Accuracy - The difference between a measured value of a feature and its true value on the Earth's surface.

Accuracy, Absolute - Accuracy determined by reference to a specified value.

Accuracy, Relative - Accuracy representing the effect of random errors in points on a map with respect to another.

Aerial Photography - The process of taking photography from an aircraft using specialized photographic equipment.

Aerial Triangulation - (a.k.a. aerotriangulation) The mathematical process of establishing precise and accurate relationships between the individual photographic film coordinate systems and a defined datum and projection.

Airborne Global Positioning System (ABGPS) - See GPS, Airborne

Base Map - A map on which information can be placed for geographical correlation. Also can be used as a "base" upon which to build other information. Digital Orthophotography serves as an excellent base map due to its accuracy and visual characteristics.

Breakline - A line with a series of vertices that defines points of inflection in the topographic surface of the earth (i.e., places where there are sharp changes in the direction of slope on the earth's surface). Breaklines are used in conjunction with masspoints to form a DEM or DTM.

C-Factor - The empirical ratio between the contour interval and photo scale. To calculate C-Factor divide the photo scale by the contour interval. Allowable systematic (not instrument) C-Factor is generally around 2100.

Cadastral Map - A map defining land ownership information. Generally used for taxation, planning, zoning and site development. Often serve as the base map for a Land Information System (LIS).

Contact Print - A photographic copy of an aerial photo negative (or positive) produced on opaque bond paper.

Contour Interval - The difference in elevation of two adjacent contour lines. Typical Contour Intervals are 1', 2', 5' and 10'.

Data Dictionary - A document that contains features, rules and item definitions for a specific set of geospatial data.

Datum - 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 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.

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.

Film Diapositive - A positive reproduction on transparent material of an aerial film negative (or positive). This product is typically used on a stereoscopic instrument for stereo compilation.

Focal Length - The distance between the nadir (center) point of the photographic lens to the photographic plane. Typical focal lengths are 153mm (6") and 305mm (12").

Geodetic Control - A network of carefully measured horizontal and vertical points. An accurate geodetic network provides the foundation for photogrammetric mapping.

Geographic Information System (GIS) - A GIS is a collaboration of software, hardware, data and people. This system is used to visualize and solve problems that are geographic by nature.

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.

Inertial Measurement Unit (IMU) - A system used to capture the orientation parameters (roll, pitch and yaw) of an aircraft at any given point in space and time. That data used in conjunction with ABGPS data can be used to supplant or validate the Aerial Triangulation solution for a project.

Land Information System (LIS) - Similar to a GIS, an LIS is designed specifically to create, visualize, analyze, report and publish land-based data such as parcel information, zoning, land use, ownership and general property information.

LiDAR (Light Detection and Ranging) - A scanning and ranging laser system that produces accurate Digital Terrain Models. Can be used for contour modeling, however, the lack of breaklines disallows the creation of cartographically aesthetic contours. Please contact Surdex to find out if your project is right for LiDAR.

Masspoint - An individual point that accurately defines that point on the Earth's surface in terms of horizontal and vertical positioning. Masspoints are used in conjunction with breaklines to form a DEM or DTM.

Metadata - Formal documentation describing the characteristics of a specific geospatial data set.

Monument - A permanent structure used to identify the location of a surveyed geodetic point. Monuments are typically constructed from concrete with a brass cap on top for identification.

Photogrammetry - The art and science of accurately measuring objects on the Earth's surface from a series of overlapping photographs (taken from aircraft or satellite).

Photo Scale - Calculated by dividing the flying height (above ground level) by the camera system's focal length. Contact Surdex to find out the appropriate photo scales for typical map products.

Planimetric Feature - Features of a map that represent everything except relief; both man-made and natural features like vegetation are considered planimetric. Examples include building footprints, edge of street pavement, sidewalks, etc.

Stereo Compilation - The process of mapping planimetric and/or topographic data in a 3-D environment from two overlapping

Topographic Feature - The features of a map that represent the natural features of the Earth's surface; representing relief. Those features collectively form a "model" of the surface.

Topology - Topology is a mathematical procedure for explicitly defining spatial relationships. Topology expresses different types of spatial relationships as features (e.g., polygon features for areas and lines for linear features).