Glossary

Accuracy is the degree to which information on a map or in a digital database matches true or accepted values. Accuracy is an issue pertaining to the quality of data and the number of errors contained in a dataset or map. In discussing a GIS database, it is possible to consider horizontal and vertical accuracy with respect to geographic position, as well as attribute, conceptual, and logical accuracy. The effect of inaccuracy and error on a GIS solution is the subject of sensitivity analysis . Accuracy, or error, is distinguished from precision , which concerns the level of measurement or detail of data in a database.

Algorithm is a procedure used to solve a mathematical or computational problem or to address a data processing issue. In the latter sense, an algorithm is a set of step-by-step commands or instructions designed to reach a particular goal.

AM/FM is an acronym for automated mapping and facilities management. AM/FM applications are a specialized GIS. The systems are used by public and private utilities, city and county governments, and other organizations that require careful management of a widely distributed collection of capital equipment, stock, or "facilities." AM/FM most commonly describes databases designed to manage networks of roads, pipes, and wires. These GIS applications are the particular concern of AM/FM INTERNATIONAL, a professional organization headquartered in Aurora, Colorado.

ANSI is an abbreviation for American National Standards Institute. ANSI standards have been established for many elements of computer systems to aid research and development. The existence of standards allows designers to develop general solutions to common problems.

ASCII is an abbreviation for American Standard Code for Information Interchange. The ASCII format provides computer systems with a common language for exchanging information. Although most GIS software systems make use of proprietary binary codes, almost all systems have import-export capabilities for translating between ASCII and binary formats.

Attribute data is descriptive information about features or elements of a database. For a database feature like census tract, attributes might include many demographic facts including total population, average income, and age. In statistical parlance, an attribute is a "variable," whereas the database feature represents an "observation" of the variable.

Base Maps provide the background upon which thematic data is overlayed and analyzed. As inputs into a GIS, the term base map is usually applied to those sources of information about relatively permanent, sometimes timeless, features including topography, pedology, geology, cadastral divisions, and political divisions. Within a GIS database, such information may become part of a land base to which other information is indexed and referenced.

Benchmark Tests are standardized routines for comparing the performance of competing hardware and software under conditions intended to reflect common patterns of use. Widely used benchmark tests have been developed to compare the speed of computers. But more specific benchmark tests also may be developed by a single organization to test new equipment under conditions close to those which will be encountered in day-to-day use.

Binary code and binary files are information and commands stored and used by hardware and software in their most elemental form--strings of on-off signals to an electronic processor. Many systems of binary encoding of data and commands are proprietary and unique to particular hardware and software systems. They are usually the most compact means of storing data, and commands stored in binary form can execute very rapidly, but binary files often are difficult to transfer between differing computer systems. Often binary files are translated into ASCII form for transfer between computers.

Bundled and Unbundled refers to the way software and hardware is sold. Often software is sold together--bundled--with hardware as a complete ready-to-use turnkey system. It is more common recently to purchase hardware and software separately, in unbundled form.

CAD or CADD refers to computer-aided design and drafting. CAD systems are used to create maps and plans and are closely related to GIS systems. Although most CAD systems lack certain features essential to GIS analysis, such as the power to manage different spatial coordinate systems and database capabilities, many CAD systems have been developed into full GIS with the addition of necessary functions.

Cadastral Survey is the means by which private and public land is defined, divided, traced, and recorded. The term derives from the French cadastre, a register of the survey of lands and is, in effect, the public record of the extent, value, and ownership of land for purposes of taxation.

Cartesian Coordinates are a system of positional reference in which location is measured along two or three orthogonal (perpendicular) axes. Every location can be defined uniquely by its X, Y, and Z coordinates. Locations in the coordinate system can be established using any unit of measurement such as meters, feet, and miles. Cartesian coordinates differ from latitude- longitude coordinates in that the latter comprise a spherical (rather than planar) reference system.

Centroid is the term given to the center of an area, region, or polygon. In the case of irregularly shaped polygons, the centroid is derived mathematically and is weighted to approximate a sort of "center of gravity." Centroids are important in GIS because these discrete X-Y locations are often used to index or reference the polygon within which they are located. Sometimes attribute information is "attached," "hung," or "hooked" to the centroid location.

COGO is an abbreviation for coordinate geometry, a method of data input and analysis similar in principle to metes-and-bounds surveying. Input begins at a point, moves at a given angle or in a certain direction for a set distance, and continues in the same fashion until a geographic feature is completely outlined.

Connectivity is a topological property relating to how geographical features are attached to one another functionally, spatially, or logically. In an water distribution system, connectivity would refer to the way pipes, valves, and reservoirs are attached, implying that water could be "traced" from its source in the network, from connection to connection, to any given final point. Functional, spatial, and logical connectivity are examples of relationships that can be represented and analyzed in a GIS database.

Continuous Map is a means of storing a digital map as if it represented a continuous, unbroken surface. Many large GIS databases are broken into subfiles which represent smaller sections of a larger area. These separate subfiles are referred to as tiles or facets. A continuous map allows the user to move in any and all directions without moving from file to file. Maintaining the illusion of a continuous map in a computer is a challenging problem because, of course, computers typically store data in strings, in linear order, on magnetic or optical storage media.

Conversion is the process of transferring data derived from existing records and maps to a digital database. Conversion is a major input problem and can consume the greatest share of time in a GIS project.

Coverage is a term applied in the ARC system to the various layers of geographic features.

DBMS stands for database management system. A DBMS is an organizational plan for the use of information within a single project, or within one unit or the whole of an organization.

DEM is short for digital elevation model, a data exchange format developed by the United States Geological Survey for geographical and topographical data.

Digitize is the process of converting source materials, prepared manually, into the digital codes stored and processed by computers. Digitizing involves tracing map features into a computer using a digitizing tablet, graphics tablet, mouse, or keyboard cursor.

DIME is the abbreviation for dual independent map encoding. This is a digital map format developed by the U.S. Bureau of the Census for population census. DIME format was superseded byTIGER format in the 1990 census.

DLG stands for digital line graph, a form of digital map developed by the United States Geological Survey. DLGs supply users with the digital version of information printed on USGS topographical quadrangle maps.

DPI is an abbreviation for dots (or pixels) per inch, a measure of the resolution of graphic displays.

DTM stands for digital terrain model, a method of transforming elevation data into a contoured surface or a three-dimensional display.

DXF is an abbreviation for drawing interchange format, a file exchange format developed by Autodesk Inc. for its AutoCAD drafting software. DXF files are ASCII records of all objects in a drawing file. DXF has been adopted more recently by GIS systems for exchanging map files.

Edge Matching is an important part of the creation of a digital map or GIS database. One digital map may encompass many paper maps. But when the paper maps are laid edge-to-edge, features running across the boundaries of the map sheets are not always properly aligned. These misalignments occur for many reasons including survey error, cartographic generalization, and problems of map projection and compilation. Edge matching is a term applied to a variety of techniques employed to resolve these inconsistencies, including warping and rubber sheeting.

Ethernet is a type of local-area network used for high-speed communication among computers.

Facet is a subset of a GIS database that contains information about one subarea of the overall digital map. Facets are an effective way of dividing a continuous map into units which can be easily created, edited, and analyzed. The terms facet and tile are synonymous and are, in some ways, the opposite of a continuous map.

GIS is the abbreviation for geographic information system. GIS are special-purpose digital databases in which a common spatial coordinate system is the primary means of reference. GIS contain subsystems for: 1) data input; 2) data storage, retrieval, and representation; 3) data management, transformation, and analysis; and 4) data reporting and product generation. It is useful to view GIS as a process rather than a thing. A GIS supports data collection, analysis, and decision making and is far more than a software or hardware product. Other terms for GIS, and special-purpose GIS, include: Land-Base Information System, Land Record System, Land Information System, Land Management System, Multipurpose Cadastre, and AM/FM System.

GPS is an acronym for global positioning system. Developed for the military for navigation and surveying, the GPS relies on satellites (and ground stations) for precise determination of location. Although GPS can be used to determine location very precisely (within centimeters given the correct controls and proper use), it does not solve all the problems of locational determination in GIS databases.

Hierarchical Database stores related information in terms of pre-defined categorical relationships in a "tree-like" fashion. Information is traced from a major group, to a subgroup, and to further subgroups. Much like tracing a family tree, data can be traced through parents along paths through the hierarchy. Users must keep track of the hierarchical structure in order to make use of the data. The relational database provides an alternative means of organizing datasets.

Host Computer is a machine to which other computers (guests) are connected so that the host can manage time-intensive computing tasks. The guest computers pass information and requests to the host as its services are required. Host-guest systems are used in GIS because large or fast host computers can assume infrequently needed functions so that guest computers are free to continue with less demanding processing.

Infrastructure refers to the roads, cables, wires, pipes, bridges, canals, reservoirs, and sewers that support economy and society. Infrastructure development and management are important in urban and rural areas at the local, state, regional, national, or international levels.

LAN stands for local-area network, a system for connecting computers so they can communicate with one another.

Land Base includes the natural and man-made environmental features within which infrastructure is developed or by which natural resources are indexed and analyzed.

Landsat is a system of satellites which scan the earth at a variety of wavelengths. The satellites return information that can be used to inventory and analyze a variety of natural and human resources.

Layer is a subdivision of a CAD or GIS database containing related data. Layers can be visualized as "transparencies" which allow the user to view and analyze information selectively by theme. Some GIS build their databases as a series of layers covering a single area. Layers are fundamental to overlay analysis.

Libraries contain frequently used symbols, subroutines, and commands. Rather than recreating a symbol everytime it is required, a user can call the symbol from a library using a unique identification code. The same is true of subroutines, sometimes referred to as macros.

LIS is the acronym for land information system. LIS and GIS are virtually synonymous, although it could be maintained that an LIS is but one component of a GIS.

Macros are collections of frequently used commands which are grouped together in a file. Instead of entering commands individually, users can call the macro and the commands stored therein will be executed automatically. Macros may be stored in libraries .

Map Scale is the relationship between distance on a map and the corresponding distance on the earth's surface. Map scale is often recorded as a representative fraction such as 1:1,000,000 (1 unit on the map represents a million units on the earth's surface) or 1:24,000 (1 unit on the map represents 24,000 units on the earth's surface). The terms "large" and "small" refer to the relative magnitude of the representative fraction. Since 1/1,000,000 is a smaller fraction than 1/24,000, the former is said to be a smaller scale. Small scales are often used to map large areas because each map unit covers a larger earth distance. Large-scale maps are employed for detailed maps of smaller areas.

Mass Storage refers to computer equipment designed to record, store, and read "masses" of data. A variety of magnetic and optical media are currently available for mass storage, including magnetic disks, magnetic tapes, magnetic cassettes, and optical disks. The selection of a suitable mass storage device for a GIS database usually involves establishing a compromise among the factors of size, speed, and cost.

Metes and Bounds is a traditional method of land surveying in which the boundaries of land parcels are recorded in terms of relative direction and distance. Local landmarks and natural environmental features may serve as points of origin and destination along a given traverse.

Monumentation in surveying refers to the practice of marking known horizontal and vertical control points with permanent structures such as concrete pedestals and metal plaques. Once surveyed and marked, these monuments can be used for further surveying and for the alignment of land-parcel boundaries and infrastructure.

Network is a system of spatially or functionally interconnected geographic features such as a system of roads, wires, pipes, or cables. With respect to computers, networks describe communication connections among machines.

Network Analysis is a range of techniques employed by engineers and planners to study the properties of networks including connectivity, capacity, and rates of flow. Network analysis can be used to estimate the capacity of a road network or to plan for additions to an electrical distribution system.

OEM is an abbreviation for original equipment manufacturer. OEMs manufacture computer equipment which they sell to other computer makers and vendors who, in turn, sell it to end users. The plotters in the lab are manufactured by Hewlett-Packard--in this case an OEM--who sells them to IBM. IBM repackages the plotters and sells them to users.

Online indicates when equipment such as plotters, printers, and digitizers is turned on and is actively communicating with a computer, or is controlled by that computer. Computers in a network can also be said to be online when the network connection operative.

Operating Systems are the software systems which manage the interaction between users and hardware. Operating systems are employed by users to tell a computer what to do with the resources at its disposal in order to run programs, store data, or send files to peripherals such as plotters and printers. Popular operating systems include DOS, VM and MVS (or IBM mainframes), VMS (for DEC machines), UNIX (including proprietary variations such as AIX, Ultrix, and UNICOS), OS/2, and Apple OS.

Paneling is a means of subdividing the display of a GIS database or digital map. Paneling allows users to view selectively the different sections of a digital map.

Parcel is a fundamental cadastral unit: a piece of land which can be owned, sold, and developed. Parcels have legal descriptions which not only describe their boundaries but also contain information concerning rights and interests.

Peripherals are items of computer equipment such as plotters, printers, scanners, digitizers, and graphics displays. Although essential to a complete GIS system, these pieces of equipment are considered peripheral to central computer resources such as the processor, memory, keyboard, and display screen.

Photogrammetry uses aerial photographs to produce planimetric and topographic maps of the earth's surface and of features of the built environment. Effective photogrammetry makes use of ground control by which aerial photographs are carefully compared and registered to the locations and characteristics of features identified in ground-level surveys.

Platform is a another term for computer hardware, including microcomputers, workstations, and mainframe computers. When discussing software, platform independence implies the software can be run on any computer.

Precision refers to the level of measurement and exactness of description in a GIS database. Precise locational data may measure position to a fraction of a unit. Precise attribute information may specify the characteristics of features in great detail. It is important to realize, however, that precise data--no matter how carefully measured--may be inaccurate. Surveyors may make mistakes or data may be entered into the database incorrectly. Therefore, a distinction is made between precision and accuracy .

Sensitivity analysis can be used in a prototype GIS project to estimate the degree of precision required to reach a particular analytical objective.

Protocol is a set of rules or standards which govern communication between computers and between computers and peripherals.

Raster displays and databases build all geographic features from grid cells in a matrix. A raster display builds an image from pixels, pels, or elements of coarse or fine resolution. A raster database maintains a similar "picture" of reality in which each cell records some sort of information averaged over the cell's area. The size of the cell may again be coarse or fine, ranging from centimeters to kilometers. Many satellites, like Landsat and SPOT, transmit raster images of the earth's surface. Reflectance at a certain wavelength is measured for each cell in an image. The cells may cover areas on the earth's surface several hundreds of meters square, the area covered being a function of a particular satellite's resolution.

Relational Database stores data in such a way that it can be added to, and used independently of, all other data stored in the database. Users can query a relational database without knowing how the information has been organized. Although relational databases have the advantages of ease-of-use and analytical flexibility, their weakness can be slower retrieval speed. SQL is one example of a relational database.

Resolution is the smallest detectable distance between features recorded on a digitizer, displayed on a graphics screen, or drawn by a plotter.

RFI stands for request for information. An RFI is one way for a company or government organization to gather information from vendors about products and services. An RFI may be a written document (somewhat like an RFP ), a letter outlining general project concepts, or personal communication.

RFP means request for proposal. An RFP is a formal invitation issued by a business or government agency requesting interested vendors to submit written proposals meeting a particular set of requirements. If interested in bidding for the project, vendors respond with a description of the techniques they would employ to meet the requirements, a plan of work, and a detailed budget for the project, along with supporting information.

Rubber Sheeting is a technique for edge matching and is another name for warping .

Scanning is a process by which data and maps are converted to digital form using optical or video equipment. Scanning differs from digitizing in that entire pages of data or map sheets are captured as images all at once. Digitizing involves entering discreet points and digits one at a time. Scanning may eventually lower the cost of adding information to a database. At the moment, scanning and digitizing take about the same amount of time. This is because scanned images, once in a computer, still require coding--the operator must indicate the relationships and data to be stored and where to store the information. This operation is normally part of the digitizing process. In the future, the use of artificially intelligent coding algorithms may make scanning faster and less expensive.

Sensitivity Analysis is a term applied to a range of techniques employed to assess how solutions derived from GIS systems are affected by the accuracy and platforms.

TIGER is an acronym for topologically integrated geographic encoding and referencing file. This is a type of digital map developed by the U.S. Bureau of the Census to support the 1990 population census. Census maps in TIGER format succeed the previous DIME format. TIGER files are available for every county in the United States and for the millions of census blocks in urban areas. Although the accuracy of TIGER files varies from county to county, partly for reasons beyond the control of the Bureau, they are likely to improve in coming decades. The TIGER files are a particularly important resource for many urban GIS.

Tile is a subset of a GIS database that contains information about one subarea of the overall digital map. Tiles are an effective way of dividing a continuous map into units which can be easily created, edited, and analyzed. The terms tile and facet are synonymous and are, in some ways, the opposite of a continuous map.

Topology refers to properties of geometric forms that remain invariant when the forms are deformed or transformed by bending, stretching, and shrinking. Among the topological properties of concern in GIS are connectivity, order, and neighborhood.

Township and Range is a cadastral system established by the United States Public Land Survey of 1785 to partition public land so that it could be sold or deeded. Land parcels across large areas of the United States are recorded using the township and range system, although not in Texas.

Translation is the process of converting data or commands from one computer format to another, or from one computer language to another.

Turnkey System is a complete hardware and software system designed, built, installed, and tested by a vendor for a customer. Supposedly, the system will be fully operational once it is installed and tested. The term derives from the idea that the customer need only "turn the key," or flip on a switch, and the system will be ready to use. Turnkey systems are often preferred by customers who do not wish, or lack the expertise, to become encumbered in the complexities of system installation, configuration, and testing.

UTM Coordinate System, based on the Universal Transverse Mercator map projection, is a planar locational reference system which provides positional descriptions accurate to 1 meter in 2,500 across the entire earth's surface except the poles. At the poles, the Universal Polar Stereographic projection is used. The UTM system divides the earth's surface into a grid in which each cell, excluding overlap with its neighbors, is 6 degrees east to west, and 8 degrees north to south (with the exception of the row from 72-84 degrees north latitude). For any position in the UTM grid, X-Y coordinates can be determined in eastings and northings. Eastings are in meters with respect to a central meridian drawn through the center of each grid zone (and given an arbitrary easting of 500,000 meters). In the northern hemisphere, northings are read in meters from the equator (0 meters). In the southern hemisphere, the equator is given the false northing of 10 million meters.

Vector displays and databases build all geographic features from points, that is from discrete X-Y locations. Lines are constructed from strings of points, and polygons (regions) are built from lines which close. Vector methods are sometimes contrasted with raster techniques which record geographic features within a matrix of grid cells. The choice between vector and raster GIS has much to do with the application being considered since both methods have strengths and weaknesses. Many current GIS permit transformation between vector and raster input and output.

VM stands for virtual machine, the name and concept behind one of IBM's two principal mainframe operating systems. In the VM environment, hundreds of individual users can make use of a large mainframe computer, and the operating system will maintain the illusion that all the computer's resources are available for each individual user alone. Thus, the user can customize the user's own virtual machine (or account) to access particular resources without having to worry about all the other demands being placed on the computer.

Warping is a method which can overcome some of the problems of edge matching . It can also be used to resolve registration problems arising from digitizing source maps of different scales, different projections, and varying coordinate systems.

XEDIT is the name of the full-screen text editor which is standard to the VM operating system. Xedit is used to create and edit data files and source-code files.



Converted on 16 July 1994. KEF.