How to Clip Vector Data in QGIS
What you'll learn
- The basic concept of clipping vector data and what it is used for
- Step-by-step instructions for clipping in QGIS
- When to use Clip versus Intersection, and how to apply each
Recommended for
- Anyone who wants to extract only the data for a specific region or area
- Anyone who wants to reduce data size by trimming away unneeded areas
- Anyone who wants to narrow down the target area as a preprocessing step before analysis
Introduction
When you analyze data in GIS, you often need to extract only the area you need from a large dataset. For example, you may want to analyze only the stores in Tokyo from a nationwide store dataset, or only the roads within a specific municipality from a nationwide road dataset.
The vector geoprocessing operation Clip helps in exactly these situations. Clip is one of the basic functions you use most often in QGIS, and it is essential for preprocessing and narrowing down the analysis area.
This article covers everything from the concept of clipping to the steps for running it, plus how it differs from a similar operation, intersection.
What is clipping?
Clip is a spatial operation that cuts out vector data along a specific boundary.
It works like a cookie cutter pressing shapes out of dough. Clip cuts out the input data (points, lines, or polygons) to match the shape of the boundary (a polygon).
Layers required for clipping
Clipping needs two layers:
- Input layer: The data you want to clip. Points, lines, and polygons all work as input.
- Overlay layer (clip layer): A polygon layer that defines the area to cut out. Only the features of the input layer inside this boundary remain in the result.
The example below clips railway lines to the boundary of Tokyo. It uses two datasets from the National Land Numerical Information download site: railway lines for all of Japan (line data) and the administrative areas of Tokyo (polygon data).
Because the railway line data covers the whole country, the layer spans all of Japan as soon as you load it.
Run a clip
From the menu, choose Vector → Geoprocessing Tools → Clip.
You can also open the Processing Toolbox on the right and search for clip.
Set the following options for the clip.
- Input layer: Select the vector layer you want to clip.
- Overlay layer (clip layer): Select the polygon layer that serves as the boundary.
- Clipped: Specify the output destination. To keep it as a temporary layer, choose Create Temporary Layer. To save it as a file, click the ... button and specify the location and file name.
Check the result
When the clip runs, QGIS cuts the input layer to the area of the overlay layer.
The railway lines are clipped to the area of Tokyo.
In the attribute table, you can see that the attributes of the original data carry over unchanged.
Choosing between clip and intersection
Clip is handy for cutting out an area, but note that the result does not inherit the attributes of the overlay layer. If you want to tell, for example, which ward (such as Toshima or Nerima) a railway line belongs to, Clip alone cannot answer that.
When you want to keep the attributes of the overlay layer while cutting out the data, use Intersection instead of Clip. Like Clip, Intersection cuts the data to the polygon area, and it also carries over the attributes of the polygon features, so you can tell which municipality each railway line belongs to.
Conclusion
This article introduced Clip, which can cut data to the area of any polygon layer.
Clip is a basic but important vector geoprocessing operation in QGIS. Try it when you want to reduce the size of your data by removing areas you do not need.
Data sources
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