Combining attribute data from two geographic layers based on their spatial relationship — such as which points fall within which polygon.
A spatial join transfers attributes from one layer to another based on geographic proximity or containment, rather than a shared key value. Common spatial join types include: point-in-polygon (which polygon contains each point), nearest neighbour (which feature is closest), and intersect (which features overlap).
Example: A dataset of crime incidents (points) needs to be enriched with neighbourhood names (polygons). A spatial join assigns each incident the name of the neighbourhood polygon it falls within. The result is a single dataset with both the incident details and the neighbourhood name — without any shared key between the original datasets.
Spatial joins are one of the most powerful GIS operations because they enable integration of datasets that have no common identifier, only a shared geography.