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A GIS links sets of features and their attributes and manages them together in units called themes. A theme consists of a collection of geographic features (such as roads, rivers, parcels, wildlife sightings, schools, or parks) and the attributes for those features.
The figure below is the View document from the U. S. migration patterns project. A view is a map display that shows the geographic distribution of data (themes). This map shows which states people moved to and which states they moved from during the period 1985-1990. (Foreign immigration was not considered.) Blue states experienced losses; red states experienced gains. Yellow states had little change.
A map which visually represents a set of data in this way is called a thematic map.
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