Geographic Information Systems: A Glossary Charles R. Blinn, Lloyd P. Queen and Les W. Maki

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• accuracy. Conformity with a standard or correctness in 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.
• ADMATCH (address matching). A computer program that can match street addresses with census tracts and/or block codes and that can convert street addresses to coordinates for computer mapping.
• algorithm. Statement of the steps to be followed in the solution of a problem.
• arc. A line established by connecting a set of points. It has length but no area and often serves as one side of a polygon. Arcs may begin and end with nodes and have points of inflection defined by vertices.
• area. (1) A polygon that encloses a homogeneous unit (e.g., lake, country, state, county). (2) A level of spatial measurement that references the size or extent of a two-dimensional defined space.

• aspect. Horizontal direction in which a slope faces. Commonly expressed in degrees clockwise from north (e.g., a southeast facing slope has an aspect of 135 degrees). Contrast with slope.
• attribute. A numeric and/or text description of a spatial entity (e.g., attributes of a parcel might include address, owner's name, and property value). Attribute data is typically stored in tabular format.
• automated mapping/facilities management (AM/FM). A system that integrates automated mapping with facilities management (e.g., management of power lines, utilities, energy services). AM/FM provides digital or computer-based storage, management, retrieval, and display of urban data for cadastral mapping and utilities management.
• azimuth. The horizontal direction measured clockwise from north (e.g., due south has the azimuth 180 degrees).
• base layer. A layer containing a variety of features often used for locational reference (e.g., section corners, political boundaries, and other major features) or to establish geodetic control which ensures geographic fidelity between data sets in the GIS.
• base map. Mapped data which seldom change and which are used repeatedly for locational reference and control. It also establishes the lowest common denominator for map scale, coordinate system, and projection for other maps in a GIS database. Contrast with thematic map.
• benchmark. A reference point for measurements, normally one that is used for elevation reference.
• benchmark test. A test to evaluate the capabilities of a computer system in terms of performance and ability to meet customer requirements.
• Boolean operators. Search strategy for information retrieval based on the use of the logical operators AND (union), OR (intersection), NOT (complement), and XOR (exclusion) to represent symbolic relationships.
• browse. System capability to search for an undefined feature or set of features in a database.

• buffer. An area of specified distance (radius) around a map item or items. See proximity analysis.
• cadastral parcel. The smallest legally defined piece of land. Used for recording ownership and related attributes.
• cadastre. A survey that creates, defines, retraces, or reestablishes the boundaries and subdivisions of public lands and private estates. The ownership, characteristics, and value of private lands are recorded for taxation purposes.
• Cartesian coordinates. A plane coordinate system in which the locations of points in space are expressed by reference to two or three planes, called the "coordinate planes" (x,y or x,y,z).
• cartographic modeling. The use of basic GIS manipulation functions or tools in a logical sequence to solve spatial problems.
• cartography. The science of map making, including the art and technology of map making.

• centroid. The mathematical or geographical center point of a polygon or the midpoint of a line. It is described as an x,y coordinate.
• choropleth map. A map with areas colored or shaded such that the darkness or lightness of an area symbol is proportional to the density of the mapped phenomena or is symbolic of the class.
• classification. Process of assigning individual observations or features into groups, categories, or classes.

• clip. The process of extracting a portion of a coverage or map from a larger coverage, much like a cookie cutter.
• coincidence analysis. Process of examining the co-occurrence of mapped phenomena.
• computer aided design (CAD). Software with the capability of performing standard engineering drawing and architecture design functions. It is graphically oriented rather than geographically based. Unlike a GIS, the database is generally not structured to allow spatial analysis. However, some data developed in CAD systems can be incorporated into a GIS.
• computer aided mapping (CAM). Software with the capability of performing standard mapping functions, typically using a vector format. Unlike a GIS, it cannot analyze or process the database.
• conflation. Procedure of reconciling the position of corresponding features in different themes so that the corresponding features overlay precisely.
• connectivity analysis. See network analysis.
• contiguity analysis. See neighborhood analysis.
• contour. An imaginary line on the ground, all points of which are at the same elevation, usually expressed as a length or elevation above some datum such as mean sea level.
• control point. A permanently fixed point on the ground, the location of which has been accurately determined and recorded for reference and future use.
• coordinate. Coordinates are used to represent location on the earth's surface relative to other locations in either 2 (x,y) or 3 (x,y,z) dimensions.
• coordinate geometry (COGO). The conversion of surveying data into geographic locations.
• coordinate system. 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 relative position of points within that particular reference frame or system.
• corridor analysis. A type of proximity analysis that utilizes specified distances or buffers along a line (e.g., amount of timber within 50 feet of proposed power transmission line). See buffer.
• cost-benefit analysis. A comparison of the costs and benefits of the current system or processes versus a proposed GIS. A cost-benefit analysis is developed to assist with making an acquisition decision.
• coverage. The graphic and attribute data related to a particular data theme for a study area. A coverage usually represents a single layer or theme.
• cross tabulation. Comparison of attribute data by location in two or more map layers. Results are given in a tabular report format.
• data (singular datum). Facts about real-world entities, organized for analysis. Includes results of observations or measurements of such entities. Three components of a datum are of direct relevance to GIS: (1) attribute information that describes the substance, characteristics, variables, values, and similar qualities of the entity; (2) geographical information that describes the position of the entity in space relative to other things in space; and (3) temporal information that describes the instant or period of time during which the entity is at a defined location or in an observed state or condition (attribute).
• data capture. Operations that are required to encode data in a computer readable digital form (e.g., digitizing, scanning). See also data input.
• data conversion. The translation of data from one storage format or media to another for the purpose of transferring it from one GIS to another. It is also the process of transforming maps from manual to digital form.
• data dictionary. A directory of all data items, giving the name and structure of each. It does not contain the actual data. The contents of a data dictionary are sometimes called metadata.
• data encoding. To apply a code to represent individual data or groups of data.
• data input (data capture). Series of operations required to enter map or attribute data into a computer. Geographic data are generally entered into a GIS database via a digitizer, scanner, or keyboard.
• data model. The conceptual organization of a database. It can be thought of as the style of structuring, describing, and manipulating the data in a database.
• data quality. The degree of excellence exhibited by the data in relation to a correct portrayal of the actual phenomena.
• data standardization. The process of achieving agreement on common data definitions, representation, and structures to which all data layers and items must conform.
• database. A logical collection of files managed as a unit. A GIS database includes data about both the position and the attributes of geographic features.
• database management system (DBMS). A systematic approach to maintaining, accessing, reporting, and analyzing attribute data. GIS packages may use DBMS to handle some data management tasks.
• database structure. The physical organization of data elements assigned to files and relationships among files.
• datum. A base or reference elevation used as the origin to define subsequent elevation. Often refers to mean sea level.
• delineation. The legal description of the locations of points that mark the boundaries of a cadastral parcel.
• demarcation. The field measurements (e.g., land survey) that determine the physical locations of boundary markers of cadastral parcels.
• derived map. A map created as the result of analyzing, altering, or combining a pre-existing map or a series of maps in the GIS.
• digital data. Collection of similar and related data records that are converted into a form for use by a computer.
• digital elevation model (DEM). A data file of a topographic surface arranged as a set of x,y,z coordinates where z represents surface elevation. It is the digital equivalent of the elevation data portrayed on a topographic base map.
• digital exchange format (DXF). A standard format for exchanging digital cartographic (map) files and associated spatial data between different GIS systems.
• digital line graph (DLG). A file or data structure from the US Geological Survey (USGS) that includes digital non-topographic information from the USGS map base categories such as transportation, hydrology, and public land survey boundaries.
• digital terrain model (DTM). See digital elevation model.
• digitize. A process used to create digital data from non-digital or analog data. See data conversion.

• digitizer. A device consisting of a table and a cursor (often with crosshairs and keys) that is used for capturing and recording the locations of map features as x,y Cartesian coordinates.
• dissolve. The process of removing boundaries between adjacent polygons having the same values for a specified attribute.
• distance analysis. Geographic computations based on Euclidean and/or non-Euclidean distances between features.
• drum plotter. A device with a rotating cylindrical drawing surface and paper reels for plotting graphic images on a continuous roll of paper.
• easting. One of two geographic coordinate values (the other being northing) used to specify the exact location of a feature on a map. Eastings and northings are specified as x,y coordinates respectively for universal transverse mercator and state plane coordinate systems.

• edge matching. The comparison and graphic adjustment of features that cross adjoining map sheets to ensure that the features intersect the boundary at a common, coincident location. A "seamless" database is thereby created.
• electrostatic printer . A "Xerox-like" device for printing graphic images by placing small electrical charges on the paper so that a dark or colored powder, or toner, will adhere in these spots.
• encode. Convert data to a form that is suitable for entry into a computer.
• entity. Object or feature about which information is stored. The information describes where the entity is (location — stored as points, lines, or polygons) and what the entity is (identity — e.g., lake, tree, house). An entity is usually a person, place, thing, or event. It may be tangible or intangible and can be further defined by attribute data.
• export. Process of transferring data or software from one GIS system to another.
• feature. A geographic component of the earth's surface that has both spatial and attribute data associated with it (e.g., well, road, lake).
• feature oriented. The use of points, lines, and polygons to represent real world entities in a GIS database.
• field. See item.
• file. A collection of related records treated as a unit.
• flatbed plotter. A device with a flat drawing surface for pen-plotting a graphic image from a list of point coordinates and pen codes.

• gap. The distance between two entities (usually lines) on a digitized map. Gaps may indicate errors made while digitizing or scanning a map. See sliver.
• generalize. A reduction of detail through resampling to larger spacing or a reduction in the number of points or vertices in a line.
• geocode. The process of creating an x,y coordinate location from another geographic location description, such as an address. The term evolved from geographic coding.
• geodesy (also geodetic, of or pertaining to geodesy). Science of the size and shape of the earth.
• geographic coordinates. Values specifying the location of features in a standard, absolute worldwide coordinate system (e.g., latitude/longitude, state plane coordinates, universal transverse mercator).
• geographic data. Data that convey the locations and descriptions of geographic features.
• geographic information system (GIS). System of computer hardware, software, and procedures designed to support the compiling, storing, retrieving, analyzing, and display of spatially referenced data for addressing planning and management problems. In addition to these technical components, a complete GIS must also include a focus on people, organizations, and standards.
• geometric correction. Alters data to correspond with true ground or image space in a known coordinate system.
• geoprocessing. The automated manipulation and/or analysis of geographic data.
• georeferencing. A process of referencing points on the surface of the earth to points on a map. Examples of referencing systems include latitude/longitude, universal transverse mercator, and state plane coordinates.
• global positioning system (GPS). A system that uses NAVSTAR satellites to locate positions on the earth's surface. Sometimes referred to as global positioning satellites.
• grid. A data structure that uses rectangular units or grid cells arranged in rows and columns to represent map features. Also known as a raster.
• ground control point. See control point and tics.
• ground truth. To verify the correctness of remote sensing information by use of ancillary information such as field studies.

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