A step-by-step tutorial on how to crop large polygons to make a smaller one using the open-source GIS product, QGIS.
- 1. The Goal
- 2. Zoom in to area of interest
- 3. Select the polygon to be split
- 4. Make the layer editable
- 5. Use the Split Features tool
- 6. Bisected shape
- 7. Select the shape again to do more splits
- 8. Bisect the shape again
- 9. Another bisected shape
- 10. Save the edited layer
- 11. Select features to export
- 12. Save the new layer
- 13. The new layer
This example is based on creating a new shape file that includes the Caribbean islands and the coastline of the Gulf of Mexico. As the entire North American polygon is not required we will crop it to a more appropriate size.
Before starting the process save the original file to a new location as we will be making changes to it.
2. Zoom in to area of interest
Add the shapefile layer and zoom in to the relevant area, leaving plenty of room to work in
3. Select the polygon to be split
Use the select tool to select the North American land mass that is to be cropped.
4. Make the layer editable
Right click on the layer in the Layers window and turn on editing. The layer will be surrounded by small red crosses which are the individual points that make up the shape boundary.
5. Use the Split Features tool
Click on the Split Features tool (the scissors and line tool selected above) and click on the start point of the line that will be used to bisect the polygon using the left mouse button. Then move the line to a point that bisects the polygon and press the right mouse button. To make this easier the start and end points can be some distance into the sea to ensure the line completely bisects the shape.
6. Bisected shape
The North American continent has now been split into two, with our selection now the northerly part and a new shape, as indicated by the green below.
7. Select the shape again to do more splits
As we want to split the shape again (we don't need Baja California on the West coast) use the select tool to select the shape to split.
11. Select features to export
Now use the select tool and select the features, including our new shape, to export. Use the Layer -> Save Selection as Shapefile option.