GEOG 160
Mapping Our Changing World

6. Logical Consistency

PrintPrint

Including topological data in TIGER also helps the Census Bureau to ensure the quality of the database. By definition, features in the TIGER/Line files conform to a set of topological rules, including:

  1. Chains begin and end at nodes.
  2. Chains connect to each other at nodes.
  3. Chains do not extend through nodes.
  4. Chain segments are bounded by two polygons, one on either side.
  5. Chains representing the limits of files are free of gaps.

Compliance with these topological rules is an aspect of data quality called logical consistency. Features that do not conform to the rules can be identified automatically, and corrected by the Census geographers who edit the database. Given that the TIGER database covers the entire U.S. and its territories, and that the complete set of TIGER/Line files fills six CD-ROMs (almost 4 gigabytes of data), the ability to identify errors in the database automatically is crucial.

Encoding topological relationships explicitly makes it easier to automate and perform other tasks as well, such as districting and routing. Redistricting problems, for example, require the ability to dissolve boundaries between adjacent polygons (such as census tracts) that share selected attributes. Although inexpensive computers are now powerful enough to calculate topology "on the fly" for many applications, others, like routing (finding the shortest path that connects two or more specified locations) benefit from geographic databases like TIGER/Line files in which spatial relationships between features are explicitly encoded, and therefore need not be computed.

Practice Quiz

Registered Penn State students should return now to the Chapter 4 folder in ANGEL (via the Resources menu to the left) to take a self-assessment quiz about Topology.

You may take practice quizzes as many times as you wish. They are not scored and do not affect your grade in any way.