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GIS glossary: h-t


"A Practitioner's Guide to GIS Terminology"
The following terms are condensed and reproduced with permission from "A Practitioner's Guide to GIS Terminology" by Stearns J. Wood, which contains more than 10,000 terms (see GIS glossary).

hachure
Short lines used for shading and denoting surfaces in relief (as in map drawing) in the direction of greatest slope. The thickness and spacing of the lines indicate the amount of relief. The lines are short, heavy and close together for steep slopes; they are longer, lighter and more widely spaced for gentle slopes. Artistically done, hachure is effective in showing relief, although actual elevations are not indicated.

hierarchical districts
A series of jurisdictions which are superimposed with, or overlap on, other jurisdictions either smaller or larger in size and usually which have a vertical relationship in authority and responsibility, i.e., alderman districts, township, city, county, congressional district, state, national.

image enhancement
The application of any of a group of operations that alter or exaggerate the tonal differences in an image. Manipulation of the point-dependent density of an image to bring out or emphasize certain features of the image. These operations may include stretching (ratioing, contrast stretching, edge enhancement, filtering) and/or smoothing to improve the detection or identification of features of interest.

image scanning
Document scanning involves using scanners to capture a digital image of a map, manuscript, or other document. Once the image is stored, a variety of processes can be applied, depending on the image's intended use. For instance, a scanned image of building plans or a real estate deed could be stored in an image database. In the case of maps, the graphics can be converted from its raw scanned ("raster") form to lines and other object ("vector") data. As objects, they can be edited and integrated with other line, area, and point data.

infrastructure mapping
The basic, underlying framework or feature, facility, equipment, public service and installation, or supporting facility structure needed for the growth and functioning of a country, county, city, community, organization, or other governmental or administrative jurisdiction. They are the basic facilities and services needed to sustain industry, residential, and commercial activities.

Infrastructure may include, but not be limited to, the public facilities and services such as roads, streets, highways, bridges and related facilities (e.g., the paved streets, curbs and gutters, and other basic improvements below and above the ground); storm water runoff and sanitary sewer lines, systems and treatment plants; water-supply distribution lines, systems and reservoirs; electrical and communication lines such as high-tension lines, telephone, and cable television transmission lines; railroad lines; other utility systems; and public buildings and other community service buildings. Infrastructure also includes environmentally safe siting, an adequately trained labor force, and a transport network that includes an adequate commercial transportation system of roadways, rail systems, and airfreight.

infrastructure management system
A GIS application program, which links infrastructure graphics and administrative and inventory data to provide a GIS database used in the planning, design, construction, maintenance lifecycle of infrastructure, including lane-miles of roadways, bridges and viaducts, retaining walls, water systems for treatment, telephone facilities, gas facilities, electrical facilities, domestic water systems.

integrated land management system
A geographic or spatial information system containing data on land and land use especially that on real estate, vegetation, recreation, wildlife, etc., and the related analysis software to identify and avoid conflicting land uses, mitigate competing land uses and enhance land management planning, and to perform land use suitability analysis. Abbreviated ILMS. It utilizes a database describing physical or legal characteristics of land areas, sometimes called a land records system (LRS). The most common forms of land information systems describe property ownership, land use, land value, tax assessment, and property boundaries. The data management and graphic system contains basic land-related data such as entitlement, assessment, use, resource and capability; it also includes other types of data including land survey (property boundary), environmental, topological, geological, agricultural, housing, commercial and industrial, transportation, utilities and other facilities.

land development analysis system
Site Suitability Analysis - System having the means to understand the environmental constraints and opportunities of property. It involves the detailed investigation of the natural resources and processes that characterize a site and includes computer modeling and mapping techniques that geographically display the areas of the site that are most and least suitable for various planning objectives and alternatives. Impact Assessment - System to determine quantitative impact on a site or an area of proposed new development. It outlines possible mitigating measures and provides assistance in determining future land-use policies. GIS can provide data layers for proposed development alternatives including habitat type, seral stage, soils, geologic, wetlands, wildlife, and topology for predevelopment, existing, and post-development conditions.

legal area map level
A category level of a virtual map. It describes the constraints and controlling interest which exist over the land area. It describes parcel deed descriptions, subdivision plans, ordinances controlling zoning, rights-of-way and easements, and related judicial interests in the land area, which altogether comprise the legal area definitions of the area map. Legal area definitions are overlays to the base geography and, indeed, could not exist without serious conflict if precise locational methodology were not used. The classic problem is the municipal boundary description which references the "old oak tree" which happened to have died fifty years ago and today no one can determine where it grew.

knowledge base system
A computer system using rule-based logic to simulate human intelligence and to perform tasks that if performed by human beings would require high levels of knowledge and several years of specialized training. A computer-based expert system takes all the questions, rules and decisions and puts them into the system. Expert systems allow everyone access to the same level of decision-making expertise as the expert that helped build the knowledge base in the system and makes their expertise available on demand to all users of the application program. It is a system with focuses on computer science and applications to assist knowledge workers (e.g., doctors, engineers, etc.). The expert system primarily involves parallel processing utilizing combinatorial mathematics to permit the development of "search engines" which speeds up the processing of logical inferences to expand the domain of knowledge-based systems. Also known as symbolic reasoning and artificial intelligence and is synonymous for Expert System.

map accuracy
(1) The closeness of results of observations, computations, or estimates of graphic map features to their true value or position. Relative accuracy is a measure of the accuracy of individual features on a map when compared to other features on the same map. Absolute accuracy is a measure of the location of features on a map compared to their true position on the face of the Earth. Mapping accuracy standards generally are stated as acceptable error and the proportion of measured features that must meet the criteria. In the case of some plotting and display devices, accuracy refers to tolerance in the display of graphic features relative to the original coordinate file.

(2) The level of allowable error of maps, as applied by National Map Accuracy Standards, is determined by comparing the positions of well-defined points whose locations or elevations are shown on the map with corresponding positions as determined by surveys of a higher accuracy. The accuracy of any map is equal to the error inherent in it as due to the curvature and changing elevations contained in each map from which the map was made, added to or corrected by the map preparation techniques used in joining the individual maps.

map projection
(1) A representation on a plane of the surface of a round body. A mathematical model is used to transform positions on the surface of the earth, which is curved, onto a flat map surface. It provides systematic drawing of lines of a plane surface to represent the parallels of latitude and the meridians of longitude of the earth.

(2) A systematic conversion of locations on the Earth's surface from spherical to planar coordinates. Because the earth is three-dimensional, some method must be used to depict a map in two dimensions. A mathematical model transforms the locations of features on the Earth's 3-dimensional surface to locations on a two-dimensional map surface. Some projections preserve shape; others preserve accuracy of area, distance, or direction; however, any such representation distorts some parameter of the Earth's surface.

multipurpose cadastre system
An integrated land information system containing legal (e.g., property ownership or cadastre), physical (e.g., topography, man-made features), and cultural (e.g., land use, demographics) information in a common and accurate reference framework. The reference framework typically is established with rigorous geodetic and survey control standards, such as the state plane and latitude/longitude coordinate systems. The Cadastre is made up of multiple independent, interrelated layers commonly used to describe the graphic component of a GIS database. Each layer contains a set of homogeneous map features registered positionally to other database layers through a common coordinate system. Data are separated into layers based on logical relationships and the graphic portrayal of sets of features.

navigable database
The database designed and prepared for Intelligent Vehicle Highway Systems to support such systems for "smart cars" and "smart highways." These databases contain first the geometry element layer of the roadway that contains links, nodes, shape points, relative elevations, and connectivity. The second navigation element layer contains physical and logical classification of the roadway environment, directionality, dividers, barriers, turn restrictions, freeway exit numbers, and exact freeway sign text speed limits, and others. Additional element layers include points-of-interest, path, geopolitical, cartography, geocoding, and special commercial/business listings.

network analysis
The GIS process providing analytical techniques for geographic or statistical analysis of relationships and flow that are dependent on the connectivity of segments and nodes in a linear system, such as the circuits of a road or utility network. Abbreviated NA. NA process provides a set of techniques for the analysis of systems represented by networks and allows the ability for: a) analytical techniques concerned with the relationships between locations on a network, such as the calculation of optimal routes through road networks, capacities of network systems, best location for facilities along networks, etc.; b) analytical techniques to determine the characteristics and relationship of a linear network used to describe the urban environment in terms of the street and non-street feature network; c) analytical capabilities involving network simulation of and use in vehicle routing, optimum facilities siting, polygon redistricting, route selection, and time/distance flow analysis; d) simple forms of network analysis are covered in shortest route and connectivity. More complex analyses are frequently carried out on network data by electrical, water, and gas utilities, telephone and cable companies, fire departments and rescue agencies, telecommunications companies, large retail business and product distributors, etc. The techniques and analysis these companies use and carry out include the simulation of flows in complex networks, load balancing in electrical distribution, traffic analysis, computation of pressure loss in water lines, maximum flow analysis, traveling salesman problem, allocation of resources, and optimum path.

nongraphic data
Digital representations of the characteristics, qualities, or relationships of map features and geographic locations, usually stored in conventional alphanumeric format. It is digital data that cannot be displayed graphically as symbols or annotation on a map. This is usually attribute information, which accompanies maps in tabular form. The term nongraphic is often used to differentiate those data that do not describe the graphic images of the map features. Nongraphic data are often called textual data or attributes. They are related to geographic locations or graphic elements and linked to them in the GIS through common identifiers or other mechanisms. Often, nongraphic data are managed separately from the graphic data due to their different characteristics or their maintenance and use in other systems.

open systems
Computing environments that allow users to access and utilize data and software across multiple platforms. Incompatible hardware platforms, operating systems, and GIS application software are tied together through the use of industry-standard system components. Signifies the development and adoption of standards to facilitate systems integration creating the means for resource sharing, an environment in which hardware, software, and data can be efficiently accessed and applied by users in an organization. It is a vendor independent system that is designed to interconnect with a variety of products that are commonly available. It implies that standards for such a system are determined from a consensus of interested parties rather than one or two vendors.

orthophotograph
An aerial photograph in which the displacement of images has been removed and that has the distortion due to tilt, curvature, and ground relief corrected. It is a "scale corrected" aerial image, depicting ground features in their exact ground positions, in which distortion caused by camera and flight characteristics and relief displacement have been removed using photogrammetric techniques.

overlay analysis
(1) The process of combining spatial information from two or more maps to derive a map consisting of new spatial boundaries.

(2) The summary of areas by one or more geographically defined zones (i.e., census tract, precinct, etc.) such that areas common to each other and those which have overlapping polygon areas are distinguished and summarized.

(3) The manipulation of spatial data organized in layers to create combined spatial features according to logical conditions specified in Boolean algebra.

parcel maps
The basic drawings (maps) of the land cadastre (ownership boundaries) for all public and private lands. Parcel maps are typically maintained at a variety of scales, and can be either very precise or very general "cartoon maps" which show only schematic layouts of the basic land cadastre. They are commonly related in some manner to legal description documents, which typically constitute the record for describing boundary ownership for a given parcel (e.g., metes and bounds descriptions). Parcel maps of an area provide more than a representation of ownership. The maps serve a wide range of purposes if they are constructed with a known level of positional accuracy and integrated with other cultural and physical information. Such databases are frequently used as a multipurpose cadastre.

planimetric map
map features
A map showing only the horizontal position of features on the Earth's surface which show geographic objects, natural and cultural physical features, and entities without topographic features such as roads, buildings, and water bodies that are visible and identifiable on aerial photographs, but which can be compiled into map features through photogrammetric or surveying procedures. A planimetrically accurate map showing planimetric detail and other general features shows accurate horizontal distances between features. Map features show roadway feature details as roads, sidewalks, streets, highways and alleys including curb lines, edge of paved surfaces or edge of traveled way, and general feature details as building footprints, reservoirs, tanks, docks, piers, airports, bridges, overpasses, underpasses, railroads, parking lots, driveways, other impervious surfaces, streams, lakes, drainage courses, holding basins, shorelines, other watercourses, vegetation outlines, elevations, fence lines, drainage, and other similar construction or terrain features.

polygon map
polygon mapping
Defines the borders of homogeneous features as well as the characteristics associated with those features which identify special land related information (e.g., a soils map describes the boundary characteristics of soil types from which the following types of information can be obtained: areas which have unstable land for construction; areas that may be within a floodplain; wetland areas; areas with high potential productivity for farming, etc.) Polygon mapping is the cartographic display of regularly or irregularly shaped polygons and their attributes. Typically, this capability includes shading, symbology and numeric labeling, as well as other map cosmetic functions for generating alphanumeric labeling of polygons.

quadrangle (quad) maps
A rectangular, or nearly rectangular, area covered by a map. The outline is generally defined by latitude or longitude. Typically refers to USGS topographic maps or to a map sheet published by the U.S. Geological Survey. Also known as a topographic or topo map.

raster data
One of the three types of spatial data in a GIS (the others being image and vector data). Raster data represents geographic space as a matrix of cells; map features are defined by numeric values assigned to the cells. Cell data are arranged in a regular grid pattern in which each unit (or cell) in the grid is assigned an identifying value based on its characteristics. Information obtained from image sources such as remote sensing from photography and satellite. Raster data thus refers to data in the form of parallel scan-line segments, grid cells or pixels. Also known as Cell System or Grid System.

remote sensing
The act of detection, acquisition and recording, identification and interpretation, or analysis of geographic imagery or data and information about the properties of objects or conditions or phenomenon using a sensing and recording device from a distance without having the sensor in direct physical contact with the object of the study. Typically, data and information is gathered using remote sensing methods, which might be hand-held, airborne, or by satellite sensor. It includes photography, aerial photography or satellite imagery, radar, and satellite imaging extended from points in space. Satellites with sensors sensitive to various bands of the electromagnetic spectrum provide a remote sensing platform for LANDSAT analysis of elements as, e.g., land use, geology and soils data. Remote sensing is a discipline that evolved from photogrammetry to remote sensing of the Earth's resources using aerial or space photographs, electronic scanners and sensors, and other devices to collect data about the Earth's surface or subsurface at a substantial distance from the targeted area.

resolution (spatial)
A measure of the accuracy or detail of a graphic display, expressed as dots per inch, pixels per line, lines per millimeter, etc. It is a measure of how fine an image is, usually expressed in dots per inch (dpi). The minimum difference or distance between two independently measured or computed values or objects that can be distinguished by the measurement or analytical method, or sensor being considered or used. It provides a limit to precision and accuracy. Often called spatial resolution but also applies to spectral and temporal aspects of remote sensing imaging systems.

Resolution is the accuracy at which a given map scale can depict the location and shape of map features; the larger the map scale, the higher the possible resolution. As map scale decreases, resolution diminishes and feature boundaries must be smoothed, simplified, or not shown at all. It is the size of the smallest feature that can be represented in a surface. For example, small areas may have to be represented as points.

route analysis
A method of determining the minimum distance between points to determine efficiency of pedestrian and vehicle paths, travel times of routes, and most effective routes and means of linear distribution. Used for emergency vehicle dispatch, routing of buses and maintenance vehicles. The data representing several variables is analyzed to produce an optimum location for a route. Utilizing a digitized network, line segments and nodes are chained together to determine the minimum distance between point A and point B and thus measure the efficiency of vehicle (or other phenomena) paths.

scientific/environmental data needs
Natural resource-oriented users will find that up-to-date satellite imagery, aerial photographs, as well as topographic, climatic and hydrographic data are all useful in environmental planning and resource monitoring. The increasing concern with the environment makes the environmental hazards data such as toxic material locations especially useful. Scientific/environmental tasks include agricultural planning, environmental hazard analysis, environmental impact assessment, environmental monitoring, fire control and modeling, geological surveying, hazardous material inventory, land use planning, resource inventory, timber harvest planning, water runoff calculations, wetlands inventory, wildlife habitat analysis.

spatial analysis
Analytical techniques to determine the spatial distribution of a variable, the relationship between the spatial distribution of variables, and the association of the variables of an area. Spatial analysis is often referred to as modeling. It refers to the analysis of phenomena distributed in space and having physical dimensions (the location of, proximity to, or orientation of objects with respect to one another; relating to an area of a map as in spatial information and spatial analysis; referenced or relating to a specific location on the Earth's surface).

Spatial analysis is the process of extracting or creating new information about a set of geographic features to perform routine examination, assessment, evaluation, analysis or modeling of data in a geographic area based on pre-established and computerized criteria and standards. Spatial analysis is a process of modeling, examining, and interpreting model results useful for evaluating suitability and capability, for estimating and predicting, and for interpreting and understanding.

In GIS, there are four traditional types of spatial analysis: spatial overlay and contiguity analysis, surface analysis, linear analysis, and raster analysis. It includes such GIS functions as topological overlay, buffer generation, and spatial or network modeling. Abbreviated SA.

spatial data
Consists of information about the relationships of entities in space, facts about the real world organized geographically, the location, shape of, and relationships among geographic features, which are usually stored as coordinates and topology. This is in contrast with spatially referenced data which are thematic or applied data (such as addresses coded by distribution routes planned by street address). Discrete symbols (numbers, letters, or special characters) used to describe some entities are organized according to the location of that entity in the three-dimensional world.

tesselation data model
A geographic data model where the basic logical unit is a location in space shown by a single cell or polygon in a mesh usually represented or classified as a regular grid or rectangular pixel; however, it can be represented by an infinitely repeatable pattern of a regular polygon or polyhedron. It can also be classified as hierarchical or as irregular. The presence of specific entities are recorded as descriptors of each location.

thematic map
A map that displays the spatial distribution of an attribute that relates to a single topic, theme, or subject of discourse. Usually, a thematic map displays a single attribute (a "univariate map") such as soil type, vegetation, geology, land use, or landownership. For attributes such as soil type or land use ("nominal" variables), shaded maps that highlight regions ("polygons") by employing different colors or patterns are generally wanted. For other attributes (like population density - a "metric" variable), a shaded map in which each shade corresponds to a range of population densities is generally wanted. Thematic maps are used to display geographical concepts such as density, distribution, relative magnitudes, gradients, spatial relationships and movements. Also called geographic, special purpose, distribution, parametric, or planimetric maps.

topographic map
A map depicting terrain relief showing ground elevation, usually through either contour lines or spot elevations. The map represents the horizontal and vertical positions of the features represented. It is a graphic representation delineating natural and man-made features of an area or region in a way that shows their relative positions and elevations.

End of GIS Glossary





GIS glossary

The GIS terms under the entries GIS glossary: a-g and GIS glossary: h-t are condensed and reproduced with permission from "A Practitioner's Guide to GIS Terminology" by Stearns J. Wood. Compiled over 30 years and first published in 1984, the book contains more than 10,000 terms embracing all aspects of geoprocessing and geoanalysis, spatial and network analysis, resource management, facilities management, automated mapping, computer-aided design and drafting, database management systems, open systems connectivity and geographic information system computer technology. Selected terms from geography, cartography, computer science, urban and regional information systems, remote sensing and GPS are also included. See GIS glossary: a-g and GIS glossary: h-t.




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