Angle of Illumination: The angle at which the illumination source is directed from. Terrain images are often illuminated from the northeast, southeast, southwest, and northwest. To minimize the possibility of terrain inversion (where hills look like valleys and the reverse), it is conventional to illuminate terrain from the northwest.
Bathymetry: The representation of the surface under water bodies.
Breaklines: Spot elevations collected along linear features.
Contours: A line along with an elevation value (number) that remains equal (the same).
Contour Interval: The interval or difference in magnitude between two sequential contour lines on a map.
Delaunay Triangulation: A circle surrounding each triangle side on a TIN surface does not contain any other vertex.
Digital Elevation Model: Any raster representation of a terrain surface. Specifically, in relation to the NSDI, a DEM is a data product of the U.S. Geological Survey. Here we consider the characteristics of DEMs produced by the USGS.
Digital Raster Graphics (DRGs): Scanned raster images of USGS 1:24,000 topographic maps.
Interpolation: The process of estimating an unknown value from neighboring known values. It is a process used to create gridded surfaces for many kinds of data, not just elevation.
Inverse Distance Weighted: An algorithm that assumes that estimated elevations are more similar to nearby elevations than to distant elevations. The algorithm estimates the value z of a point P as a function of the z-values of the nearest n points. The more distant a point, the less it influences the estimate.
Isarithm: A line that connects locations of the same value.
LIDAR: LIght Detection And Ranging. Like radar (RAdio Detecting And Ranging), lidar instruments transmit and receive energy pulses, and enable distance measurement by keeping track of the time elapsed between transmission and reception. Instead of radio waves, however, lidar instruments emit laser light (laser stands for Light Amplifications by Stimulated Emission of Radiation).
Mass Points: Spot elevations produced photogrammetrically.
Multibeam: A multibeam instrument mounted in the ship's hull calculates ocean depths by measuring the time elapsed between the sound bursts it emits and the return of echoes from the sea floor.
National Spatial Data Infrastructure (NDSI): "The technology, policies, standards and human resources necessary to acquire, process, store, distribute, and improve utilization of geospatial data" (White House, 1994). See Chapter 4.4 for more.
Neighborhood Method: A calculation of the slope at one grid point by comparing the elevations of the eight grid points that surround it (i.e., those in its neighborhood).
Quadrangles: A four-sided polygon where each 1:24,000 quad covers 7.5 minutes longitude by 7.5 minutes latitude, with its shapes and area coverage varying depending on its location on Earth.
Raster: Involves sampling attributes for a set of cells having a fixed size.
Reference Maps: Maps that help people look up facts. They are designed to serve many different purposes such as locating place names and features, estimating distances, directions, and areas, and determining preferred routes from starting points to a destination. They can also be used as base maps.
Shaded Relief: A method of assigning gray values to pixels in ways that make it appear that a DEM is illuminated from above.
Side Scan Sonar: Instruments are mounted on both sides of a submerged "towfish" tethered to the ship. Unlike multibeam, side scan sonar measures the strength of echoes, not their timing. Instead of depth data, therefore, side scanning produces images that resemble black-and-white photographs of the sea floor.
Slope: A measure of change in elevation.
Sonar: (SOund NAvigation and Ranging) Echo sounders for deepwater surveys beginning in the 1920s.
Spatial Dependence: Nearby locations are more likely to have similar elevations than are distant locations.
Tiles: Regions that correspond to the 7.5-minute topographic quadrangles. Users often download data that is bound to the region in a tile for each quadrangle.
Topography: A science that studies the surface of the Earth.
Topographic Maps: Maps that show and name many of the visible characteristics of the landscape, as well as political and administrative boundaries.
Triangulated Irregular Network (TIN): a vector representation of a continuous surface that consists entirely of triangular facets. Vector: Involves sampling either specific point locations, point intervals along the length of linear entities, or points surrounding the perimeter of areal entities, resulting in point, line, and polygon features. Vertical Exaggeration: The effect of elevations exaggerated several times to make the terrain more pronounced.