Overlay
The Overlay tools take two layers and combine them by where they sit on the map: keep what falls inside, keep what two layers share, remove what one layer covers, merge both into one shape.
Each tool has an input layer, the one you are working on, and an overlay layer, the one you are cutting or comparing with. The order matters for Clip and Difference, and not for Intersection and Union.
Start here
Section titled “Start here”Cuts your layer to a study area. Keep the roads inside a municipality, the parcels inside a flood zone, the sample points inside a forest.
Put the layer you want to keep in Input layer and the boundary in Overlay layer. The result carries the input layer’s attributes, unchanged. Features that straddle the boundary are cut at it.
Intersection
Section titled “Intersection”Keeps only the areas covered by both polygon layers, and merges the attributes of both onto the result. This is the tool for questions like “which part of each parcel is in the flood zone, and what are the parcel reference and the flood level there”.
Choose Clip when the second layer is only a cookie cutter, and Intersection when you need what both layers say about the overlap.
Difference
Section titled “Difference”Removes the overlay layer’s footprint from your input layer. Land outside protected areas, parcels not already built on, the part of a catchment that is not urban. The result keeps the input layer’s attributes.
Swapping the two layers gives a completely different answer, so check which one is which before you run it.
All the tools
Section titled “All the tools”| Tool | What it does |
|---|---|
| Clip | Cuts the input layer to the area covered by the overlay layer. Keeps the input attributes. |
| Intersection | Keeps the areas where both polygon layers overlap, merging the attributes of both. |
| Difference | Removes the overlay layer’s area from the input layer. Keeps the input attributes. |
| Union | Merges two polygon layers into a single combined geometry. |
Geometry types. Clip accepts any geometry as input. Intersection, Difference and Union work on polygons.
Invalid geometries. Self-intersecting polygons, a common issue in data digitised by hand, can make an overlay fail or return an odd shape. Running the input through Simplify or Smooth in Geometry often clears it up.
Attributes on Union. Union answers a question about shape, not about data. Use Intersection when you need attributes on the result.