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Glossary of GIS Terminology Part 2

Contour - (noun) An imaginary line on the ground, all points of which are at the same elevation above or below a specific datum surface, usually mean sea level.
 
Contour map - A topographic map that uses contour lines to portray relief. Contour lines join points of equal elevation.
 
Contrast - The difference between bright and dark values in the display or printout of a continuous tone (usually grayscale) image. The stronger the contrast, the more difference between the brightest and darkest values. Most images benefit from a process of contrast enhancement which artificially increases the contrast. Many images look better when they are produced with the Normalized contrast model, which assigns intensity increments according to a normal distribution curve.
 
Control point - Points and/or cells which are used to establish map coordinate control for un-calibrated objects. In the manual mosaic process, a control point is a feature in a piece of the mosaic (such as a road intersection) for which the map coordinates are known. In the raster-to-vector calibration process, a control point is a feature that is co-located between the un-calibrated raster object, and the calibrated vector object overlay. A control point may be something like a bend in a river or road intersection that shows on both a raster object and an overlying vector object.
 
Convolution - Mathematically determining the data value for a new cell in an m x n neighborhood of cells. Raster filtering, resampling, and other raster processes use convolution. Convolution processes should never be applied to raster objects that contain categorical data. Convolution is only appropriate for continuous data.
 
Coordinate - A value measured along an axis of a coordinate system.
 
Coordinate filtering - A process of weeding out superfluous coordinates. It is used to generalize maps and to reduce requirements for computer storage.
 
Coordinate pair - Set of Cartesian coordinates describing the location of a point, line or area (polygon) feature in relation to the common coordinate system of the database.
 
Coordinate systems - A particular kind of reference frame or system, such as plane rectangular coordinates or spherical coordinates, which use linear or angular quantities to designate the position of points within that particular reference frame or system.
 
Coordinate transformation - Conversion of coordinates from one system to another system.
 
Co-registration - The condition in which associated raster and vector objects overlay each other with correct orientation and geometry so that corresponding internal features align.
 
Corridors generation - To generate corridors of given width around existing points, lines, or areas.
 
CPS code - (GPS) A sequence of 1023 pseudo-random, binary, C modulations on the GPS carrier at a chip rate of 1.023 MHz Also known as the civilian code.
 
Cross-hatching - The technique of shading areas on a map with a given pattern of lines or symbols.
 
Cubic convolution or cubic interpolation - A computationally intense type of convolution used in raster resampling which determines a new cell value by fitting a cubic polynomial surface to a 4 x 4 neighborhood of cells. The simpler and faster bilinear interpolation process normally produces results that are almost as good.
 
Culture - Features constructed by humans that are under, on, or above the ground and which are delineated on a map. These include roads, trails, buildings, canals, and sewer systems.
 
Cursor - A shape on a video display that indicates position. A cursor may appear as a blinking line on the text screen or as a crosshair on an image display screen. The cursor usually represents the point at which some action will take place.
 
Cursor coordinate - 1) The current coordinate where the cursor locates; 2) Addressable coordinate range of the cursor, which might or might not be the entire screen area.
 
Cursor hot spot - The pixel location on a cursor shape at which the cursor activity takes place. For example, the hot spot on an arrow cursor is at the point of the arrow, while the hot spot for a cross-hair cursor is at the intersection of the cross-hairs.
 
Data - Things known about real world entities; results of observations or measurements of such features. A single datum has three potential components: attribute information that describes the substance, characteristics, variables, values and similar qualities of the datum; geographical information that describes the position of the datum in space relative to other data; and temporal information that describes the instant or period for which the datum is valid.
 
Data coverage - The data set of one data type that covers the whole of the management jurisdiction concerned, e.g., the whole of a national forest.
 
Data Definition Language (DDL) - Repository of information about the definition, structure, and usage of data. It does not contain the actual data.
 
Data dictionary - (DD) Repository of information about the definition, structure and usage of data. It does not contain the actual data.
 
Data element - A specific item of information appearing in a set of data.
 
Data entry - A process of key entering tabular data into a structured file. This is typically accomplished by using a CRT terminal with a formatted screen.
 
Data fragmentation - In relational databases, data may be partitioned into row-and column subsets (called sub relations) of a given relation by means of projection and selection. This is necessary if logical data elements are replicated in physical storage. A sub relation is called a horizontal or a vertical fragmentation according as it is derived from the parent relation through selection or projection, respectively. The parent relation can be constructed from its sub relations by join or union operations.
 
Data input - Entering data into a computer; geographic data is generally entered into a GIS database via a digitizer or a scanner.
 
Data layer - Refers to data having similar characteristics being contained in the same plane or overlay (e.g., roads, rivers). Usually information contained in a data layer is related and is designed to be used with other layers.
 
Data mask - (Image processing) A processing barrier that only allows data values in a chosen range to pass. User might choose a data mask to block all values outside a selected color range, eliminating all image features except those of the color the user wants to use in a process. The mask blocks off the parts that the user wants to exclude from the current analysis.
 
Data type - (Map layer) The name of a data set based on the nature of the real world entities or conditions described by the data; e.g., forest stand data, soil type data, campsite location data, habitat type data, insect damage map, ownership boundaries, etc. When the data are represented and stored on maps the term map layer is synonymous with data type.
 
Datum - A mathematical description of a smooth surface that closely fits the mean sea-level surface for an area of interest. A datum is derived from a chosen ellipsoid, and provides the surface to which a cartographer refers ground control measurements. Maps of large extent must use consistent parameters for ellipsoid and datum to insure consistency between the map projection and ground control. (See also Ellipsoid).
 
Degree - A unit of measurement equal to 1/360 of a circle. A degree of latitude on the Earth’s surface is about 69 miles. A degree of longitude varies from about 69 miles at the equator to zero at the poles, but any point on the surface rotates through a degree of longitude in about 4 minutes of time.
 
DEM - Digital Elevation Model. A file with terrain elevations recorded for the intersection of a fine-grained grid and organized by quadrangle as the digital equivalent of the elevation data on a topographic base map. USGS geographic elevation data distributed in raster form on open reel magnetic tapes. There are 2 basic types - 1) The DMA type created by the Defense Mapping Agency in both a fixed cell size and a 3 x 3 arc second cell size distributed in 1 x 1 degree files. 2) A newer format for those 7.5’ USGS quadrangles that have been processed into 1 x 1 arc second elevation cells.
 
Demographic map - Any map that shows primarily political or social data such as political divisions, populations or occupations.
 
Denormalization - Normalization of the database may be regarded as optimizing for update at the cost of retrieval. A fully normalized database tends to require less processing on update but more on retrieval. To avoid this problem, denormalization can be performed so that retrieval processing becomes faster.
 
Depressionless elevation - (watershed analysis) A version of the elevation surface that has all depressions (puddles, ponds, and potholes) filled.
 
Derived map - A map created as the result of analyzing, altering or combining maps for a master database.
 
Dichotomous data - Data comprised of only one subject, which is either present or absent.
 
Differential positioning - (GPS) Precise measurement of the relative positions of two receivers tracking the same GPS signals.
 
Digital or Digital data - Information stored and processed with numerical digits, often in base 2. Digital information processing is constrained by the finite set of numbers a system uses, such that every data value is forced into its nearest representation. For example, a digital temperature sign at the local bank may have no way to deal with fractions of a degree: it can show 75 or 76, but not 75 and 2/3. At some point, every digital system faces the same kind of limit of accuracy. On the other hand, digital information is easy to copy, store, manipulate and reproduce dependably. (See also Analog).
 
Digitizer - A generic term for any device that converts an analog signal into digital form. Different kinds of digitizers work with input from video, audio, or field and laboratory measurements of temperature, pressure, voltage, radiation, etc. (See also Analog, Video digitizing board, X-Y digitizing tablet, and Scanner)
 
Digitizer accuracy - The maximum error in either axis between a points’s true coordinates and recorded coordinates.
 
Digitizing - Process of converting an analog image or map into a digital format usable by a computer.
 
DIME file - A geographic base file with Dual Independent Map Encoding, with the linear elements in a network coded to represent both their bounding by areal units and their cobounding by the points at which the linear links meet.
 
Dimensionality - An image contained in one color composite raster has a dimensionality of one. An image contained in a three-raster RGB set has a dimensionality of three. A LANDSAT TM image map for a single date will have a dimensionality of up to seven.
 
Distance districting - Process to measure distance and accumulate distance data through a network radiating out from a given point, associating these distance values to the point through which the measurement are taken. This procedure is used to define contour distances away from known points, and to determine districts which are created from a variety of such processes. It may be considered as proximal mapping on a network time/proximity/districting rather than distance basis.
 
Distortion elimination - The ability to remove various types of systematic distortion generated by different input methods.
 
Dither, Dithering - A method of printing that simulates continuous tone or full-color images by using patterns of discrete dots of black or cyan, magenta, yellow, and black. The dots are so small that the eye resolves them into a continuous-color image.
 
Dithered raster - A printer-ready 4-bit raster object in which each cell corresponds to one dot on the printer.
 
DLG - Digital Line Graph. A digital computer file containing lists of point coordinates describing boundaries, drainage lines, transportation routes and other linear features, organized by quadrangle as the digital equivalent of the linear hydrographic and cultural data on a topographic base map. A USGS map format usually used to distribute topographic maps in vector form.
 
DPI - Dots per inch. A measure of scanner, screen, and printing resolution. The more dots per inch, the more detail a device can process for a given area of page or display. On the other hand, the more dots per inch, the higher the demands on machine storage and processing (files get large and processing slows down).
 
Drift - In Kriging, drift is defined as a systematic increase or decrease in the value of the regionalized variable in a particular direction. Drift occurs at all scales, but it is important in universal kriging for the drift to be seen to be continuous and slowly varying at the scale of working so that it can be represented by an analytical expression. (See Kriging)
 
DTM - Digital Terrain Model. A land surface represented in digital form by an elevation grid or lists of three-dimensional coordinates. Elevation data in a 3 x 3 arc second grid form or a similar rectilinear form created by the Defense Mapping Agency.
 
DXF - A spatial data format developed by Autodesk, Inc. for its popular AutoCAD system. Several mapping programs that are based on AutoCAD also use the DXF (Drawing Exchange Format) format.
 
Easting - A rectangular (x, y) coordinate measurement of distance east from a north-south reference line, usually a meridian used as the axis of origin within a map zone or projection. False easting is an adjustment constant added to coordinate values to eliminate negative numbers.
 
Edge - A line linking two nodes in a graphic representation of a data structure; also called a side, or an arc in graph theory.
 
Edge matching - The comparison and graphic adjustment of features to obtain agreement along the edges of adjoining map sheets.
 
Electromagnetic spectrum - The entire spectrum, considered as a continuum, of all kinds of electromagnetic and visible radiation, from gamma rays having a wavelength of 0.001 angstrom to long waves having a wavelength of more than 1 million km. Remote sensing devices typically record electromagnetic bands in the region of optical light and may include the near infrared. (See also spectral band)
 
Elevation polygons - Areas of uniform or near-uniform elevation derived from a digital elevation model.
 
Ellipsoid - An ellipse rotated about its shorter axis. Since the earth is an irregular ellipsoid, and not a true sphere, many methods have been developed to describe its ellipsoidal deviations. Cartographers have a selection of ellipsoids from which to choose; most of which have best-fit properties for certain portions of the globe. In 1924, an International ellipsoid was defined which described the Earth ellipsoid as a flattening of 1 part in 297. Historically, ellipsoids were derived from careful surface measurements. More recently, satellite data has been used to construct ellipsoid models that relate coordinate measurements to the Earth’s center of mass.
 
EPPL7 - Enhancement of Environmental Planning and Programming Language. A raster-based GIS that operates on grid-cell files stored in a combination of raster and run-length formats. It is a PC-based GIS and was developed by the Land Management Information Center, Minnesota State Planning Agency.
 
ERDAS - Earth Resources Data Analysis System. A first-generation, raster-oriented microcomputer image processing and GIS system marketed by ERDAS, Inc.
 
Feature - A feature is a contiguous area (it may have holes in it) of homogeneous material, distinct from other areas of the same material. When two features of the same type touch they are considered a single larger feature. Features may represent anything the user chooses to isolate and identify, like ponds, lakes, agricultural fields, a biological entity in an NMR scan, or bones in a CAT scan. All of the instances of a named feature in an image form a class.
 
Feature mapping - A classification process for isolating, identifying, and classifying areas in multivariable images. Some GIS and image processing package can do most of the classification automatically for high-quality materials like satellite multispectral images. With more user interaction, the process may also successfully map features in images created from 35mm slides or even noisy air video.
 
Federal Information Processing Standards (FIPS) - Official source within the Federal Government for information processing standards. Particularly, FIPS 173 establishes standards for the formatting and exchange of spatial data. 
 
Fourier transform (Fourier analysis) - A type of two-way frequency transform for identifying and removing unwanted spatial frequency components in an image. The Fourier process is normally used to identify and remove systematic noise spikes, such as regular lines that may have been introduced by a faulty image collection device. The process works as a two-way operation. First the forward transform creates a pair of intermediate raster objects that reveal abnormal data variability. After the values in those raster objects have been edited to remove the data spikes, the inverse transformation creates a raster object with the same image as the original, but with lines or other noise removed.
 
Fuzzy C-means – It is a method of clustering which allows one point to belong to one or more clusters
 
Gap - The distance between two graphic entities (usually lines) on a digitized map. Gaps may arise through errors made while digitizing or scanning the lines on a map.
 
Geocode - A data value, assigned to a spatial object, which provides information on the geographic location of the object and is used as a key to access data relating to the object.
 
Geocoding - Translating geographic coordinates for map units, lines and points into (x,y) digits or grid cells.

Geodesy - Geodesy is the science of accurately measuring and understanding the Earth's geometric shape, orientation in space, and gravity field.

Geodetic Elevation Model of Planetary Relief - A method for modeling planetary relief based on a recursive tesselation of a regular octahedron (8 faces) or icosahedron (20 faces) into equilateral triangular facets. Horizontal coordinates are implicit in the hierarchy of nested triangles and only elevations are stored, using single bit flags to quantize height changes.
 
Geographic calibration - Bringing a raster or vector object into alignment with some geographic coordinate system. Raster and vector objects in a GIS file may have attached subobjects that contain the information needed to relate every raster cell or vector coordinate point to some geographic coordinate system (like a particular map projection). Geographic calibration may be established when creating an object (like extracting an image map from a LANDSAT or SPOT satellite image), or an image may be calibrated by entering control point lists or by associating it with some calibrated object (like overlaying a calibrated vector to calibrate a raster).
 
Geographic data - A geographic data set is a collection of data that are individually or collectively attached to geographic location. Spatial data is often used synonymously with geographic data.
 
Geographic direction - Direction measured relative to a north meridian, in contrast to direction based upon magnetic north or grid north. 
 
Geophysics - The branch of physics concerned with the forces that act in and on the Earth and with the changes caused by these forces.
 
Georeference system - An (x, y) or (x, y, z) coordinate system that locates points on the surface of the earth as a reference to points on a map. Systems include latitude-longitude, Universal Transverse Mercator, and State Plane Coordinates.
 
GeoVision - A GIS marketed by GeoVision Corporation. It has raster, vector, and quadtree data structure, and operates on UNIX operating systems. It can be linked to the Oracle relational database management system.
 
GFIS - Geographic Facilities Information System. A set of software that can be used to interactively create, manipulate, and maintain models of facilities or events that are geographically oriented. GFIS is developed by IBM.

GIS - Geographic Information System. A Geographic Information System is a computer system designed to allow users to collect, manage, and analyze large volumes of spatially referenced and associated attribute data. The major components of a GIS are: a user interface system; data base management capabilities; data base creation/data entry capacity; spatial data manipulation and analysis packages; and display/product generation functions.
 
GKS - Graphics Kernal System. A set of software primitives for allowing device-independent graphics programming.
 
Gnomonic projection - A type of perspective azimuthal map projection. The point of the projection is the center of the sphere. Gnomonic is the only projection on which all great circles represented are straight lines.
 
GPS - Global Positioning System is a space-based satellite navigation system that provides location and time information in all weather conditions, anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites. Ground-based GPS receivers can automatically derive accurate surface coordinates for all kinds of GIS, mapping, and surveying data collection.
 
Gradient analysis - Process to determining the maximum rate of change of surface altitude in a DEM.
 
Gradient filtering - A method of edge detection using two filters. One filter enhances horizontal edges and the other enhances vertical edges. The results are then combined into a single output cell value. Gradient filtering usually produces better results than a single high-pass filter. (See also Filtering)
 
GRASS - Geographic Resources Analysis Support System. A public domain GIS system developed at the U.S. Army’s Construction Engineering Research Laboratory (CERL) in Urbana, Illinois. It is now managed by an inter-agency governmental committee. The system was designed for minicomputers and has since been implemented on various microcomputers. It is a raster-based system that can interface to vector data sets for data preparation and import/export.
 
Grid map - A map in which the information is carried in the form of grid cells. (See Raster)
 
Grid or Grid format - A data structure in which data is encoded and stored as a regular unit of area, usually square or rectangular in shape, and called grids or cells. Data are somewhat generalized.
 
HIS - Hue, Intensity, and Saturation. Sometimes called HLS, for Hue, Luminance and Saturation. The system of defining video output color from hue, intensity and saturation characteristics. HIS controls are commonly used with color television sets. Most computer displays use RGB (red/green/blue) color mixing information instead. The hue-intensity-saturation color model uses a double cone (with its greatest circumference at the midpoint of the z-axis) to describe specific colors. On any horizontal slice of the cone, the hue varies around the slice and the saturation increases outward from the center. Intensity is the z-axis of the model. The shades of gray are found along the z-axis, where hue and saturation equal zero.
 
Histogram - A graph of the number of times a value occurs across a range of possible values. One axis of a raster histogram shows how many times a cell value is found in the raster. The other axis shows the range of possible data values (like 0 to 255 for 8-bitcell values).
 
Hue - One of the three coordinates needed to specify a color in the HIS color domain. Hue tells the color such as red, blue, or green. (See also HIS)
 
Hypsometric curve - Set of contour lines that describe the altitude surface.
 
Hypsometry - The measurement of the elevation of the Earth’s surface with respect to sea level.
 
Image - Any analog or digital two-dimensional array of values whose spatial interconnections convey useful information. Image has a wider sense than photographs, TV, or human vision. A photographic print is an image but an image is not necessarily a photograph. What a human sees can be called an image but image is not restricted to describing human vision. The raster scan of a color map is an image, but not its converted vector object derivative. However, the display of a vector object on the monitor creates an image of that vector file. Properly, that which is stored in a raster object can be called an image, but that which is stored in a vector, CAD, text, or database object cannot.


Image analysis - A technique to locate edges or boundaries in digital image data.
 
Image map or calibrated image map - An image that has been processed to be like a map in appearance, scale, geometry, boundary, and projection with a degree of precision that satisfies the user. Measurements made from an image map yield results equal to those made from the corresponding planimetric, topographic, or other map. Similarly, either the image map or the conventional map can be overlaid and matched with the other. For example, a
7.5’ image map prepared in MIPS from LANDSAT or SPOT satellite images will accurately match the corresponding USGS topographic map. Similarly, the color scan of a topographic map that has been assembled by tiling and calibrated to map coordinates is an image map. (See also Nominal image map)
 
Image processing system - A set of analytical routines designed for the restoration, enhancement and computer-assisted interpretation of digital images, most particularly of remotely sensed data.
 
INGRES - INteractive Graphics and REtrieval System. A relational database management system by Relational Technology, Inc. (RTI) of Berkeley, California. Mainframe and PC-based versions are available. INGRES has two query languages: SQL and QUEL for database handling.
 
Integrated map - A natural features map where each map unit has multiple labels addressing a variety of geographic subjects. For example, one map unit may contain Bedrock Geology, Soil Type A, Vegetation Type 1, Slope 5%-10%, etc.

Intensity - One of the three coordinates needed to define a color in the HIS color domain. Intensity represents a color’s brightness or average luminance, or radiance level. Intensity data is very similar to the information in black and white representations of color images. (See also HIS)
 
Interpolation - Applying mathematical estimation techniques to sets of numbers to find intermediate values or Estimation of a variable at an unmeasured location from observed values at surrounding locations
 
ISO - International Standards Organization. The ISO defines and publishes standards that can be adopted by many manufacturers. For example, one ISO standard defines the format for erasable optical disks, so they can be used on drives from any manufacturer that supports the standard.
 
Isoline - line connecting points of equal value.

UPDATED:  July 9, 2017