Geographic Information Systems: An Introduction, 3rd Edition

14.7 Geometric Data Search and Retrieval

Geometric data may be searched using specified criteria, employing a dedicated query language or SQL for spatial data. Searches start from coordinate values, and in principle, any part of a database may be searched. Seamless databases permit searches anywhere within a geographical area. Databases based on map sheets, however, limit searches to one map sheet at a time, although contiguous map sheets may be combined for search purposes. In this case, the combined map sheets function as a partial copy of the database as a whole. For example, a search may be conducted within an area specified by the coordinates of its lower left and upper right corners, without involving the rest of the database.

Geometric data usually carry thematic codes for various types of objects, such as roads, administrative borders, water supply systems, buildings, properties, and so on. In searching these codes, the various map themes may be turned off and on, or shown in different colors, for the different types of objects.

A typical task may require the display of all map details within the limits of x minimum = 10,000 to x maximum = 12,000 and y minimum = 5000 to y maximum = 7000. The query program then reads and tests all coordinate values in the database against these constraints and retains those that lie within the limits stated. Proximity operations often necessitate searches in map data.

In such searches it is often presupposed that certain spatial relations ...