How to crop large polygons in QGis

How to crop large polygons in QGis

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

The Goal

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

ZoomInToAreaOfInterest

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

SelectThePolygon

Use the select tool to select the North American land mass that is to be cropped.

 

4. Make the layer editable

MakeTheLayerEditable

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

UseTheSplotFeaturesTool

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

BisectTheShape

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.

 

8. Bisect the shape again

BisectTheShapeAgain

And bisect the shape once more to create an appropriate split.

 

9. Another bisected shape

AnotherBisectedShape

And the shape is split again.

 

10. Save the edited layer

SaveTheEditedLayer

Now turn off editing as per stop 4 above and save the layer.

 

11. Select features to export

SelectFeaturesToExport

Now use the select tool and select the features, including our new shape, to export. Use the Layer -> Save Selection as Shapefile option.

 

12. Save the new layer

SaveTheNewLayer

And keep the defaults (WGS 84).

 

13. The new layer

TheNewLayer

We can check the new layer by adding it to QGis and taking a look.


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