Overlay, Topological

Abstract

A topological overlay is a geoprocess whereby separate layers sharing the same spatial extent are merged in different ways. It is the most required and common geographic information system (GIS) technique in spatial data processing. In GIS, questions on spatial relationships (e.g., on top of or within) among features can be answered with the use of a topological overlay. It allows users to combine two or more layers to intersect, erase, or modify datasets, resulting in a new feature with the updated attributes of the input and overlay dataset values. The logic behind how a topological overlay works is based on the rules of Boolean logic. Exemplary methods of a topological overlay include erase, identity, intersect, symmetric difference, union, update, and so on. A topological overlay is different from a graphical overlay (image overlay), which neither requires any attribute updates nor creates a new dataset. Topological overlay among different feature types is possible: a point‐in‐polygon, a line‐in‐polygon, and a polygon‐on‐polygon overlay. Although overlay works in both vector and raster domains, a topological overlay is mainly applied to vector formats because it requires the rebuilding of topologic structure for node, arc, and polygon.