Openstreetmap, but I really would like to work with a small part of them, is there a way to generate a shapefile of a small area.

I'm using MacOSX or Unix for this.

Edit: Apparently the coastline file from Openstreetmap wasn't one big polygon so everything worked fine. Without cutting polygons.

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According to the docs, it's not "one big polygon," but has already been "divided into 100km x 100km squares." Do you need to clip out smaller portions of those square features or would it suffice to select those squares covering your study region? – whuber♦ Feb 10 '11 at 14:31
It would be enough if I could export a couple of those 100km squares. Not optimal, if I'm limited to such a big grid, but very doable. – Erik Johansson Feb 17 '11 at 0:27

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With ogr2ogr, you can specify a bounding box that you'd like to clip by with -clipsrc x_min y_min x_max y_max, so for example, to only get the northern hemisphere of a global dataset you could do:

ogr2ogr -f "ESRI Shapefile" borders_north.shp borders.shp -clipsrc -180 0 180 90

See the ogr2ogr documentation for further options, including clipping to another geometry.

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If you have an old GDAL version (<1.7) this "works": ogr2ogr -f "ESRI Shapefile" borders_north.shp borders.shp -spat -180 0 180 90 It will not clip so if you have one big geometry you will still have to upgrade, – Erik Johansson Dec 12 '11 at 12:29
The file is of course in spherical mercator, so you will have to covnert the coordinates in someway. – Erik Johansson Dec 13 '11 at 10:39
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You can install and use qgis to load the file you downloaded; use the mouse or query the data for the features that you would like to select; and export the selected features to a new shapefile.

QGIS is a cross-platform (Linux, Windows, Mac) open source application with many common GIS features and functions.

A link containing tutorials that illustrates these steps in qgis is available here http://qgis.spatialthoughts.com/

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