Introducing Geographic Information Systems with ArcGIS: A Workbook Approach to Learning GIS, 3rd Edition

Geodatabases and Attributes

We said that a GIS was the marriage of a geographic database to an attribute database. In a geodatabase, each row in the attribute database refers to an “object” that is a point feature, line feature, or polygon feature.

One of the indications of maturity of the GIS field is the growing emphasis on attribute data correctness and integrity. All large databases, spatial and otherwise, contain errors. With geodatabases, a number of built-in capabilities promote data quality. For example, suppose that you are building a database of the roads in your county. One attribute in the database is the material the road is made of. Perhaps you know that the only allowed materials are concrete, asphalt, macadam, and gravel. With geodatabases you could allow a data entry person to only select among these four. First, this makes data entry faster. Second, it avoids the possibility that someone will type in “asfault” instead of “asphalt.” The items concrete, asphalt, macadam, and gravel constitute a domain for the roads feature. You could also set a default value for roads. If no other value is entered, the value of the attribute would automatically be set to “concrete.”

Subtypes

Geodatabases go even further in promoting data integrity. Within feature type, say roads, you can define subtypes of roads. Perhaps your planning agency has classed all the roads as freeway, major, or minor. You could set up the database so that “concrete” was the default material for freeways, ...