Unit 41: Using Boolean Search Techniques
Chris Allen · 1998
Unit 41 - Using Boolean Search Techniques Written by Chris Allen, The Clark Labs, Clark University Context It is common to use a GIS to locate areas which satisfy a set of criteria. For example, we might be looking for all properties which are zoned for commercial development and which are located within 100 meters of a sewer line. A typical way to go about this is to prepare a data layer for each criterion in which only areas which satisfy the criterion are represented. In this case, we prepare one data layer which shows all commercially zoned properties and a second data layer showing only areas within 100 meters of a sewer line. By using an OVERLAY operation (see Units 34 and 35) between the two data layers we can determine which parts of our study area, if any, satisfy both criteria. Often there are many criteria to be satisfied and a series of such analytical steps is required. In a raster system, it is not possible to create a new raster layer which contains ONLY the features of interest - one cannot remove or copy out a subset of pixels without changing the spatial characteristics of the dataset (e.g., number of rows and columns, and adjacency relationships). In a raster system, the differentiation of features of interest is performed by setting the attribute value of unselected feature pixels to zero or some other background value. The situation where only two states of a variable are possible (in this case, criterion satisfied and criterion not satisfied) places us in the arena of boolean analysis. Boolean analysis is named after mathematician and logician George Boole, who devised rules and methodologies for combining such two-state variables. In boolean search we are generally most concerned with the AND operator (sometimes called the logical AND operator to indicate that we are referring to the word AND in this special context of specific rules defining the ways in which variables are combined). The logical AND operator produces a true result from the phrase A AND B only if both A and B are true . In GIS this methodology is used in a multiplication overlay between layers containing only zeroes (representing areas where conditions are false or criterion is not satisfied ) and ones (representing areas where conditions are true or criterion is satisfied ). When such boolean layers are multiplied together in an overlay operation, the only areas of ones in the output image occur where ones are present in both input images (i.e, 1 X 1 = 1, but 1 X 0 = 0, 0 X 1 = 0, and 0 X 0 = 0). The practical use of this technique is described more fully below. In raster and vector GIS, the concepts underlying boolean search are similar but vary in the details. Conceptually, one can think of a data layer or series of data layers which describe different characteristics of the same spatial area; each layer represents the areas which satisfy The NCGIA Core Curriculum for Technical Programs Page 1