This Glossary is intended to serve the user who may be unfamiliar with some of the terminology appearing in GIS documentation. Definitions of other commonly used GIS terms have also been included.
Rapid advances in computer technology make it virtually impossible, however, to provide universally recognized definitions for the wide range of applications and operations being conducted with geographic information systems today.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z References
- Accuracy-degree of conformity with a standard, or the degree of correctness attained in a measurement. Accuracy relates to the quality of a result, as distinguished from precision.
- Arc-represents the location of linear features or the border of polygon features.
- Area-is a closed figure whose boundary enclosed a homogeneous area, such as a water body or a state boundary.
- Aspect-horizontal direction in which a slope faces, commonly expressed as the direction clockwise from north.
- Attribute-descriptive characteristic or quality of a feature that can be assigned to one or more discrete values in a GIS.
- Base data-set of information that provides a baseline orientation for another layer of primary focus, e.g., roads, streams, and other data typically found on USGS maps.
- Cadastral National Data Infrastructure (NSDI)-defined by the FGDC Cadastral Subcommittee as a minimum set of attributes about land parcels that is used for publication and distribution of land ownership, land rights, and land boundary data, often for the purpose of taxation.
- Central meridian-for conic projections, the central meridian is the single line of longitude that is truly vertical on the map. It is usually in the middle of the map.
- Contour-a line connecting points of equal elevation.
- Control points-point locations for which map projection and database coordinate pairs are known to a high degree of accuracy. Control points are most often used to convert digitized coordinates to standard map projection coordinates.
- Coordinate pair-set of dimensional discrete values describing the location of a point, line, or polygon (area) feature in relation to the common coordinate system of the database.
- Coordinate systems-reference frame or system, such as plane rectangular coordinates or spherical coordinates, that uses linear or angular quantities to designate the position of points within that particular reference frame or system. Coordinates are used to represent locations on the earth's surface relative to other locations or fixed references.
- Coverage's-set of thematically associated data considered to be a unit and usually representative of a single them or layer (soils, streams, roads, land use, etc.), which is registered to the base map data by a common coordinate system.
- Datum-a mathematical reference framework for geodetic coordinates defined by the latitude and longitude of an initial point, the azimuth of a line from this point, and the parameters of the ellipsoid upon which the initial point is located. In regard to usage in this catalog, datum is identifying which system the coordinates are based upon (NAD27 or NAD83)
- Database-consist of one or more data sets related by a common fact or purpose.
- Database Management System (DBMS)-Software designed to access and structure a database.
- Data capture-series of operations required to encode data in a computer-readable digital form.
- Data element-specific item of information appearing in a set of data.
- Data element dictionary-description of the information contained in a data coverage, e.g., format, definition, structure, and usage.
- Data quality-refers to the degree of excellence exhibited by the data in relation to the portrayal of the actual phenomena.
- Data standardization-the process of achieving agreement on data definitions, representation, and structures to which all data layers and elements in an organization must conform.
- Data structure-organization of data, particularly the reference linkages among data elements.
- Digital data-of or relating to data presented in the form of digits--data displayed, recorded,
or stored in binary notation.
- Digital Elevation Model (DEM)-file with individual terrain elevations recorded to a single raster cell, usually spaced in a uniform horizontal grid.
- Digital Line Graph (DLG)-USGS product that includes digital information from the USGS map base categories, such as transportation, hydrography, contours, and public land survey boundaries.
- Digital Orthophoto Quarter Quadrangle (DOQQ)-an orthographic photograph provided in digital formats by the USGS, with most tilt and terrain error removed.
- Digital Raster Graphic (DRG)-digital version of USGS fine- to medium-scale maps.
- Digital Terrain Model (DTM)-land surface representation in digital form by an elevation grid or list of three-dimensional coordinates.
- Digitizing-refers to the process of manually converting an analog image or map or other graphic overlay into numerical format for use by a computer with the use of a digitizing table
or tablet and tracing the input data with a curser (also see scanning)
- Edge matching-the comparison and graphic adjustment of features to obtain agreement along the edges of adjoining map sheets.
- False easting-many projections have an origin point. The origin point is particular to each projection. The false easting is the x-coordinate value assigned relative to this origin. False easting must be in meters (i.e. the same units as the spheroid).
- False northing-similar to false easting, except that it is and arbitrary y-shift. False northing must be in meters (i.e. the same units as the spheroid).
- Feature-objects that have a geographic location that can be represented by one or more points, lines, or polygons.
- Federal Information Processing Standards-official source within the federal government for information processing standards, which were developed by the Institute for Computer Sciences and Technology, National Bureau of Standards.
- FIPS zone-refers to the appropriate FIPS code for the state plane zone.
- Format-predetermined arrangement of characters, fields, lines, punctuation, page numbers, etc.
- File Transfer Protocol (FTP)-widely accepted method of transferring files across a computer network.
- Geodetic Control-set of control points whose coordinates are established by ground survey and are often used as referencing positions for other surveys or digital georeferencing.
- Geographic Information System (GIS)-system of computer hardware, software, people, and procedures designed to support the capture, management, manipulation, analysis, modeling, and display of spatially references data for solving complex planning and management problems.
- Geographic Names Information System (GNIS)-maintained by the US Board on Geographic Names and contains information about almost 2 million physical and cultural geographic features in the United States.
- Latitude-angular distance measured in degrees north or south of the equator on the earth's surface.
- Latitude of projection's origin-identifies where to put the false easting and false northing for conic projections where two standard parallels are assigned.
- Layers-refers to the various "overlays" of data, each of which normally deals with one thematic topic. These overlays are registered to each other by the common coordinate system of the database.
- Line-Level of spatial measurement referring to one- dimensionally defined object having a length and direction, and connecting at least two points, e.g., roads, railroads, telecommunication
lines, streams, etc.
- Lineage-information about the characteristics and history of the data sources.
- Logical consistency-refers to the topological structure of the data within a database and the ability of the data structure to relate spatial and feature elements to each other without contradictions.
- Longitude-angular distance in degrees east and west of the Prime Meridian on the earth's surface.
- Map projection-mathematical model that transforms the location of features on the earth's surface to locations on a two- dimensional surface.
- Merge-the combining of two or more maps or data sets into a single, coherent map or database without redundant information.
- Metadata-data describing a GIS database or data set including, but not limited to, descriptions of data transfer media, formats, and contents, source lineage data, and any other applicable data processing algorithms or procedures.
- Normalized Difference Vegetation Index (NDVI)-a measure and monitor on plant growth, vegetation cover, and biomass production from analysis of multispectral satellite data.
- Orthophotograph-a vertical photograph with a view such that horizontal placement is depicted from a vertical viewpoint at infinity. Orthophotographs are created by using projection geometry and measurements to remove tilt, terrain, and perspective distortion from aerial photographs.
- Pixel-a picture element having both spatial and spectral properties.
- Planimetric mapping-the representation of features within a two-dimensional coordinate system in which locations are represented by x, y coordinate pairs.
- Point-a discrete location, usually depicted by a symbol or label, representing a point feature. It defines a map object whose boundary or shape is too small to be shown as a line or area, such as a well or weather station.
- Positional accuracy-term used in evaluating the overall reliability of the positions of cartographic features relative to their true positions.
- Precision-refers to the quality of the operation by which the result is obtained, as
distinguished from accuracy.
- Projection-a geometric representation of a three- dimensional spheroid on a two- dimensional surface. In regard to usage in the file description of this catalog, projection refers to the name or type of projection being used.
- Public Land Survey System (PLSS)-grid system, based on township, range, and section, used for land ownership referenced in portions of the United States.
- Quadrangle-typically refers to a topographic map sheet published by the USGS in both 7.5 and 15 minute series.
- Quadrant-refers to which geographic quadrant the coordinates belong (i.e., NW, NE, SW, SE).
- Quality Control (QC)-The process used to ensure the quality of data or operations is in keeping with standards set for the system.
- Raster data-a uniform array or grid of cells defined in row/column sequences with each cell containing a single value. Every location in the data area corresponds to a raster cell.
- Registration-the procedure used to bring two maps or data layers into concurrence via known ground location control points or the procedure of bringing a map or data layers into concurrence with the earth's surface.
- Registration tic-geographic control points for a map or data layer within a GIS representing known locations on the earth's surface. These tics allow all coverage features to be recorded in a common coordinate system (e.g., UTM or State Plane)
- Root Mean Square Error (RMSE)-measure of registration accuracy used during digitizing and coverage transformations.
- Rubber sheeting-topological process of stretching or shrinking a subarea or portion of a map or image to fit in registration with selected control points.
- Scale-ratio or fraction between the distance represented on a map, chart, photograph, etc., and the corresponding distance on the surface of the earth.
- Scanning-process of using an automated electronic input device to convert analog information (from maps, photographs, overlays, etc.) into a digital format usable by a computer (see also digitizing).
- Semi-major axis of ellipsoid-defines the size of the Earth by the radius at its widest part. The value measured by Clarke in 1866 (6,378,206 meters) is a common standard.
- Semi-minor axis of ellipsoid-defines the size of the Earth by the radius at its narrowest part. The value measured by Clarke in 1866 (6,356,584 meters) is a common standard.
- Slivers-refers to polygons formed when two adjacent polygons do not abut along a single common line and leave a small space between the larger of the two.
- Source material-data of any type required for the production of mapping, charting, and geodesy products including, but not limited to, ground-control aerial and terrestrial photographs, sketches, maps, and charts: topographic, hydrographic, hypsographic, magnetic, geodetic, oceanographic, meteorological information; intelligent documents; and written reports pertaining to natural and man-made features.
- Spatial data-data pertaining to the location of geographical entities together with their spatial dimensions. Spatial data are classified as point, line, area, or projection.
- Spheroid-spheroid upon which the projection will be based. A common standard is the Clarke 1866 if no other spheroid is specified or if it is not inherent to the projection.
- Standards-exact value, a physical entity, or an abstract concept, established and defined by authority, custom, or common consent to serve as a reference, model, or rule in measuring quantities or qualities, establishing practices or procedures, or evaluating results.
- Standard parallel-for conic projection, the standard parallel refers to the one or two tines of latitude along which the cone contracts the earth.
- Thematic categories-mapping categories, consisting of a single type of data, such as elk habitat, water quality, or timber stands, intended to be used with base data.
- TIGER-Topologically Integrated Geographic Encoding and Referencing files, a set of structures used to deliver digital vector data and attributes associated with the US Census.
- Topographic map-a map that represents the horizontal and vertical positions of features on the face of the earth. Vertical positions are defined by contours or other symbology.
- Topology-branch of geometrical mathematics concerned with order, contiguity, and relative position, rather than actual linear dimensions.
- Transformation-procedure to transfer features from one projection surface to the corresponding position on another projection surface by graphical or analytical methods.
- Triangulated Irregular Network (TIN)-data structure, normally used in connection with terrain modeling, which describes a three-dimensional surface as a series of irregularly shaped triangles.
- Units-specifies the units of the coordinates being used.
- Vector-directed line segment, with magnitude commonly represented by the coordinates for the pair of end points. Vector data refer to data in the form of an array with one dimension.
- Vegetation Fraction of Photosynthetically Active Radiation (fPAR)-measures the proportion of available radiation in the photosynthetically active wavelengths that a canopy absorbs.
- Vegetation Leaf Area Index (LAI)-descriptor of plant canopy structure as half the total developed area of green leaves per unit ground horizontal area.
- Web Coverage Service (WCS)-an Open Geospatial Consortium standard that provides a simple HTTP interface for obtaining raster data sets from the Internet. The raster data sets made available through Web Coverage Services are coverages, not to be confused with vector coverage data sets. A Web Coverage Service returns data in a format that can be used as input for analysis and modeling, unlike the Web Map Service which only returns a visual of the data.
- Web Feature Service (WFS)-an Open Geospatial Consortium standard that provides a simple HTTP interface for obtaining geographic features with geometry and attributes from the internet that clients can use in geospatial analysis, unlike the Web Map Service which only returns a visual of the data.
- Web Map Service (WMS)-an Open Geospatial Consortium standard that provides a simple HTTP interface for requesting geo-registered map images from on ore more distributed geospatial databases. A WMS request defines the geographic layer(s) and area of interest to be processed. The response to the request is one or more geo-registered map images (returned as JPEG, PNG, etc) that can be displayed in a browser application.
- X-shift-a constant in the horizontal direction to add to the input coordinates. The use of X-shift will cause a value specified by distance to be added to all coordinates.
- Y-shift-a constant in the vertical direction to add to the input coordinates. The use of Y-shift will cause a value specified by distance to be added to all coordinates.
- Zone-a number identifying the UTM or State Plane Coordinates System projections grid.
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*Other sources include the United States Geological Survey, the National Aeronautics and Space Administration, the Federal Geographic Data Committee, the US Board on Geographic Names, and the Open Geospatial Consortium.