Linking features to information

On a GIS map, there's more to a feature than its location and shape. There's all the information associated with that feature. For a road, this might include its name, speed limit, and whether it's one-way or two-way. For a city, this might include its population, demographic characteristics, number of schools, and average monthly temperatures.

A particular type, or category, of information associated with a feature in a GIS is called an attribute. For example, population can be an attribute of a city, country, continent, and other features. Feature attributes are stored in an attribute table. In an attribute table, each feature is a record (row) and each attribute is a column, or field. The attributes for all the features in a layer are stored in the same attribute table.

This attribute table for a layer of cities stores each feature's ID number, shape, name, the country in which it's located, and its status.

A feature on a GIS map is linked to its record in the attribute table by a unique numerical identifier (ID). Every feature in a layer has an identifier. It is important to understand this one-to-one relationship between feature, identifier, and attribute record. Because features on the map are linked to their records in the table, you can click a feature on the map and see the attributes stored for it in the table. When you select a record in the table, the linked feature on the map is automatically selected as well.

In this map of India, the city of Bombay is selected. Its record is also selected in the attribute table. The unique identifier for Bombay is stored in the FID field.

Links between features and attributes make it possible to ask questions about the information stored in an attribute table and display the answer on the map. This linkage makes GIS maps much more informative than static maps.