Map Overlay:

Integrating Information from Diverse Sources

Objectives:

• REMINDER: Project team formation (P0), DUE by M/T (27/28 Oct.) lab meetings; discuss in M/T lab meetings 20/21 October.
• Origins of overlay (site suitability, environmental regulation)
• Steps to compute overlay

< Additional Resources; illustrations linked, not included>

First: recognition that no one map layer is enough to make a decision. Environment is full of interactions. No one science measures exactly all the relevant information as one single value. (Still it occurs that people fail to get this far...)

McHarg's Design with Nature (1969) proclaimed that environmental design required multiple factors and overlay, though the method is a bit hazy...

<Illustration of McHarg's manual overlay technique applied to site suitability>

Many environmental regulations imply overlay logic (examples)

Steps to carry out Overlay:

• Measurement and representation of source maps
• Registration (bringing each map into a common geometric reference)

Raster implementation is relatively easy (the integration is already done)

Vector Overlay involves a geometric discovery of intersections - creates a new representation

The Common Thread between different implementations:

Discover relationships from geometry

Actually a form of spatial JOIN where the "key" is sharing position (containership)

• Simple form of container: Point in Polygon
• More complicated: Polygon Overlay

All served by one engine that performs "Planar Enforcement" on the geometry

Then, Handle the Attributes

Assumption of uniformity

Category assumed to apply even over whole polygon/cell...
Combination of attributes requires agreement of the various parties... a whole social science issue (see next lecture)

Complete combination approach: intersect everything before any analysis

Logic applied: set operations {AND, OR, NOT} "Boolean logic"

Some operations can produce results by enumerating all the combinations obtained (Change analysis and error detection, for example)