Glossary of GIS Terminology Part 1

Absolute accuracy - A measure of the difference between the locations of an object as specified in a GIS, and its true location in the real world. 

Accuracy - Degree of conformity with a standard, or the degree of correctness attained in a measurement. Accuracy relates to the quality of a result and is distinguished from precision, which relates to the quality of the operation by which the result is obtained. 

Adaptive sampling - A data sampling technique that uses accumulated knowledge from samples already taken to direct future sampling. For example, redundant sample points may be rejected during the sampling process on the grounds that they carry too little extra information. 

Additive color - Colors seen by the human eye that are created by mixing different colors of visible radiation (light) are produced in an additive manner. Colors on a display screen and colors projected onto a white screen are examples of additive color. When all wavelengths, or colors, of visible light are mixed, white light is produced. A prism separates white light into the spectrum of colors which compose it. Mixing complementary colors can also produce white light. Absence of radiation or relatively low radiation at all wavelengths of human visual sensitivity yields black. A predominance of radiation in a particular range of wavelengths yields a color whose intensity is proportional to the level of radiation. (See also subtractive color).
Address geocoding - Process of assigning alphanumeric locational identifiers (such as the municipal address or physical location) to spatially related information. For example, an address may be matched to an address range on a street segment, or a given spatial area (i.e., the limits of a polygon, a line segment, a point along the segment, or an absolute point that has been coordinated). The process implies a geographic base file which can be used to pass addresses in order to find out characteristics about the geometry.
Addressability - The number of pixels in the x and y axes on a screen.
Addressable point - A position on a visual display unit that can be specified by absolute coordinates.
Aerial photograph - A photograph taken vertically downward from the air. Aerial photograph images may be in the form of paper prints, often 9" x 9", or transparent film.
AGIS - A GIS marketed by Delta Data Systems, Inc. It has both raster and vector data acquisition and attribution capabilities in a single operating environment. The user can link dBASE III to its relational database.
Air video image - An image acquired vertically downward from the air with a color, monochrome, or color-infrared video camera and recorder.
Air videography - Making measurements from digitized frames of vertical airvideo images.
Albedo - The ratio of the light reflected by a planet or a satellite to that received by it. This definition can be generalized to any object, such as a part of the Earth’s surface or atmosphere, a leaf, a soil element, etc.
Aliasing - The appearance of jagged lines on a raster display.
Alphanumeric - Consisting of both letters and numbers, and possibly including other symbols such as punctuation marks.
AM/FM - Automated Mapping/Facilities Management means exactly that: to automate the mapping process and to manage facilities represented by items on the map. In the past, when a map was needed, a crew of surveyors, draftspersons, and geographers would combine their resources and develop a map on paper. This map was created by hand, updated by hand, and reproduced by a professional printer. Today, it can be drawn on a computer screen using a Computer Aided Drafting and Design (CADD) software program. The map program is then connected to a database containing a variety of detailed information related to items on the map. When the map is needed to answer a question, it is displayed on the screen automatically. Updates are made quickly using a digitizing table, a mouse and a keyboard. The entire map, or just portions of it, may be selected to be printed on a plotter. The process is similar to word processing for maps.
Analog - Information stored and processed with signal intensity or other measurement of a continuous physical variable. Analog information processing has the advantage of being able to translate and represent slight increments in data easily. Analog information (video, audio, or field and laboratory measurements of temperature, pressure, voltage, radiation, etc.) can be converted to its digital equivalent. (See also Digital, Digitizer).
Analog/Digital conversion - The process of converting data from analog to machine usable digital format via processes such as digitizing or scanning.
Anti-aliasing - Anti-aliasing removes or greatly smoothes the jagged, stair-step appearance of a digital line by filling in some of the intermediate and flanking cells in lower-intensity colors. (See also Aliasing)
Arc - A portion of the perimeter of a two-dimensional closed figure lying between two nodes at which two or more arcs intersect. An arc usually represents a continuous common boundary between two adjoining mapping units.
Arc data - Data representing the location of linear entities or the borders of polygon entities.
Arcsecond - The sixtieth part of a minute of angular measure often represented by the “symbol, as in 30", which is read "30 seconds". (See also Minute)
Area - A level of spatial measurement referring to a two-dimensional defined space. A polygon on the earth as projected onto a horizontal plane is an example of an area.
Argument - A component, such as a filename, that is part of a command line. The shell interprets arguments for commands to see if the syntax is correct before executing the command.
ARIES - ARIES is a modeling /decision making platform rather than a single model or collection of models. ARIES is capable of incorporating existing ecological process models where appropriate, and turns to ad hoc models where existing process models do not exist or are inadequate for local contexts. Agent-based models are used to simulate ecosystem service flows.
Arithmetic operator - In a GIS, operations performed on attributes of objects or entire maps, such as add, subtract, multiply, and divide.
Array - A data structure that treats contiguous element in the form of a grid or matrix as a series of addressable repeating patterns. Each element in an array is referenced by an index which gives the location of the element in relation to other elements.
ASCII - American Standard Code for Information Interchange (pronounced "askee"). The 256 characters that comprise a computer alphabet. The character set encoded into digital values between 0 and 127 includes the lowercase and uppercase letters, the numerals 0-9, punctuation marks, special symbols (such as @#$%^&*), and non-displaying characters often used as printer control codes. Values from 128 to 255 are not standardized, and are not part of the basic ASCII code. However, most PCs use a common extended ASCII character set for values from 128 to 255. (See also ASCII file)
ASCII file - A text-only file. Documents in most word processors are not text-only files, since they include coded header information and formatting characters. However, most word processors import ASCII files and have an Export, Save/Text-only, or print-to-file utility that converts a document into an ASCII format. (See also ASCII)
Aspect - A parameter that is associated with a feature of a topographic or other three dimensional surface that tells which direction the surface slopes. Aspect is the compass direction (usually from North) for the line of steepest slope at some selected point.
Aspect ratio - The ratio of horizontal scale to vertical scale for printing or display. For graphics and image processing, square cells/pixels are best (aspect 1:1). Some display devices have a non-square aspect, which causes images to appear stretched or distorted. Standard broadcast video has an aspect ratio of 4:3, which must be corrected in any frame grabbing or other digitization process.
Attribute - An attribute is a set or collection of data that describe the characteristics of real world entities or conditions. Attribute data are usually alphanumeric. Small amounts of attribute data are frequently used to describe the graphic representation of an entity on a map as a label, e.g., a polygon label. Large amounts of attribute data are usually maintained as separate attribute data sets, related to a map by names or codes.
Attribute query - A process to select data items from a file system based on the values of specific attributes or combination thereof, defined by arithmetic, relational, and logical expressions.
AutoCAD - A commercial Computer Aided Design (CAD) software package, developed and distributed by Autodesk, Inc.
Autocorrelation - Statistical concepts expressing the degree to which one value of an attribute co-varies with other values of the same attribute. Particularly in the spatial case, the degree to which the values of an attribute of two objects co-vary with the distance separating them. Mathematical autocorrelation techniques can be applied to overlapping image segments in processes such as mosaicking and raster-to-raster registration. For example, autocorrelation can automatically find the best seam between adjacent overlapping image segments.
AVHRR imagery - Advanced Very High Resolution Radiometer imagery produced by NOAA satellites.
AVIRIS imagery - Airborne Visible/Infrared Imaging Spectrometer imagery. Multispectral images of approximately 240 co-registered spectral bands collected by NASA aircraft.
Azimuth - Particularly for a class of map projections, the angle defined by the intersection of a map’s central line of projection with any meridian. If a map projection uses a central line that is oriented to true north, such as a standard meridian, the azimuth is zero.
Azimuthal projections - A class of map projections on which the directions of all lines radiating from a central point are the same as the directions of the corresponding lines on the sphere. Azimuthal projections are formed onto a plane which is usually tangent to the globe at a pole (a polar projection), at a point on the equator, or at any selected intermediate point. Most azimuthal maps do not have standard parallels or standard meridians. Each map has only one standard point—the center. Thus, the azimuthal are suitable for minimizing distortion in a somewhat circular region, such as Antarctica, but not for an area with predominant length in one direction.

B-Spline - The sums of other splines that by definition have the value of zero outside the interval of interest, therefore, allow local fitting from low-order polynomials in a simple way. B-Splines are often used for smoothing digitized lines to display, such as the boundaries on soil and geological maps, where cartographic conventions expect smooth, flowing lines.
Band or spectral band - A range of wavelengths of electromagnetic radiation. Remote sensing devices commonly collect images in discrete bands, such as visible red, green, and blue, and the invisible near-infrared.
Bandwidth - For monitors, bandwidth refers to the highest signal frequency a monitor’s circuit can display. The higher the bandwidth, the higher resolution and the sharper the image will be.
Barriers - Static or dynamic factors such as physical features, time, season, speed, direction and accumulation, which must be considered when performing, distance analyses.
Base data - Basic level of map data on which other information is placed for purposes of comparison or geographical correlation.
Base map - Mapped data that seldom changes and is used repeatedly.
Bearing - The horizontal angle of a line of direction, measured in the quadrant of the line as degrees east or west of the meridian of reference. Northeast, for example, has a bearing of 45 degrees east of north, whereas southwest has a bearing of 45 degrees west of south.
Benchmark test - A series of tests for ensuring that hardware and/or software meets user needs.
Bezier curve - A polynomial curve bounded by four points, which form a bounding box. Manipulating the position of the bounding points lets a user stretch and position a smooth curved line in the design of a graphic shape.
Bilinear interpolation - A mathematical method for interpolating a new cell’s value within a 2 x 2 neighborhood of cells. Bilinear interpolation is used in resampling a raster object to create a new raster object with a different cell size, orientation, or internal geometry. (See also Interpolation)
Binary - A base 2 number system that uses only the data values 0 and 1. Each place represents a power of 2, so, for example, the decimal number 13 is represented in base 2 as 1101 (= 1x8 + 1x4 +0x2 + 1x1).
Binary arithmetic - The mathematics of calculating in powers of two.
Binary data model - Among relational data models, the binary data model is that each binary relation represents a single atomic fact: a correspondence between a key and non-key attribute, where the key consists of only one attribute.
Binary file - A file containing characters that are in machine-readable form.
Binary raster object - A raster object whose cells contain only the values 0 or 1. Binary raster objects can contain a scan of black lines on white paper, the results of thresholding (method of image segmentation) a byte-oriented raster object into two data ranges, a threshold of a particular color range, or a data mask.
Bit - Abbreviation for binary digit. A single value of either 1 or 0. A contraction of the two words Binary and digit. The smallest unit of computer data. Computers normally manipulate bits at least 8 at time. A group of 8 bits is called a byte. A computer’s processing power is often measured by the number of bits it handles at once. The earliest PCs were 8-bit machines. More recently processors and data structures for 16-bit, 24-bit and 32-bit data have become common.
Bit plane - A part of a data structure of superimposed grids of cells having the values 0 or 1.
Bitmap - An image stored as a pattern of dots (or pels).
Bitmapped font - A text font in which the individual characters are defined by the positions of pixels in a reference grid. Bitmapped fonts process quickly, but when they are enlarged, they lose the illusion of smooth edges and look blocky. (See also Vector/scalable font).
Bits per pixel or pixel depth - The number of data bits each pixel represents. In 8-bit contexts, the pixel depth is 8, and each display pixel can be one of 256 possible colors or shades of gray. With a 24-bit raster (or with three co-registered 8-bit raster’s) the pixel depth is 24, and 16,777,216 colors are possible.
Boxcar classification or boxcar interpretation - The simplest form of automated image interpretation whereby three data ranges are selected for three co-registered images (like red, green, and blue). The three data ranges define a three-dimensional cube, or boxcar shape if plotted on three perpendicular axes that represent possible data values in red, green, and blue. The ranges are usually selected to represent the color variation in the three raster’s for a feature of interest (like all the dark brown areas representing bare soil).

CAD - Computer Aided Drafting/Design/Drawing. CAD originated on larger, dedicated workstations and minicomputers and has now migrated to microcomputers. In its simplest sense, CAD is used for computerized drafting. Many CAD systems also provide more advanced features like solid modeling and simulation. CAD generally lacks topology of objects and direct links to an attribute database, which are essential features in GIS modeling and analysis operations.

CAD object - A CAD object describes coordinate data. A CAD object has a free-form topology, so it may be useful for applications that do not require an exact description of the relationships between the elements in the object. CAD object topology does not reconcile things like line intersections, polygon overlapping, and polygon islands.
CAD/CAM/CAE - Computer Aided Design/Computer Aided Manufacturing/Computer Aided Engineering. Differs from a GIS in that the system can only create displays. It cannot analyze or process the base data.

Cadastral layer - A set of information depicting the pattern of land ownership rights in an area. The layer is typically based on legal descriptions tied to elements of the geodetic control network available in an area.

Calibrate - Particularly geographic calibration, to bring a raster or vector object into alignment with some geographic coordinate system. Geographic calibration may be established when creating an object (like extracting an image map from a LANDSAT or SPOT satellite image). Calibration can be added to an existing object either by entering control points or by associating it with some calibrated object (like overlaying a calibrated vector to calibrate a raster).

Calibrated image map (also 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 a GIS from LANDSAT or SPOT satellite images can 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.
Cardinal direction - The four principal directions: North, South, East and West.

Cartesian coordinates - A coordinate system in which the locations of points in space are expressed by reference to three perpendicular axes, called the coordinate axes (x, y, and z).
Cartogram - A map projection with areas or distances distorted, according to a transforming variable, to communicate by relative distance or size such concepts as travel time or population size.
Cartographic model - A flow diagram depicting a process of combining and analyzing multiple layers of mapped information to create a new synthesized map.

Cartography - The art and science of graphically representing a geographical area, usually on a flat surface such as a map or chart; it may involve the superimposition of political, cultural, or other non-geographical divisions onto the representation of a geographical area.
Cell - One value in a raster that corresponds to a specific area on the ground. A raster cell may contain a value that describes the elevation above sea level at one position in a survey site or the intensity of red radiation for a pixel in a video image. For convenience, a raster cell is usually thought of as square or rectangular, although many image collection devices actually measure circular or elliptical areas.
Cell size - The dimensions of the area on the ground to which a raster cell value applies. A cell size of 30 meters signifies that the value in each cell of the raster object applies to a 30 x 30 meter area in the study site.

Centroid - The point that may be considered as the center of a one- or two-dimensional figure, the sum of the displacements of all points in the figure from such a point being zero.
Centroid calculation and sequential numbering - Calculate a contained, representative point in a polygon and assign a unique number to the new object.
Chain - A sequence of coordinates defining a complex line or boundary.
Chain coding - A method of reducing the storage requirements of a raster.
Change image - An image produced using raster algebra that shows change over time between co-registered images (multitemporal image processing). For example, subtracting the old raster object from the new raster object could show the difference between early-season crop development and mid-season development; or between pond surface areas from year to year.

Choropleth map - A map with areas colored or shaded so that the darkness or lightness of an area symbol is proportional to the density of the mapped phenomenon. More generally known as a map of uniform values separated by abrupt boundaries, that is, adjacent areas are not necessarily close in value.
CIR image - Color-Infrared Image. Color-infrared images may be collected by an electronic scanner or a camera that uses special film with sensitivity from green through infrared. The photographic infrared radiation just beyond the range of human vision is then displayed as red. Normal red from the scene becomes green, and green becomes blue. Normal blue in the scene is filtered out and not recorded. CIR images are used to show the vigor of plant life. Healthy vegetation appears red, while distressed or damaged vegetation may look pink, tan, or yellow. (See also Color-infrared image)
Class (raster) - A set of all image features of the same type. As part of the interpretive process, the user names a class to identify the type of material it contains, like corn, bare soil, wetland, or urban.
Class (vector) - One of the possible attributes that may be assigned to a vector element, like intermittent stream or highway. Element classes can be assigned directly or taken from a selected field in an associated database object.

Classification - Grouping cells (often by spectral characteristics) from one, or more characteristically, from a set of coregistered raster objects to isolate and name image features (crop varieties, wetlands, forest, or other surface cover).
Clump - A set of contiguous line, node, and polygon elements in a vector object.
Cluster analysis - Reduction of the complexity of the data set by assigning objects to classes in a classification system. Because these clusters are derived from an analysis of the original data, they would represent natural groupings that should be more representative of reality than exogenous, hierarchical classes imposed from without.
Cluster labeling - Identifying and grouping the clusters in the cluster map raster object that results from any kind of automated image interpretation.

Cluster map - The output raster object created by clustering or by unsupervised classification. The clusters are usually identified or labeled as some useful type of material (e.g. an agricultural crop, a body tissue type, a soil type, etc.). It is important to note that this raster contains categorical data--its values cannot be subjected to further mathematical analysis because the clusters and their re-identification as material or area types do not represent data values that are mathematically continuous. For example, cells in a cluster arbitrarily assigned the value of 4 (belonging to cluster number 4) by the clustering process do not thereby represent twice as much of something as a cell assigned to cluster 2.
Clustering - A process in which multiple, spatially coincident, co-registered raster objects are reduced to a single raster object, called a cluster map. The input rasters contain analytical data (such as spectral images and elevations). In general, each clustering method compares the values in corresponding cells to all other cell values, and assigns the output cell to the group (or cluster) it most resembles.
CMY or Cyan-Magenta-Yellow - The standard set of subtractive, processing colors used in printing. Color printing devices use discrete dots of cyan, magenta and yellow (and black) to present the appearance of a full-color image to the human eye.

COGO - Coordinate Geometry. The set of mathematical tools and functions for encoding and converting bearings, distances, angles, etc. into coordinate information. This operation is normally done on an alphanumeric screen into which data is entered and the geometry is determined analytically.

Cold start - Re-boot a DOS microcomputer by turning off and turning on again.
Color balancing - Adjusting the intensities and distribution of red, green, and blue to create an image with a particular color appearance for display or printing.
Color composite - In satellite imagery, a color negative, transparency or print produced by allowing the reflectance recorded for each band of a multispectral image to be represented by a proportionate intensity of one of the primary colors.
Color depth or pixel depth - The number of data bits each pixel represents. In 8-bit contexts, the pixel depth is 8, and each display pixel can be one of 256 possible colors or shades of gray. With a 24-bit raster (or with three coregistered 8-bit rasters) the pixel depth is 24, and 16,777,216 colors are possible.
Color scheme - A predefined combination of colors that a GIS uses for its screen elements. Also, refers to the graduation of color for the GIS output display. (See also Color table or Color map)
Color separation - Manipulating a full-color image in order to extract features of one color or range of colors. The color separation process can be used to create a binary raster object from a composite color raster object (or a set of three RGB raster objects) to lift out blue line images, for example, leaving behind background colors and lines images of other colors. Printers use related color separation techniques to prepare process color separates from full-color originals.

Color-infrared (also CIR) - Color-infrared images may be collected by an electronic scanner or a camera that uses special film which is sensitive to spectral bands from green through infrared. The photographic infrared radiation just beyond the range of human vision is displayed as red. Normal red from the scene becomes green, and green becomes blue. Normal blue in the scene is filtered out and not recorded. Any physical or biological damage to growing plants which begins to cause deterioration in their vigor (their water and/or chlorophyll content) causes a rapid decrease in their reflectance of photo-infrared radiation, and increases in their red reflectance. CIR photographs show these changes much sooner and more dramatically than normal photographs or human eyesight. Healthy, green vegetation appears in bright red, while damaged, diseased, or dying vegetation appears in shades of pink, tan, and yellow. This knowledge was first used during the Second World War when color-infrared film was called camouflage detection film. It provided pre-visual detection of the changes in vegetation that was cut or damaged by military activity and it could very easily separate color-camouflage materials (like olive drab canvas) from live foliage.
Compiled map - A map based on information used primarily in the preparation of other maps. Most small-scale maps of large areas are compiled maps.
Composite color raster object - (8-bit data and device contexts) A raster object in which each cell contains a data value representing one of the 256 colors. Composite color rasters are usually compressed from a red/green/blue raster set that retains a wider range of color information. The composite raster still produces near photographic quality color displays. (See also Pseudo-color image).

Composite color video - The standard color video output of a VCR or video camera that adheres to the NTSC video standard. All the color information is contained in one composite signal (instead of in three separate RGB color signals).
Composite map - A single map created by joining together several separately digitized or scanned maps.
Contiguity analysis - Concerned with adjacency relationships between any given polygon and its neighbors. Typically this involves summarizing and relating the attributes of neighboring polygons to the polygon being examined.
UPDATED:  July 9, 2017