GIS Data is collected by data layer or "theme". . .
	. . . such as boundaries.
"Themes" define features using one or more feature "types".
	Roads are defined by LINE features.
	These Air Quality Pollution Discharge Locations are defined by POINT features.
	Hydrographic (water) features are defined by both LINE features (for streams), and AREA features (for lakes and ponds).
Each individual feature is uniquely numbered, and this number identifies a row in a table where the feature's attributes are stored.
	ID-# ID-TYPE LENGTH MATERIAL 1 landing strip 1500 feet long ashpalt 2 runway 1200 feet long gravel 3 phone line 30000 feet long fiber optic . . . . . . . . . . . .
The "themes" are then layered on top of each other ...
	When viewed in a GIS, these layers visually appear as one graphic, but are actually still independent of each other. This allows changes to specific themes, without affecting the others.
	Overlaying themes creates "maps"