GIS Glossary of Terms

Aerial Photography
The process of taking photography from an aircraft using specialized photographic equipment.
Base Map or Base Layer
The foundation of a GIS on which all other data layers are built.  The accuracy of the base map dictates the accuracy of other data used in the GIS.  Digital orthophotography is a commonly used base map for counties and cities.
Acronym for COordinate GeOmetry.  A method often used by surveyors to enter survey data and calculate precise boundaries.  Bearings and distances are used to descibe the border of a specific feature such as a parcel boundary. 
Example:     N90:0:0E 100'
                   S0:0:0E 100'
                   N90:0:0W 100'
                   N0:0:0W 100'
A reference surface for a particular coordinate system. Defines the mathematical relationship by which a feature relates to its original surface in a specific coordinate system.
Digital Elevation Model (DEM)
A 3-D model of the Earth's surface represented by masspoints and breaklines. Less rigorous (not as densely collected) than a DTM. A DEM does not support contour modeling and is primarily used for orthorectification of aerial photography to the Earth's surface.
Digital Orthophotography
A photo-quality digital image of surface features in their geometrically corrected, true map projection. The orthorectification process ties each pixel in a digital image to its true Earth location.
Digital Terrain Model (DTM)
A more rigorous 3-D model of the Earth's surface than a DEM. A DTM can be used for accurate contour modeling and can also be used for orthorectification.
Global Positioning System (GPS)
A group (constellation) of 24 satellites maintained by the U.S. Department of Defense that are used to triangulate the true position on the Earth's surface of a specific GPS receiver.
GPS, Airborne
A GPS receiver located in an airplane used to triangulate the position of the aircraft or its camera system at any given point in space or time.
Map Scale
The relationship between distance on a map and the corresponding distance on the earth's surface.  A statement of a measure on the map and the equivalent measure on the Earth's surface, often expressed as a representative fraction, such as 1:12,000 (one unit of distance on the map equals twelve thousand units of the same distance on the Earth's surface).   Map scale can also be expressed as a statement of equivalence units, such as 1 inch = 1,000 feet.
Map scales are often termed as "large" or "small."  Large scale maps are used to map smaller areas in more detail while small scale maps are employed to map larger areas with less detail.  A map scale of 1:1,200 would be larger than a map scale of 1:12,000. 
The configuration of the Earth's surface including relief and the position of  natural features.  Contours indicating lines of equal surface elevation are used to illustrate topography on a map.