Geographic Information Systems and Spatial Analysis
Geographic Information Systems and Spatial Analysis
This domain tests spatial data models, topology, overlay operations, and whether analytical results are geometrically and statistically defensible.
Core rules and concepts
- Vector models represent discrete objects as points, lines, and polygons; raster models represent continuous fields as cells. Topology encodes adjacency, connectivity, and containment.
- Distance, area, and buffer calculations require a projected coordinate system with suitable units. Geographic coordinates in degrees are not suitable for area measurement.
- Common operations include overlay (intersect, union), buffer, spatial join, dissolve, clip, and raster map algebra, reclassification, and zonal statistics.
- The modifiable areal unit problem and ecological fallacy affect interpretation of aggregated spatial data.
- Edge effects and positional error propagate through overlays, often creating sliver polygons.
Worked example
A parcel of 8,000 m² intersects a mapped flood zone. An intersect overlay creates the parcel portion inside the flood zone. If that polygon measures 3,200 m² in a projected CRS, the affected share is 3,200/8,000 = 40%. If the same polygon is measured in geographic degrees, the area value is meaningless for this comparison; reproject to an appropriate equal-area or local projected CRS first.
Traps
- Buffering features in a geographic CRS and interpreting the output distance as metres.
- Treating overlay output as exact when input boundaries have positional error.
- Confusing spatial join with attribute join.
- Ignoring sliver polygons or topological errors after overlay.