Overlay: a key tool in GIS

Objectives of lecture

• show manual form of overlay
• consider combination logic

Map Overlay: Manual approach

Maps on transparencies: same scale & projection, can be made to register

Overlay analysis by stacking them up

Ian Mc Harg (1969) Design with Nature - a cult classic

What works easily is "One strike and you're out" exclusions

More complex: ordinal scores, logical combinations (this AND that; this OR that...)

Use of addition depends on numerical scores that can be added (counterexamples)

Digital version

Automates all those steps

Other GIS operations

• (there are plenty)
• Buffers (just mentioned) converts points and lines to areas surounding them

Combination logic:

Exclusion (One strike and you're out)

Logical combinations (AND / OR, etc.)

Higher levels of numerical score permit complex models