I’ve recently been extracting some river geometries for major cities around the world. The data needs to be a list of latitude/longitude coordinates, representing the nodes on the shape for the river concerned.
I’m sure there’s easier ways to do this, but here’s my technique, shown here for Minneapolis. Click the images for larger version.
1. Extract the data from OpenStreetMap. Use the Export function, and draw out the area concerned with a bounding box. Choose OpenStreetMap XML as the format. I originally tried SVG, but this presents you with screen coordinates instead of latitude/longitude pairs.
2. Open the resulting file in Quantum GIS (QGIS). I used QGIS 1.9. You need the OpenStreetMap plugin installed, this will allow the OSM file that was created in Step 1 to be read straight in (in fact you could download the file directly from the OSM servers, if you wanted to).
3. Select the feature you are interested in. My river (actually a waterbank polygon) is a “hairy feature” as it extends well beyond the extent of the data that was downloaded. Make sure you are selecting it (feature turns yellow) rather than highlighting it for feature information (feature turns red). Otherwise, the subsequent file is, rather unhelpfully, blank.
4. Do Layer > Save Selection as Vector File. Choose “KML” as the format. You probably don’t need to change the coordinate reference system (CRS) as the data will already be in WGS 84, and this (“normal GPS-style latitude/longitude) is the CRS you want.
5. Edit the resulting file, removing the XML tags, and header/footer, and replace spaces with return characters, to leave a long list of latitude/longitudes, ready for importing into your visualisation code.