Standard (2)—A number, or set of numbers, established in an industry, a science, or a technology, setting limits on the precision or accuracy with which operations, measurements, or products are to be made.

Stereomodel—The surface area of elevation and feature models visible in three dimensions by viewing the overlapping areas of stereo imagery in an analog, analytical, or soft copy stereoplotter.

Strip—A set of overlapping photographs that can be arranged in sequence so that, except for the last photograph, part of the object space shown in one photograph is also shown in the succeeding photograph, often obtained sequentially from a moving aircraft or satellite.

Subsidence—The loss of land surface elevation due to removal of subsurface support.

Surface—A three-dimensional geographic feature represented by computer models built from uniformly or irregularly spaced points with x,y coordinates and z-values.

Systematic Error—See Error.

Three Dimensional—Having horizontal (x,y) coordinates plus elevations (z-values).

Triangulated Irregular Network (TIN)—A representation of terrain with adjacent, non-overlapping triangular surfaces. A TIN is a vector data structure generated from the mass points and breaklines in a DTM. TINs also preserve abrupt linear features and are excellent for calculations of slope, aspect, and surface area and for automated generation of topographic contours, which are all important functions in flood study engineering.

Vertical—The direction in which the force of gravity acts. Whereas the vertical is the perpendicular to an equipotential surface of gravity (e.g., the geoid), a normal is the perpendicular to a given ellipsoid.

Vertical Accuracy—See Accuracy.

Vertical Datum—See Datum.

Void—Portions of a digital elevation dataset in which no elevation data are available. In USGS DEMs, each elevation post located within a void area is assigned a discrete false value representing the void. Treatment of void areas should be documented in the metadata file.

z-Coordinate—The distance along the z-axis from the origin of a three-dimensional Cartesian coordinate system. Note, this is not the same as the elevation or height above the vertical datum.

z-Values—The elevations of the three-dimensional surface above the vertical datum at designated x,y locations.