Union (Spatial)
Spatial union combines two or more overlapping feature layers into a single output that preserves all features and attributes from every input. The resulting dataset contains the full spatial extent of all inputs with overlapping areas split into distinct features carrying attributes from both sources.
The spatial union operation merges two or more input feature layers into a single output feature layer that includes all areas from all inputs. Where features from different inputs overlap, the overlapping area is extracted as a separate feature that inherits attributes from both contributing layers. Non-overlapping areas are preserved with their original attributes and null values for attributes from the other layer.
How It Works
Union computes the geometric union of all input features, splitting overlapping polygons along their shared boundaries. Each resulting polygon fragment carries the attributes of all input features that contributed to it. For example, unioning a zoningZoningZoning is a land use planning tool that divides geographic areas into zones with specific permitted uses, building st... layer with a flood zone layer produces an output where each polygon carries both the zoning classification and the flood zone designation, enabling integrated analysis of the two regulatory frameworks.
Applications
Environmental analysts union land cover datasets from different time periods to create change polygons that carry attributes from both dates. Planners combine zoningZoningZoning is a land use planning tool that divides geographic areas into zones with specific permitted uses, building st..., utilities, and environmental constraint layers to create comprehensive regulatory maps. Conservation managers merge habitat range maps from multiple species to identify areas of biodiversity overlap. Emergency managers combine hazard layers (flood, earthquake, wildfire) to create multi-hazard assessment maps.
Considerations
Union can produce a large number of small polygon fragments, especially when combining layers with complex boundaries. The output may require dissolveDissolveDissolve merges adjacent or overlapping features that share a common attribute value into single, unified features by... operations to simplify the results. Performance can be affected by geometric complexity and the number of overlapping features.
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