Snow's Map and GIS
In 1854, John Snow mapped London cholera deaths and had the Broad Street pump handle removed. The outbreak was already declining, and Snow already believed cholera was waterborne; the map did not generate his hypothesis.
GIS does not answer questions. It makes spatial arguments visible, checkable, and falsifiable. Its true power lay in the anomalies—such as surviving brewery workers and a widow who drank the water from afar—which made miasma theory untenable.
GIS operates by stacking layers over shared coordinates. Each layer is ordinary, but their overlays reveal historical relationships.
Spatial Operations and Traps
| Operation | The question it answers |
|---|---|
| Overlay | What coincides with what? |
| Buffer | What is within X of this? |
| Least-cost path | Where would movement go given terrain? |
| Viewshed | What can be seen from here? |
| Density | Is the pattern clustered? |
Least-cost path computes routes across a cost surface of slopes and rivers. Viewshed determines line-of-sight visibility for towers and castles.
The central danger: GIS renders guesses and sketches with sub-metre precision. Avoid the ecological fallacy, assuming aggregate district data applies to individuals.