No other operation acts so clearly to integrate diverse sources of information. Over a number of years, a number of different schemes have been articulated to explain the overlay operation, most notably a 1975 paper by Hopkins and the map algebra advanced by Tomlin. These frameworks do not provide an adequate explanation of how overlay serves to integrate and what limits should be imposed on its application. With a dominance rule, one of these values become the result based on some external rule, such as the highest value for ordinal sources. A contributory rule uses all the source values to construct a composite value where each value contributes to the result equally.
Addition is the simplest model, though far from universally applicable.
An interaction rule uses all the values, but relies on the relationships between the values, not just their numerical values. This new taxonomy offers an improved explanation of GIS operations for education, training and an analytical critique of GIS practice. Gestalt, Ordinal, Linear, Factor Combination, etc. Formal interaction tables Threshold formulae Divided phase space Three Classes of Rules Connection to transformations Overlay as special case Geometric procedure assembles all the attributes for a given location. Rules of combination produce a composite result.