Topological Overlay:

Co-Occurence mapping in a vector GIS is accomplished by topological overlaying. Any number of maps may be overlayed to show features occurring at the same location. To accomplish this, the GIS first stacks maps on top of one another and finds all new intersecting lines. Second, new nodes (point features where three or more arcs, or lines, come together) are set at these new intersections. Lastly, the topologic structure of the data is rebuilt and the multifactor attributes are attached to the new area features.

Boolean Analysis

In a raster GIS, boolean operations on different map layers allow the user to conduct co-occurence analyses. For example, determining which woodlots are growing on Soil A (good soil) could show where to expect the highest levels of timber production. Using the boolean intersection, called the "and" function, areas where woodlots and Soil A co-occur may be mapped, thereby depicting the areas of highest production potential. The graphic illustrates several other common boolean functions.