Geographic Information Systems: An Introduction, 3rd Edition

14.2 Logic Operations

Logical searches in databases normally employ set algebra or Boolean algebra. Set algebra uses the three operators equal to, greater than, and less than, and combinations thereof:

=, >, <, ≥, ≤, < >

These operators are included under SQL.

Practical applications include:

• Identifying extrema, such as finding attribute minima or maxima within various polygons and, as a result, delineating a new thematic layer (new row in the attribute table)
• Selection or isolation, where particular values are selected for sub-sequent ranking in a new thematic layer

Boolean algebra uses the AND, OR, NOR, and NOT operators to test whether a statement is true or false. AND, OR, and NOT are used in SQL. For two items, A and B, we might have any of the following statements:

A AND B, A OR B, A NOR B, A NOT B

Figure 14.1

Logical operations shown in a Venn diagram. Similar operations can be carried out on geographical data. The shaded areas represent true statements.

Such statements may be illustrated in a Venn diagram, which is a schematic representation of a set in which magnitudes illustrated by surfaces are superimposed, as shown in Figure 14.1. The shaded areas represent true statements. This technique is well suited to analyzing geographical data. For example, potential conflicts between forestry and cattle farming can be illuminated by assigning A to potentially productive ...