Accuracy—The closeness of an estimated value (e.g., measured or computed) to a standard or accepted (true) value of a particular quantity. The true values of locations and elevations, relative to established datums, are rarely, if ever, known. All spatial coordinates are computed measurements; therefore accuracy itself can only be estimated, never known absolutely. The quantification of error and the language of accuracy assessment rely heavily on principles of statistics and probability.
• Absolute Accuracy—The value expressed in feet or meters that reports the uncertainty in vertical or horizontal positions due to systematic and random errors in measurements of the location of any point on a geospatial dataset relative to the defined vertical or horizontal datum at a stated confidence level. The absolute vertical accuracy is normally different than the absolute horizontal accuracy.
• Accuracyr—The NSSDA reporting standard in the horizontal component that equals the radius of a circle of uncertainty, such that the true or theoretical horizontal location of the point falls within that circle 95 percent of the time. Accuracyr = 1.7308 × RMSEr.
• Accuracyz—The NSSDA reporting standard in the vertical component that equals the linear uncertainty value, such that the true or theoretical vertical location of the point falls within that linear uncertainty value 95 percent of the time. Accuracyz = 1.9600 × RMSEz.
• Horizontal Accuracy—The positional accuracy of a dataset with respect to a horizontal datum. The horizontal accuracy reporting standard (Accuracyr) is defined above.
• Positional Accuracy—The accuracy of the position of features, including horizontal and/or vertical positions.
• Relative Accuracy—The value expressed in feet or meters that reports the uncertainty in vertical or horizontal positions due to random errors in measurements in the location of any point on a geospatial dataset relative to any other point on the same dataset at the 95 percent confidence level. Relative accuracy may also be referred to as point-to-point accuracy. The general measure of relative accuracy is an evaluation of the random errors (systematic errors and blunders removed) in
Portions of this glossary reprinted from Appendix B of Maune, 2007, with permission from the American Society for Photogrammetry and Remote Sensing.
determining the positional orientation (e.g., distance, azimuth) of one point or feature with respect to another.
• Vertical Accuracy—The measure of the positional accuracy of a dataset with respect to a specified vertical datum. The vertical accuracy reporting standard (Accuracyz) is defined above.
Adjustment—The process of changing the values of a given set of quantities so that results calculated using the changed set will be better than those calculated using the original set. The concept of “better” is vague. The most common interpretation is that the sum of the squares of differences between results obtained by measurement and results obtained by calculation shall be a minimum. With this criterion, the method of least squares is the required process.
Aerial Triangulation (Aerotriangulation)—The process of measuring a number of points on overlapping images and/or ground control points to determine the most probable values of exterior orientation elements of aerial photographs. The output of this process includes ground space coordinates for all points measured on at least two images.
Arc-Second (or Second of Arc)—1/60 of a minute of arc, or 1/3,600 of a degree.
Attitude—The position of a body defined by the angles between the axes of the coordinate system of the body and the axes of an external coordinate system. In photogrammetry, the attitude is the angular orientation of a camera (roll, pitch, yaw), or of the photograph taken with that camera, with respect to some external reference system. With lidar (light detection and ranging) and IFSAR (interferometric synthetic aperture radar), the attitude is normally defined as the roll, pitch, and heading of the instrument at the instant an active pulse is emitted from the sensor.
Autocorrelation—A method for self-comparison of a string or sequence of numeric data.2
Bankline—A break in land surface slope adjacent to a stream that separates the steeper slope of the stream channel bank normally containing the flow from the flatter slope of the adjacent land area that is inundated only during floods.
Bare-Earth—Digital elevation data of the terrain, free from vegetation, buildings, and other man-made structures. Elevations of the ground.
Base Flood—A flood that has a 1-percent chance of being equaled or exceeded in any given year. Often called the 100-year flood.
Base Flood Elevation (BFE)—An elevation that has a 1-percent chance of being equaled or exceeded in any given year by a base flood.
Bathymetry—The measurement and study of water depths. Traditionally bathymetry has been expressed with contours and hydrography with spot depths. Bathymetry may not meet hydrographic standards, because it may not show all of the bottom characteristics important to the mariner who is navigating.
Bench Mark (Benchmark or BM)—A relatively permanent, natural or artificial, material object bearing a marked point whose elevation above or below an adopted vertical datum is known.
• Tidal Bench Mark—A bench mark whose elevation has been determined with respect to mean sea level at a nearby tide gauge. The tidal bench mark is used as reference for that tide gauge.
Breakline—Linear data structure that represents a distinct or abrupt change in the terrain. Breaklines comprise a series of vertices with z-values (elevations) attached.
Calibration—The process of identifying and correcting for systematic errors in hardware, software, or procedures; determining the systematic errors in a measuring device by comparing its measurements with the markings or measurements of a device that is considered correct. Airborne sensors can be calibrated geometrically and/or radiometrically.
• Camera Calibration—The geometric calibration of a conventional film mapping camera includes determination of the following quantities: (1) the calibrated focal length, (2) the location of the principal point with respect to the fiducial marks, (3) the location of the point of symmetry, (4) the distortion effective in the focal plane of the camera and referred to the particular calibrated focal length, (5) the resolution of the lens system, (6) the degree of flatness of the focal plane, (7) the opening and closing cycle of the shutter as a function of time, and (8) the locations of fiducial marks—all of which help to ensure that micrometer measurements made on aerial film will translate correctly into accurate ground coordinates via photogrammetric calculations. Depending on camera design, digital cameras have different forms of geometric calibration and also include radiometric calibration (e.g., spectral response of charge-coupled device [CCD] sensors over the spectral range of the sensor) and determination of the pixel-to-pixel uniformity.
• Lidar System Calibration—Factory calibration includes both radiometric and geometric calibration unique to each manufacturer’s hardware and tuned to meet the performance specifications for the model being calibrated; factory recalibration is normally performed every 24 months. The “lever-arm” calibration determines the sensor-to-GPS (Global Positioning System) antenna offset vector (lever arm) components relative to the antenna phase center; the offset vector components are
redetermined each time the sensor or aircraft GPS antenna is moved or repositioned in any way. Field calibration is normally performed for each project, or even daily, to determine corrections to the roll, pitch, and scale calibration parameters.
Check Point (Checkpoint)—One of the surveyed points in the sample used to estimate the positional accuracy of the dataset against an independent source of higher accuracy.
Confidence Level—The probability that errors are within a range of given values.
Contour Interval—The difference in elevation (z-values) between two adjacent contours.
Contours—Lines of equal elevation on a surface. An imaginary line on the ground, all points of which are at the same elevation above or below a specified reference surface (vertical datum).
Control Point—Stationary point with accurately surveyed horizontal (x,y), vertical (z), or horizontal and vertical (x,y,z) coordinates. When used for aerotriangulation, control points either are chosen as photo-identifiable points or are “paneled” prior to flying.
Coordinates—A set of N numbers designating the location of a point in N-dimensional space. Horizontal coordinates are two-dimensional coordinates, normally expressed as x,y coordinates, eastings and northings, or longitude and latitude (geographic coordinates). A vertical coordinate may be one-dimensional (i.e., the vertical distance of a point above or below a reference surface [vertical datum] such as the elevation of a bench mark without known x,y coordinates). However, most vertical coordinates are specified as three-dimensional coordinates (i.e., x,y coordinates and z-values).
Correlation—The extent to which one randomly varying quantity can be expressed as a function of another or to which both quantities can be expressed as function of a third, nonrandom quantity. See also Image Correlation. With IFSAR, interferometric correlation is a measure of the similarity of the signal received at the two antennae.
Data Model—The conceptual view of what information is to be represented. For example, the surface of the earth can be represented as a grid of posts of varying heights (i.e., raster data model) or as lines of equal elevation (i.e., vector data model).
Datum—Any quantity or set of such quantities that may serve as a basis for calculation of other quantities. See Table 2.3 for a listing of 26 different vertical datums included in the National Geodetic Survey (NGS) Vertical Datum Transformation Tool (Vdatum).
• Geodetic Datum—A set of constants specifying the coordinate system used for geodetic control (i.e., for calculating coordinates of points on the earth). At least eight constants are needed to form a complete datum: three to specify the location
of the origin of the coordinate system, three to specify the orientation of the coordinate system, and two to specify the dimensions of the reference ellipsoid.
• Horizontal Datum—A geodetic datum specifying the coordinate system in which horizontal control points are located. The North American Datum of 1983 (NAD83) is the official horizontal datum in the United States.
• Hydrographic Datum—A datum for depths (soundings), depth contours, and elevations of foreshore and offshore features; also called chart datum.
• Mean Sea Level (MSL)—A tidal datum computed as the arithmetic mean of hourly heights observed over a specific 19-year Metonic cycle. Shorter series are specified in the name (e.g., monthly mean sea level, yearly mean sea level).
• Vertical Datum—A set of constants defining a height (elevation) system containing a coordinate system and points that have been consistently determined by observations, corrections, and computations. The North American Vertical Datum of 1988 (NAVD88) is the official vertical datum in the United States.
Decorrelation (IFSAR)—A measure of the dissimilarity of a signal received at two antennae caused by geometry, thermal noise, and other factors.
Digital Elevation Model (DEM)—A model containing elevations at points arranged in a raster data structure—a regularly-spaced x,y grid, where the intervals of ∆x and ∆y are normally in linear units (feet or meters) or geographic units (degrees or fractions of a degree of latitude or longitude).
Digital Orthophoto—A digital photograph prepared from a perspective photograph by digitally removing displacements of points caused by tilt, relief, and perspective. A “true digital orthophoto” is defined as one in which the sides of vertical features are not visible, as though looking straight down on each natural and man-made feature from infinity; this feature is especially desired in urban areas with skyscrapers and tall buildings, the sides of which are normally photographed with aerial photography.
Digital Surface Model (DSM)—A model that includes features above the ground such as buildings and vegetation. It is used to distinguish a bare-earth elevation model from a non-bare-earth elevation model. The term DSM is generally applied regardless of whether the data are in gridded or mass point format.
Digital Terrain Model (DTM)—A data structure made up of x,y points with z-values representing elevations. Unlike the DEM, these may be irregularly or randomly spaced mass points. Direct observations of elevation at a particular location can be incorporated without interpolation, and the density of points can be adjusted so as to best characterize the actual terrain. Fewer points can describe very flat or evenly sloping ground; more points can be captured to describe very complicated terrain. A DTM is often more expensive and
time consuming to collect than a DEM but is considered technically superior for most engineering analyses because it retains natural features of the terrain.
Direct Georeferencing—The direct measurement of the position (x,y,z coordinates) of the camera focal point and the angular orientation (roll, pitch, heading) at the instant an aerial photograph is taken, to aid or replace aerial triangulation. The term is also applicable to the position and orientation of airborne lidar or IFSAR sensors.
Drape—The superimposition of two-dimensional features over a three-dimensional surface, normally for viewing of all features in three-dimensional perspective, for three-dimensional fly-throughs or walk-throughs in virtual reality.
Elevation—The distance measured upward along a plumb line between a point and the geoid. The elevation of a point is normally the same as its orthometric height, defined as H in the equation: H = h − N.
Ellipsoid—A spheroid that has been slightly flattened at the north and south poles.
Ellipsoid Height—See Height.
Error—The difference between the observed value of a quantity and the theoretical or defined value of that quantity. In the computation of root mean square errors (RMSEs), x, y, and z errors are the differences in x or y coordinates or z-values between a sample dataset and a dataset of higher accuracy for the same sample points.
• Random Error—An error produced by irregular causes whose effects upon individual observations are governed by no known law that connects them with circumstances and so cannot be corrected by the use of standardized adjustments.
• Systematic Error—An error whose algebraic sign and, to some extent, magnitude bear a fixed relation to some condition or set of conditions. Systematic errors follow some fixed pattern and are introduced by data collection procedures and systems. Systematic error artifacts include vertical elevation shifts; misinterpretations of terrain surfaces due to trees, buildings, and shadows; fictitious ridges, tops, and benches; and striations. A systematic error is, in theory at least, predictable and therefore is not random; such errors are regular and so can be determined a priori. They are generally eliminated from a set of observations prior to RMSE calculations and before applying the method of least squares to eliminate or reduce random errors.
Field of View—The angular extent of the portion of object space surveyed by an aerial camera or lidar sensor, measured in degrees.
Flood Insurance Rate Map (FIRM)—An official map of a community on which the Federal Emergency Management Agency (FEMA) has delineated both the flood hazard areas and the risk premium zones applicable to the community.
Floodplain—The low-lying area along a river, stream, or coast that is subject to flooding; any land area susceptible to being inundated by water from any source.
Floodway—The channel and portion of the adjoining area required to discharge the base flood without increasing flood heights more than 1 foot.
Footprint—Different definitions in geospatial community, depending on usage:
• Building Footprint—The outline of a building, normally as viewed orthogonally from above.
• Footprint (general usage)—The beam size or surface area measured by a single beam from an active sensor such as IFSAR, lidar, or sonar.
• Lidar Horizontal Footprint—The area illuminated by a laser beam on the face of a horizontal surface, typically based on the full width at half maximum (FWHM) points of the beam or alternative criteria such as 1/e or 1/e2 of the maximum irradiance or amplitude. These different definitions are used because a lidar beam diverges or spreads, does not have a constant spatial energy distribution, and decays similarly to a Gaussian distribution away from the center of a beam.
• Lidar Vertical Footprint—The area illuminated by a laser pulse (beam) on the face of a vertical surface, based on the FWHM points of the beam. This term is used only when the lidar sensor is tilted into a forward-looking position so that consecutive scan lines “walk up” a vertical surface.
Foreshortening—A phenomenon that occurs when slopes facing toward the radar will be imaged at nearly the same time with very similar ranges, depending on the relative angle of incidence of the radar beam. These sloping features appear closer together in planimetric view, compressed or bunched, compared to their actual position; they also appear bright due to strong backscatter. Slopes facing away conversely are dark and expanded or stretched compared to their actual positions.
Ground Sample Distance (GSD)—The size of a pixel projected to the ground surface, as reported in linear units per pixel.
Height—The distance, measured along a perpendicular, between a point and a reference surface (e.g., height of an airplane above the ground surface). The distance, measured upward along a plumb line (line of force), between a point and a reference surface of constant geopotential. Elevation is preferred if the reference surface is the geoid. Height systems are called by different names depending on the geopotential number (C) and gravity (G) selected. When G is computed using the Helmert height reduction formula, which is what was used in NAVD88, the heights are called Helmert orthometric heights. When G is computed using the international formula for normal gravity, the heights are called normal orthometric heights. When G is equal to normal gravity at 45 degrees latitude, the heights
are called normal dynamic heights, which is what was used in IGLD85 (International Great Lakes Datum of 1985).
• Ellipsoid Height—The height above or below the reference ellipsoid (i.e., the distance between a point on the earth’s surface and the ellipsoidal surface, as measured along the normal [perpendicular] to the ellipsoid at the point and taken positive upward from the ellipsoid). Defined as h in the equation: h = H + N; same as ellipsoidal height and geodetic height.
• Orthometric Height (Elevation)—What most people think of as height above mean sea level. The height above the geoid as measured along the plumbline between the geoid and a point on the earth’s surface, taken positive upward from the geoid. Defined as H in the equation: H = h − N. The difference between adjusted orthometric heights is computed using a normal gravity formula. The orthometric height (H) and the geopotential number (C) are related through the following equation: C = G * H, where G is the gravity value estimated for a particular system.
Infrared—The portion of the invisible spectrum consisting of electromagnetic radiation with wavelengths in the range from 750 nanometers to 1 millimeter.
Interferometer—An instrument that measures differences between the phases of two electromagnetic signals originating from a common source that have traversed different paths. The phase differences are measured by combining the two signals. The amplitude of the combined signal is a function of the phase difference between the two signals. The phenomenon of fluctuations in the amplitude of the combined signals in response to phase changes in the input signals is sometimes referred to as interference.
Interferometric Synthetic Aperture Radar (IFSAR)—An airborne or spaceborne interferometer radar system, flown aboard rotary or fixed-wing aircraft or space-based platforms, used to acquire three-dimensional coordinates of terrain and terrain features that are both man-made and naturally occurring. IFSAR systems form synthetic aperture images of terrain surfaces from two spatially separated antennae over an imaged swath that may be located to the left, right, or both sides of the imaging platform.
Interpolation—The estimation of z-values at a point with x,y coordinates, based on the known z-values of surrounding points.
Layover—An extreme case of foreshortening that occurs when the slope of the terrain is greater than the angle of incidence of the radar beam. The top of the object is imaged before the bottom, and the feature appears inverted or laid over in the image. Layover effects preclude useful determination of elevation.
Leveling—The process of finding differences of elevation.
Lidar—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. The word “lidar” (lowercase letters) is the appropriate form because it is directly analogous to radar and to a lesser extent sonar.
Map—A representation, usually on a plane surface and at an established scale, of the physical features (natural, artificial, or both) of a part or the whole of the earth’s surface. Features are identified by means of signs and symbols, and geographical orientation is indicated.
• Planimetric Map—A map that shows only the horizontal positions of the features represented. It does not show relief (elevations) in measurable form.
• Topographic Map—A map showing the horizontal and vertical locations of natural and artificial features. It is distinguished from a planimetric map by the presence of numbered contour lines or comparable symbols to indicate elevations of mountains, valleys, and plains; in the case of hydrographic charts, symbols and numbers are used to show depths in bodies of water.
Map Projection—A function relating three-dimensional coordinates of points on a curved surface (usually an ellipsoid or sphere) to two-dimensional coordinates of points on a plane map.
Mass Points—Irregularly spaced points, each with x,y coordinates and z-value, typically (but not always) used to form a triangulated irregular network (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 automatically (e.g., by lidar or IFSAR scanners), mass point spacing and pattern depend on the characteristics of the technologies used to acquire the data.
Mean (Arithmetic Mean)—The average of all numbers in a dataset.
Mean Sea Level—The average location of the interface between ocean and atmosphere, over a period of time sufficiently long so that all random and periodic variations of short duration average to zero. The U.S. National Ocean Service has set 19 years as the period suitable for measurement of mean sea level at tide gauges.
Model—A copy of a physical object such as the earth, normally at reduced scale.
• Mathematical Model—Mathematical reconstruction of a physical object such as the earth, normally for computer display and analyses.
Orientation—The rotation or set of rotations needed to make the axes of one rectangular Cartesian coordinate system parallel to the axes of another.
Orthometric Height—See Height.
Orthophoto—A photograph prepared from a perspective photograph by removing displacements of points caused by tilt, relief, and perspective.
Orthorectification—A process through which an aerial photo can be resampled into a scale-constant image map, in which effects of tilt and relief displacement are removed by using the orientation information derived from aerotriangulation and an elevation model representing the terrain.
Parallax—The apparent shift of an object against a background due to a change in the observer’s position.
Parallel—A line that has the same latitude at every point.
Peak—A point around which all slopes are negative.
Photogrammetry—The science of deducing the physical three-dimensional measurements of objects from measurements on stereo photographs that photograph an area from two different perspectives.
Pixel—A two-dimensional raster cell, normally used for computer display of imagery or coded feature data.
Position—The location of a point on the surface of the earth, expressed in terms of one of several coordinate systems. Examples are geographic position (latitude, longitude, and altitude), universal transverse mercator (UTM) northing, easting and height, or State Plane northing, easting, and height.
Post spacing—The ground distance interval of cells in a uniform elevation grid.
Precision—A statistical measure of the tendency for independent, repeated measurements of a value to produce the same result.
Profile—The side view of a cross section of a terrain surface. In U.S. Geological Survey (USGS) DEMs, profiles are the basic building blocks of an elevation grid and are defined as one-dimensional arrays (i.e., arrays of n columns by 1 row, where n is the length of the profile).
Projection—A function relating points on one surface to points on another so that every point on the first surface corresponds exactly to one point on the second surface. A map projection is a special case in requiring that one of the surfaces be a spheroid or ellipsoid and the other be a developable surface (normally a plane, cylinder, or cone that can be “cut” and flattened into a plane).
Pulse Energy—The total energy content of a laser pulse measured in microjoules (μJ).
Quadrangle (Quad)—A map or plot of a rectangular or nearly rectangular area usually bounded by given meridians of longitude and parallels of latitude.
Quality Assurance (QA)—Steps taken (1) to ensure that the government receives the quality products it pays for and/or (2) to ensure that an organization’s quality program works effectively. Quality programs include quality control procedures for specific products as well as overall quality plans that typically mandate an organization’s communication procedures, document and data control procedures, quality audit procedures, and training programs necessary for delivery of quality products and services.
Quality Control (QC)—Steps taken by data producers to ensure delivery of products that satisfy standards, guidelines, and specifications identified in the scope of work. These steps typically include production flow charts with built-in procedures to ensure quality at each step of the work flow, in-process quality reviews, and/or final quality inspections prior to delivery of products to a client.
Radar—Radio detection and ranging. An instrument for determining the distance and direction to an object by measuring the time needed for radio signals to travel from the instrument to the object and back, and by measuring the angle through which the instrument’s antenna has traveled.
Radar, Synthetic Aperture—A radar containing a moving or scanning antenna; the signals received are combined to produce a signal equivalent to that which would have been received by a larger, stationary antenna.
Range, IFSAR—The distance in a direction perpendicular to the flight path (cross-path direction) imaging the terrain below. Range or cross-track resolution is achieved by finely gating the received echo in time.
Rectification—The process of producing, from a tilted or oblique photograph, a photograph from which displacement caused by tilt has been removed. Orthorectification, in addition to correcting tilt displacement, also corrects for perspective and relief displacement.
Relative Accuracy—An evaluation of the amount of error in determining the location of one point or feature with respect to another; see also Accuracy.
Relief—Topography. The deviation of a surface, or portions thereof, from some surface such as a reference ellipsoid.
Relief Displacement—The displacement of an image outward from the center of an aerial photograph, caused by the elevation (relief) of features above an established base elevation.
Resolution—In the context of gridded elevation data, resolution is synonymous with the horizontal post spacing. Sometimes used to state the number of points in x and y directions in a lattice (e.g., 1,201 * 1,201 mesh points in a USGS one-degree DEM).
• Radiometric Resolution—The ability of a sensor to detect differences in energy magnitude. Sensors with low radiometric resolution are able to detect only relatively large differences in the amount of energy received; sensors with high radiometric resolution are able to detect relatively small differences.
• Spatial Resolution—A measure of the finest detail distinguishable in an image.3
• Temporal Resolution—The frequency at which data are captured for a specific place on the earth. The more frequently they are captured, the better or finer is the temporal resolution said to be. Temporal resolution is relevant when using imagery or elevations datasets captured successively over time to detect changes to the landscape.
Root Mean Square Error (RMSE)—The square root of the average of the set of squared differences between dataset coordinate values and coordinate values from an independent source of higher accuracy for identical points. The vertical RMSE (RMSEz), for example, is calculated as the square root of
Zn = the set of N z-values (elevations) being evaluated, normally interpolated (for TINs and DEMs) from dataset elevations of points surrounding the x,y coordinates of checkpoints;
Z'n = the corresponding set of checkpoint elevations for the points being evaluated;
N = the number of checkpoints; and
n = the identification number of each of the checkpoints from 1 through N.
Scale (Map)—A number, constant for a given map, that is representative of the ratios of small distances on the map to the corresponding actual distances. Map scale is normally presented as a fraction expressed as, for example, 1/50,000 or 1:50,000. Because 1/50,000 of something is smaller than 1:20,000 of something, a 1:50,000-scale maps is considered to be a smaller-scale map than a 1:20,000-scale map.
Scan Rate—The number of times per second a scanning device samples its field of view, measured in hertz.
Sea Level—In general, the reference elevation of the surface of the sea from which elevations are measured. This term is used as a curtailed form of “mean sea level.”
Shadow—Area produced when a radar beam is blocked from reaching parts of the terrain obscured by other objects. These areas appear in the image as dark or void. Elevation values cannot be determined.
Shoreline—The boundary line between a body of water and the land, in particular, the boundary line between the water and the line marking the extent of high water or mean high water. (Mean high water is a tidal datum computed as the arithmetic mean of the high-water heights observed over a specific 19-year Metonic cycle. For stations with shorter series, a comparison of simultaneous observations is made with a primary control tide station in order to derive the equivalent of the 19-year value.)
Side Lap—The overlap between adjoining swaths of lidar data or adjoining strips of aerial photography.
Slope—The measure of change in elevation over distance, expressed either in degrees or as a percent. For example, a rise of 4 meters over a distance of 100 meters describes a 2.3-degree or 4 percent slope; the maximum rate of change in elevation, either from cell to cell in a gridded surface or of a triangle in a TIN. Every cell in a DEM or triangle in a TIN has a slope value; the lower the slope value, the flatter is the terrain; the higher the slope value, the steeper is the terrain.
Soft Copy Photogrammetry—Stereo photogrammetric procedures that utilize digital imagery in digital stereo photogrammetric workstations (DSPWs)—also called soft copy workstations—that have significant advantages compared to analytical stereoplotters. These advantages include automatic digital image correlation, efficient production of DEMs and digital orthophotos, and superposition of all types of geospatial data over digital imagery. For DEM generation, superimposition means that all elevation mass points, breaklines, and contours can be reviewed in stereo against the actual ground form, and old three-dimensional data can be superimposed on new stereo models to see where DEMs, breaklines, or contours need to be revised.
Spectral Resolution—A description of the way an optical sensor responds to various wavelengths of light. High spectral resolution means that the sensor distinguishes between very narrow bands of wavelength; low spectral resolution means that the sensor records the energy in a wide band of wavelengths as a single measurement.
Standard (1)—An agreed-upon procedure in a particular industry or profession that is to be followed in producing a particular product or result.
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.