National Digital Elevation Program Organization

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Digital Elevation Glossary of Terms
Derived from NDEP Guidelines for Digital Elevation Data
Part 4 Glossary of Terms and other USGS sources

A-D

Bounding Rectangle
The smallest rectangle that surrounds a bounded region and contains every point in the region. The sides of a bounding rectangle are parallel to the coordinate axes. Used in this context to define the geographic extent of a project.

Data Model--Mass Points/Variable Spacing
Irregularly spaced points, each with an x/y location and a z-value, used to form a TIN. When generated manually, mass points are ideally chosen to depict the most significant variations in the slope or aspect of TIN triangles. However, when generated by automated methods. For example, by LIDAR or IFSAR scanners, mass point spacing and pattern depend on characteristics of the technologies used to acquire the data. Mass points are most often used to make a TIN, but not always. They can be used as XYZ point data for interpolation of a grid without an intermediate TIN stage.

Data Model--Mass Points/Breaklines
Linear features that describe a change in the smoothness or continuity of the surface. The two most common forms of breaklines are as follows:

Data Model--Soft Breaklines
Ensure that known z-values along a linear feature are maintained, and they ensure that linear features and polygon edges are maintained in a TIN surface model, as described below, by enforcing the breakline as TIN edges, but they do not define interruptions in surface smoothness. Soft breaklines are generally synonymous with 3-D breaklines because they are depicted with series of x/y/z coordinates.

Data Model--Hard Breaklines
Define interruptions in surface smoothness. They are used to define streams, shorelines, dams, ridges, building footprints, and other locations with abrupt surface changes. Although some hard breaklines are 3-D breaklines, they are often depicted as 2-D breaklines because features such as shorelines and building footprints are normally depicted with series of x/y coordinates only.

Data Model--Multi-Res Grid
Begin by reading .Regular Grid., below) Grid resolution . the width and height of the cells . determines the precision of the grid representation. The size of a grid cell depends upon the data resolution required for the most detailed analysis and level of detail of the input data used to create the grid. The cell must be small enough to capture the required detail, but large enough so that computer storage and analysis can be performed efficiently. Multi-Res gridding is a technique that accomplishes efficiency and delivers required detail in a given geographic extent by having more than one resolution (width and height of each cell). For example, the drainage basin on a particular quadrangle is gridded at 1-meter resolution, while the remainder of the quadrangle extent is gridded at 10-meters.

Data Model--Regular Grid
A grid is a rectangular array of cells, each of which stores an elevation value. In the case of a regular grid, each cell stores the elevation value for the centroid of the cell. The cell is a uniform unit that represents a defined area of the earth, such as a square meter or square yard. For most terrain models, grid cells are usually square, having the same height and width, and organized in rows and columns.

Data Model--TIN Contours
Contours made from a Triangulated Irregular Network (TIN) . A TIN is set of adjacent, nonoverlapping triangles computed from irregularly spaced points with x/y coordinates and z-values. The TIN data structure is based on irregularly spaced point, line, and polygon data interpreted as mass points and breaklines. The TIN model stores the topological relationship between triangles and their adjacent neighbors. The TIN data structure allows for the efficient generation of surface models for the analysis and display of terrain and other types of surfaces. A TIN surface can be created from one or more of the following sources: point, line and polygon data; contour maps; stereoplotter data; LIDAR, IFSAR, or Sonar data; randomly distributed points in ASCII files; breakline data; and DEM lattices. TINs usually require fewer data points than DEMs or DTMs, while capturing critical points that define terrain discontinuities and are topologically encoded so that adjacency and proximity analyses can be performed. TINs have several other advantages over DEMs and DTMs; but they are probably best known for their superiority in surface modeling, For example, calculation of slope, aspect, surface area and length; volumetric and cut-fill analysis; generation of contours; interpolation of surface z-values; generation of profiles over multiple surfaces; intervisibility analysis; and 3-D visualization, simulation, and fly-throughs.

E-R

Ellipsoid
A closed surface whose planar sections are either ellipses or circles. The earth's ellipsoid is a biaxial ellipsoid of revolution (defined by its major axis "a" and its minor axis "b") obtained by rotating an ellipse about its minor (shorter) axis.

Geoid
The equipotential (level) surface of the earth's gravity field that, on average, coincides with the mean sea level in the open undisturbed ocean. In practical terms, the geoid is the imaginary surface where the oceans would seek mean sea level if allowed to continue into all land areas so as to encircle the earth. The geoid undulates up and down with local variations in the mass and density of the earth. The local direction of gravity is always perpendicular to the geoid.

Horizontal Datum
A geodetic datum specifying the coordinate system in which horizontal control points are located.

Nominal Post Spacing (Horizontal Resolution)
The smallest distance between two discrete points that can be explicitly represented in a gridded elevation dataset. It is important to note that features of a size equal to, or even greater than the post spacing, may not be detected or explicitly represented in a gridded model. For gridded elevation data the horizontal post spacing may be referenced as the cell size, the grid spacing, the posting interval, or the ground sample distance. Horizontal post spacing should be documented in the metadata file.

S-Z

Surface Description--Bare Earth Surface
Digital elevation data of the terrain, free from vegetation, buildings, and other man-made structures. Elevations of the ground.

Surface Description--Bathymetric Surface
The underwater terrain model.

Surface Description--Constant Elevation

Surface Description--Multiple Returns
Light Detection and Ranging (LIDAR) uses an instrument that measures distance to a reflecting object by emitting timed pulses of light and measuring the time between emission and reception of reflected pulses. The measured time interval is converted to distance. Each single pulse of laser light may generate multiple returns (currently up to five). As a light beam travels to the ground, it may have distended to a foot in diameter by the time it reaches any objects. If part of the beam hits the roof edge of a house and the rest of the beam travels to the ground, then a sensor will record the reflection from the roof edge (first return) and the reflection from the ground (last return). First-return data is excellent for recording tree tops or buildings, while last return data is the most desirable for bare-earth surface modeling.

Surface Description--Reflective Surface
LIDAR data processing software can categorize detected laser pulses according to the reflectivity of the target surface. The output has the detail of high-resolution photographs, yet images can be taken at night and the data are already in digital form. Highly reflective materials yield a strong (intense) return signal, while the converse is also true. Factors affecting reflectivity include elevation, composition, density, and orientation to the sensor. Examples of low and high reflectivity:

High Reflectivity: Light surfaces

Water (wavy conditions)

Low Reflectivity: Dark surfaces

Coal, iron oxide

Wet surfaces, mud

Still water Vertical Accuracy (RMSE)
Measure of the positional accuracy of a dataset with respect to a specified vertical datum.

Vertical Datum
A set of fundamental elevations to which other elevations are referred.

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