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Spring 2011 Edition

Glossary of Terrain Terms

Anchor point Anchor points remain throughout all pyramid levels of a terrain dataset. They are never filtered or thinned away (e.g., benchmarks, bathymetry control points).
Breakline A line in a TIN or terrain dataset that represents a distinct interruption in the slope of a surface by a ridge, road, or stream, for example. Triangles in a TIN or terrain dataset may not cross a breakline (i.e., breaklines are enforced as triangle edges). Z-values along a breakline can be constant or variable.
Clipping polygon Polygons are used to define boundaries for terrain surfaces when a data area has an irregular shape. Without a clip polygon, the data area would be convex and produce errors in the surface. The triangulation would create long sliver triangles while attempting to connect points around the exterior of the area and generate errors in the surface.
Constrained Delaunay A constrained Delaunay triangulation method follows traditional Delaunay rules everywhere except along breaklines. Using a traditional Delaunay triangulation method, breaklines are densified to ensure that the resultant triangulation remains Delaunay conforming. Therefore, one input breakline segment can result in multiple triangle edges. Using a constrained Delaunay triangulation, no densification occurs, and each breakline segment is added as a single edge. (See also Delaunay triangulation.)
Delaunay triangulation A technique for creating a mesh of contiguous, nonoverlapping triangles from a dataset of points. Each triangle's circumscribing circle contains no points from the dataset in its interior. Delaunay triangulation was named for the Russian mathematician Boris Nikolaevich Delaunay.
Face In a TIN or terrain dataset, a planar surface of a triangle bounded by three edges and three nodes. Faces do not overlap. Each face is adjacent to three other faces on the surface. A face defines a plane with an aspect and slope.
LAS LAS (spell L-A-S) is an open/published standard file format for the interchange of lidar data. It is a binary file format that maintains specific information related to lidar data. It is a way for vendors and clients to interchange data and maintain all information specific to that data.
Lidar Lidar (light detection and ranging) is a remote-sensing technique that uses laser light to densely sample the surface of the earth with x,y,z measurements. Lidar datasets produce mass point datasets that can be visualized and analyzed using a terrain dataset.
Mass points A dataset containing points that are typically derived from data sources such as lidar, sonar, or photogrammetric data.
Node One of the three corner points of a triangle in a TIN or terrain dataset, topologically linked to all triangles that meet there. Each sample point in a TIN or terrain dataset becomes a node in the triangulation that can store elevation z-values and tag values. Tags represent user-defined criteria about area.
Overview terrain The coarsest representation of the terrain dataset, it is designed to draw rapidly when the display is zoomed at or beyond its full extent.
Point spacing The most common distance between points used to construct a terrain dataset.
Sonar Sonar (sound navigation and ranging) is an underwater mapping technique that uses sound waves to map the shape of the ocean floor. Sonar datasets produce mass point datasets that can be visualized and analyzed using a terrain dataset.
Surface feature type Defines the role the feature class plays in defining the terrain dataset surface. The surface feature type (e.g., mass points, breaklines, various polygon types) are indicated when a feature class is added to a terrain.
Terrain dataset A terrain dataset is a multiresolution, TIN-based surface built from measurements stored as features in a geodatabase.
Terrain pyramid Levels of detail generated for a terrain dataset to improve efficiency. Terrain pyramids are generated through the process of point reduction or point thinning to reduce the number of measurements needed to represent a surface for a given area. Two pyramid types exist: z-tolerance and window size.
Triangle A face on a TIN surface. Each triangle on a TIN surface is defined by three edges and three nodes and is adjacent to from one to three other triangles on the surface. TIN triangles can be used to derive aspect and slope information and can be attributed with tag values. Tags represent user-defined criteria about area.
Triangulated irregular network (TIN) A vector data structure that partitions geographic space into contiguous, nonoverlapping triangles. The vertices of each triangle are sample data points with x-, y-, and z-values. These sample points are connected by lines to form Delaunay triangles. TINs are used to store and display surface models and as a background structure built on demand by terrains.
Window-size pyramid The window-size pyramid type thins points based on horizontal sample density.
Z-tolerance pyramid The z-tolerance pyramid type thins points based on vertical accuracy constraints.
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