Overlay — Operator family
Overlay Operators combine two layers as sets of areas. They are the spatial form of and, or and not, and they turn a list of conditions into areas that meet them.
Part of the Operators families. Used in: Network gaps and stepping stones (Task), Nature content layer (Task).
| Operator | Set logic | Typical use |
|---|---|---|
| Intersection | A and B | areas that meet two conditions |
| Union | A or B | areas that meet at least one condition, or alternatives |
| Difference | A not B | remove what is excluded (a negative constraint) |
| Symmetric difference | A or B, not both | one of two conditions is acceptable, both together is not |
| Clip | A, cut to the outline of B | intersection that keeps only A’s attributes |
Intersection
Operator Schema: the parts of A that are also in B. The result carries the attributes of both layers.
Tools: QGIS Intersection (Tool - QGIS).
Union
Operator Schema: all parts of A and of B, split where they overlap, so every piece knows whether it belongs to A, B or both. The attributes of both layers are kept.
Tools: QGIS Union (Tool - QGIS). To merge layers without splitting them, use merge instead.
Difference
Operator Schema: the parts of A that are not in B. Order matters: A minus B is not B minus A.
Tools: QGIS Difference (Tool - QGIS).
Symmetric difference
Operator Schema: the parts in A or B but not in both. Used for “either condition is acceptable, both together are not”.
Tools: QGIS Symmetrical difference (Vector overlay).
Clip
Operator Schema: the parts of A inside B, keeping only A’s attributes. It looks like an intersection on the map; the difference is in the attribute table.
Tools: QGIS Clip (Tool - QGIS). Clipping to a rectangle (a study area) is also part of clip to extent.
From a text to Overlay Operators
A description such as “within 100 m of existing nature, on farmland, but not on roads or in towns” breaks down into:
- positive constraints, combined with intersection (and) or union (or): within 100 m of nature, on farmland;
- negative constraints, combined with union and then removed with difference: roads, towns.
The result is positive constraints minus negative constraints.