Spatial Analysis and Overlay Techniques

Spatial analysis is the set of techniques that let a GIS answer questions a simple map cannot: not just where things are, but how they relate to one another in space. The foundational technique is overlay analysis, in which two or more data layers covering the same area are combined so that the resulting layer's geometry and attributes reflect the spatial relationships among the inputs. Common vector overlay operations include intersect, which keeps only the geometry and attributes common to all input layers; union, which combines the full extent of all inputs while preserving each one's attributes; and erase (or difference), which removes the area of one layer from another. The conceptual origin of overlay analysis is often traced to landscape architect Ian McHarg's Design with Nature (1969), which popularized a manual technique of stacking hand-drawn, semi-transparent map layers — slope, soil type, flood zones, wildlife habitat — over a light table to visually identify land best suited, or least suited, to a proposed development, a method GIS software would later automate directly on digital layers.