Geographic Information Systems (GIS) model selected aspects of reality.
Overlay analysis is one of the spatial GIS operations. Overlay analysis integrates spatial data with attribute data. (Attributes are information about each map feature.) Overlay analysis does this by combining information from one GIS layer with another GIS layer to derive or infer an attribute for one of the layers.
At its simplest, overlay analysis can be a visual operation, but analytical operations require one or more data layers to be joined physically. This overlay, or spatial join, can integrate data of different types, such as soils, vegetation, land ownership, jurisdictions, etc. with assessor's parcels.
Results of overlay analysis rely on the spatial accuracy of the GIS layers. If the layers don't line up well, then the attributes inferred by the overlay may be incorrect. That is, the results are only as good as the GIS data used for the analysis. Our Department Disclaimer lists potential sources for errors that could affect data derived by overlay analysis.
We use overlay analysis to gather attributes displayed on this web site, including the following:
In addition to overlay analysis, other spatial GIS operations include:
- Query and Analysis, i.e. Retrieval/classification/measurement functions
- Answering analytical questions (distance, location, what's there, etc.)
- Continuity analysis
- Buffering
- Proximity analysis