Essential terrain dataset vocabulary

This topic provides an overview of some of the terminology you are going to encounter when working with terrain datasets in ArcGIS.

Terrain dataset terminology

Term	Description
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 used as a background structure built on demand by terrains.
Terrain dataset	A terrain dataset is a multiresolution, TIN-based surface built from measurements stored as features in a geodatabase.
LAS datasets	A LAS dataset stores reference to one or more LAS files on disk, as well as to features containing surface constraints. The LAS dataset allows you to examine LAS files quickly and easily, providing detailed statistics and area coverage of the lidar data contailed in the LAS files.
Terrain dataset versus TIN	TIN and terrain datasets are both excellent ways of displaying surface data. Each surface format has its own purpose with its own set of benefits. The following list compares TIN and terrain datasets. A terrain has the support of the geodatabase, whereas TINs are stored directly on disk and cannot be stored in a geodatabase. The terrain dataset maintains ties to the source measurements from which it was created. TINs, however, lose ties to their source data upon creation. A TIN has a recommended size limit of 5 million points, whereas terrain datasets do not have a size limit. A TIN can be visualized in 3D using ArcScene, whereas a terrain dataset cannot. Terrains are edited by modifying source measurements. TINs are edited by modifying the 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 is named for the Russian mathematician Boris Nikolaevich Delaunay.
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 resulting 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.
ArcGIS 3D Analyst extension	An extension to ArcGIS. The ArcGIS 3D Analyst extension provides tools for creating, visualizing, and analyzing GIS data in a three-dimensional (3D) context.
Masspoints	A dataset containing points that are typically derived from data sources such as lidar, sonar, and photogrammetric data.
Multipoint	A multipoint feature class stores many point records in one database row.
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. This reduces the number of measurements needed to represent a surface for a given area. Two pyramid types exist: z-tolerance and window size.
Z-tolerance pyramid	The z-tolerance pyramid type thins points based on vertical accuracy constraints.
Window size pyramid	The window size pyramid type thins points based on horizontal sample density.
Breakline	A line in a TIN or terrain dataset that represents a distinct interruption in the slope of a surface, such as a ridge, road, or stream. No triangle in a TIN or terrain dataset may cross a breakline (in other words, 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. They're needed when a data area has an irregular shape. Without a clip polygon, the data area would be convex, producing errors in the surface. The triangulation would want to create long sliver triangles to connect the points around the exterior, generating errors in the surface.
Face	In a TIN or terrain dataset, the 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.
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.
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 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.
Point spacing	The most common distance between points used to construct the terrain dataset.
Group	Groups are used to define multiple levels of detail for line and polygon features that contribute to a terrain dataset.
Anchor point	Anchor points remain throughout all pyramid levels of a terrain dataset. They are never filtered or thinned away.
Overview terrain	The overview terrain is the coarsest representation of the terrain dataset and is intended for fast drawing when zoomed at or beyond full extent.
Feature dataset	A container for datasets that share the same spatial reference; that is, they share a coordinate system, and their features fall within a common geographic area. Feature classes with different geometry types can be stored in a feature dataset.
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.
LAS format	LAS 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.
Sonar	Sonar (sound navigation and ranging) is an underwater mapping technique that uses sound waves to map out 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	When adding a feature class to a terrain, you need to indicate its surface feature type. This defines the role the feature class plays in defining the terrain dataset surface. There are mass points, breaklines, and several polygon types.