Overlay
Definition
Overlay is the core GIS operation of combining two or more spatial layers to derive new information based on their spatial relationship—intersections, unions, differences, and identity. Vector overlays compute topology between polygons, lines, and points; raster overlays combine cells with map algebra. Overlays power suitability modeling, impact assessment, and data integration from disparate sources. Accuracy hinges on clean geometries, common coordinate systems, and thoughtful handling of attribute conflicts and slivers. Temporal overlay—aligning datasets by valid time—prevents spurious results when boundaries change. While conceptually simple, overlay is where errors from scale mismatch, projection, and topology often surface.
Application
Planners intersect flood zones with parcels to set building rules. Ecologists overlay habitat with roads to locate conflict hotspots. Public health overlays clinics with population needs to target investments. Insurers combine hazard layers with portfolios to price risk. Researchers integrate socioeconomic and environmental measures for equity indices.
FAQ
How do we prevent sliver polygons?
Snap and clean geometries, set tolerances, and dissolve where appropriate. Track area changes so slivers don’t distort statistics.
Can overlays mix rasters and vectors?
Yes via zonal statistics or rasterization. Choose cell sizes carefully to avoid aliasing small features.
What about different vintages of boundaries?
Use correspondence files or rebase indicators to a common vintage; document the temporal assumptions to maintain comparability.
How to handle conflicting attributes?
Define precedence rules or aggregate functions before running overlay. Ambiguity should be resolved by policy, not ad hoc after the fact.