Spatial Analysis: Buffering, Map Overlay & Surface Modeling

Explanation

Spatial analysis extracts novel geographic insights by combining disparate spatial layers. In vector overlay, intersecting a soil classification polygon layer with a rainfall polygon layer creates a new composite attribute table enabling agricultural suitability modeling. In raster analysis, Map Algebra performs mathematical expressions across cell grids. Surface modeling operates on DEMs to compute slope steepness and aspect, vital for landslide susceptibility mapping and identifying sun-facing slopes for solar farm development.

Key points

• Vector Overlay Operations: Point-in-Polygon, Line-in-Polygon, Polygon-on-Polygon.
• Overlay Boolean Operators: Intersect (AND: common areas retained), Union (OR: all areas combined), Identity, Erase (NOT: cut-out).
• Map Algebra (Raster Analysis): Mathematical operations on raster cell grids: Local (cell-by-cell), Focal (neighborhood window), Zonal (zones), Global operations.
• Surface Analysis: Derived from Digital Elevation Models (DEM): Slope (steepness in degrees/percentage), Aspect (downslope compass direction), Hillshade, and Viewshed.

Example

Code examples

/*
Vector Overlay Operations Matrix:
Operation | Input A (Land Cover) | Input B (Flood Zone) | Output Result
Intersect | Agriculture (Forest) | High Flood           | Flooded Farmland only (A AND B)
Union     | All Land Covers      | All Flood Zones      | All combined geometries (A OR B)
Erase     | Urban Areas          | River Waterways      | Urban land with rivers removed
*/

Exam questions

• [10 Marks] Explain vector overlay operations (Intersect, Union, Identity, Erase) with neat illustrative diagrams.
• [8 Marks] What is Map Algebra in raster analysis? Explain how Slope, Aspect, and Viewshed are calculated from a Digital Elevation Model (DEM).