How do I Clip Data?
This post covers a very useful skill – clipping subsets of vector data from larger datasets (clipping raster data is covered in another post – Updated May 2021 for Pro).
As seen is some earlier posts, most of the data on the J: drive is at a national scale. In many cases, though, you’ll be working at a much larger scale (and by that I mean a smaller area in the Cartography sense of scale). The Clip tool is what we’ll use to do this. Like a cookie cutter, the Clip tool uses the boundary of the polygon to clip out the data inside the polygon; this can be done on points, lines or polygons, but we must have a polygon layer that defines the area we’re clipping.
Sometimes you’ve already got a polygon layer that you can use – that’s what we’ll talk about here. Other times, you need to create a new polygon layer – this post covers how to create a new feature class or shapefile.
Now onto clipping. If, for example, we wanted to clip out all of the roads within the Canterbury region, I would add the road_cl.shp (road centrelines – a massive layer at the best of times) layer from J:\Data\Toposhapefiles:
Next, I need a polygon layer that shows the extent of the Canterbury region. I’ll add the regions.shp layer from J:\Data\Administrative_Boundaries. Note that this gives me all the regions but I want to use only the Canterbury Region, so I’ll go into the Attribute table and select the Canterbury record – note how it’s outlined in blue. With most tools in ArcGIS, if a tool is run on a layer that has any features selected, it will only work on those features. (Alternatively, with the Canterbury region selected, I could right-click on the layer name and go to Data > Export Data to create a new layer with just the Canterbury region in it and use that as my boundary.)
Now I’ll run the Clip tool. You can find it in two places, either:
- ArcToolbox > Analysis Tools > Extract > Clip,
- Search for it on the Geoprocessing pane, or
- Or, since it’s such a popular tool, it’s also found on the Analysis tab with the often used Tools section:
From any path, you set the Input Features as the layer you want to clip, specify your polygon boundary layer as the Clip Features and set the output location and name:
Notice that I’ve got both layers added from the J: drive but am saving the output to my H: drive – no problem doing this. Click Run and off it goes. When finished, the new layer is added to your map:
Now I can remove the unneeded roads_cl.shp layer from my map and just work with the clipped data.
Clipping is a fundamentally useful tool and one often used at the beginning of a project. It allows you to work with a subset of larger dataset and can be used with points (e.g. water wells), lines (e.g powerlines or roads) or polygons (e.g. landcover, soils). Clipping grids is a different process and warrants its own separate post.