Understanding Geographical Information Systems

Dr Munir Morad, Kingston University

According to the International GIS Dictionary, GIS is a "computer system for capturing, managing, integrating, manipulating, analysing and displaying data which is spatially referenced to the Earth." (R McDonnell & K Kemp. 1995. International GIS Dictionary. Cambridge: GeoInformation International). GIS draws on several related disciplines.

What distinguishes GIS from other forms of information systems, such as databases and spreadsheets, is that GIS deals with spatial information. GIS has the capability to relate layers of data for the same points in space, combining, analysing and, finally, mapping out the results. Spatial information uses location, within a coordinate system, as its reference base. The most common representation of spatial information is a map on which the location of any point could be given using latitude and longitude, or local grid references such as the National Grid.

Some applications of GIS are obvious, for example water supply companies use GIS as a spatial database of pipes and manholes; local governments can use GIS to manage and update property boundaries, emergency operations and environmental resources. GIS may also be used to map out the provision of services, such as health care and primary education, taking into account population distribution and access to facilities. Increasingly, GIS is being used to assist businesses in identifying their potential markets and maintaining a spatial database of their customers.

In broad terms, a Geographic Information System could be defined as a set of principles and techniques employed to achieve one (or both) of the following objectives:

• Finding suitable locations that have the relevant attributes. For example, finding a suitable location where an airport, a commercial forest or a retail outlet can be established. This is usually achieved through the use of Boolean (logical) operations.
• Querying the geographical attributes of a specified location. For example, examining the roads in a particular locality, to check road density or find the shortest path, and so on. This is often achieved by ‘clicking’ onto the location or object of interest, and examining the contents of the database for that location or object.

GIS data are usually stored in more than one layer in order to overcome the technical problems caused by handling very large amounts of information at once. It is easier to work with complex spatial problems one layer at a time, to enable the revision of the data without having to overhaul the entire information system. This is a fundamental aspect of GIS, and working with layers of geographical information is known as data integration. For example, a comprehensive GIS of a Welsh county might consist of several layers of data:

• layer 1 property boundaries and land use types (as area maps)
• layer 2 road and railway networks (as line maps)
• layer 3 terrain characteristics (as spot-height or contour maps)
• layer 4 hospital and school locations (as point maps)

Spatial data may be represented in GIS in one (or both) of the two following formats:

• Vector model, as geometric objects: points, lines, polygons.
• Raster model, as image files composed of grid-cells known as pixels.

The importance of geospatial data

The nature of any natural or economic activity with a spatial dimension cannot be properly understood without reference to its spatial qualities. Spatial data have two essential parts: location and attributes.

• A GIS requires locational references. Typical locational references are latitude and longitude and national grid references such as the National Grid. However, other geospatial codes can also be used to identify location, such as postcodes.
• Attributes. Any locality would have a number of characteristics or properties associated with it. These attributes are usually kept in tables, containing such information as vegetation types, population, annual income, and so on.

GIS systems store and process data in two formats, vector and raster.

In the vector data model, the world is represented as a mosaic of interconnecting lines and points representing the location and boundaries of geographical entities. In vector data models, the data are represented as:

• arcs (lines)
• polygons (traversed areas)
• points (labelled nodes)
• nodes (intersection points)

The raster (or grid-cell) data model has come out of aerial and satellite imaging technology, which represents geographical objects as grid-cell structures known as pixels.

Each data model has particular strengths and weaknesses, and the type of model used is determined by the nature of the work being undertaken and the data available.

The main advantage of the vector data format is that it allows precise representation of points, boundaries, and linear features. This makes it particularly useful for analysis tasks that require accurate positioning, for example in engineering or cadastral boundary databases. It is also possible in a vector-based GIS to define the spatial relationship (ie the connectivity and adjacency) between coverage features. This aspect of GIS is known as topology, and is important for such purposes as network analysis (for example to find an optimal path between two nodes in a complex transport network). By contrast, raster-based GIS defines the position of features in terms of x,y coordinates where topological associations are more difficult to represent. However, the main disadvantage of vector data is that the boundaries of the resulting map polygons are discrete (enclosed by well-defined boundary lines), whereas in reality the map polygons may represent continuous gradation or gradual change, as in soil maps.

Grid Data Models use a raster matrix (a grid of image cells) to represent information. Grid data are also known as raster data. The resolution (visual definition) of the raster depends on its pixel (cell) size. In other words, pixel resolution represents the size of the ground area covered by each pixel in the image. The smaller the cell size, the higher the resolution. The raster data model is, therefore, good for representing indistinct boundaries, such as thematic information on soil types, soil moisture, vegetation, ground temperatures, and so on. Furthermore, as reconnaissance satellites and aerial surveys use raster-based scanners, the information (ie scanned images) can be directly incorporated into GIS programmes capable of working with raster data. However, the higher the grid resolution, the larger the data file is going to be. This is the main limitation of raster based GIS.

The question of which data model to use in GIS depends on the nature and objective of the GIS project. Primarily the model type will depend on the nature of the data. Issues of concern are the volume of the data generated, ease of analysis and accuracy. Generally, vector data sets are economical in terms of file size, and have a high level of positional precision, but are relatively difficult to use in mathematical computations. On the other hand, grid data sets tend to take up more file space and have a coarser resolution, but are easier to work with mathematically.

Geospatial data processing

There are several techniques for processing and analysing geospatial data in GIS, some of these are explained below.

Data capture. A GIS cannot analyse the information in a map, if the data are not already in digital form which the computer can recognise. Maps can be digitised (hand-traced with at computer mouse) to collect the coordinates of the map features. Electronic scanning devices can also be used to convert map lines and points to digital information.

Information retrieval. With a GIS we can point at a location, object, or area on the screen and retrieve recorded information about it from the Database Management System (DBMS) which holds the information abut the map’s features.

Spatial measurements. GIS makes spatial measurements easy to perform. Spatial measurements can be the distance between two points, the area of a polygon or the length of a line or boundary. Calculations can be of a simple nature, such as measuring areas on one map, or more complex, such as measuring overlapping areas on two or more maps.

Overlays (data integration). Overlaying is an important procedure in GIS analysis. Overlaying involves superimposing two or more map layers to produce a new map layer. Consider the following example: a new genetically engineered variety of wheat grows well in dry environments, with long growing seasons and alkaline soils. Given the availability of data on the length of the growing season, moisture regime and soil alkalinity, where is the best place to plant the wheat? The answer can be found by overlaying (superimposing) several maps showing (separately) water-budget, growing season length, soil pH, sodium content, and so on. The GIS analysis can establish the locations where all the favourable soil conditions coincide, as the places where the wheat will grow best.

Spatial interpolation. A GIS can be used to estimate the characteristics of terrain or ecological conditions from a limited number of field measurements. For example, a rainfall map can be created from the interpolation of rainfall point measurements taken at a number of different locations on a map.

Buffering and corridors. Buffers are used when the distance from a point or line is important to measure. For example a classic use of GIS in earth sciences involves estimating the damage caused by an earthquake, in terms of distance belts away from the epicentre. If the influence of the earthquake is grouped into four classes then the buffers around the epicentre might look like a map of semi-concentric zones of earthquake damage.

Network analysis. GIS can handle complex network problems, such as road network analysis. A GIS can work out travel times and the shortest path from A to B. This facility can be built into more complicated models that might require estimates of travel time, accessibility or impedance along a route system. An example is how a road network can be used to calculate the risks of accidents. There are, of course, other types of network analysis, involving stream networks. For example, GIS could be used to model the flow of water through a river system, to plan a flood warning system. Real time data would be transmitted by flood warning monitors, such as rain gauges and river height alarms, which could be received and passed to a GIS system to assess the hazard. If the amount and intensity of rain exceeds a certain limit, determined by the GIS flood model for the area, a flood protection plan could be put into operation with computer-generated maps demarcating the vulnerable areas at any point in time.

Registration and warping. Before they can be used in GIS, maps and images should be geometrically rectified. The process begins by registering the raw maps and images to known (control) coordinates. The registration record is then used to warp (or georectify) the map or image in need of rectification. The latter step is also known as rubber-sheeting.

Map projection. Map projection is a fundamental aspect of mapmaking. A map projection is essentially a geometric model that transforms the locations of features on the Earth's surface to corresponding locations on a two-dimensional map. It is impossible to project a spheroid perfectly onto a plane; but some projections can preserve shape, while others preserve area, distance, or direction. Different projections are used for different types of maps because each projection is particularly appropriate for certain uses. For example, a projection that preserves the shape of the continents may distort their relative size.

Digital terrain analysis. GIS can build three dimensional models, where the topography of a geographical location can be represented with an x,y,z data model known as Digital Terrain (or Elevation) Model (DTM/DEM). The x and y dimensions of a DTM represent the horizontal plane, and z represents spot heights for the respective x,y coordinates. As illustrated in Figure 10, the data are represented by a DEM array (grid cells) or a Triangulated Irregular Network (TIN). The data sets derived from a Digital Terrain Model can be used to analyse environmental phenomena or engineering projects that are influenced by elevation, aspect or slope, as in forestry and stream sedimentation studies. The visualisation (display) power of the computer allows the terrain data to be visualised in three dimensional form, often from any angle of view (this is known as point-of-view analysis). For example, using a DTM, civil engineers can assess how well a new structure (say, an open-cast mine) is screened by the surrounding terrain, and estimate how much additional screening might be needed to conceal the structure or reduce noise levels.

Global Positioning System (GPS) and GIS

GPS is a system of Earth-orbiting satellites, each providing precise time and position information which enables GPS receiving devices to compute positions on the Earth. Signals must be received from at least three satellites in order to establish the receiver’s position in latitude and longitude (or a national coordinate system provided with the equipment). Measurement from a fourth satellite is required to calculate altitude (height) position.

GPS configuration consists of three parts:

• Twenty four US Navy satellites orbiting the earth.
• Portable mobile receiver (aerial plus portable console).
• Base station receiver positioned at a known geographic position, in order to perform differential correction of the raw satellite signals.

Each satellite in orbit carries a computer and an accurate (atomic) clock. The computer onboard calculates the satellite's orbit a week or so into the future, predicts ionospheric conditions over that period. By consulting its clock and computer-generated data, the satellite can tell where it is in the sky at any given microsecond, and transmits its position and the current time continuously.

Mobile GPS receivers in the filed do not provide accurate readings of position. The reason for this apparent inaccuracy is a US policy called selective availability (SA). The satellites are programmed to ‘dither’ the transmitted values of their clock and locational data. This is done mainly to prevent a foreign military power from using the GPS for hostile purposes.

Therefore, where greater positional accuracy is required, the underlying carrier wave of the signals can be analysed after capture, and compared with signals received simultaneously at a control ground station. The difference between these two sets of data yields more accurate information. This procedure is known as differential correction.

Remote Sensing and GIS

Remote sensing is the study of phenomena sensed from air or satellites. The use of remote sensing provides scientists with an opportunity to interpret data as seen from space. This facility is particularly useful in surveying very large areas at relatively little cost. Remote sensing is therefore widely used in such locations as the arctic zones, deserts, forested areas, marshlands and mountainous regions, where small-scale maps of an adequate quality can be reproduced relatively cheaply from satellite images. In general, remote sensing is suitable for studying extensive areas with difficult or inhospitable terrain, or where conventional surveys may be very costly.

Satellite and aerial remote sensing provide an almost infinite pool of information. They provide the experienced interpreter with knowledge of ground conditions, including geomorphology, vegetation, soils and mineral composition. A great deal of time and cost can be saved, through remote sensing, by reducing the necessary amount of field work. Taken at regular intervals and over a long period of time, satellite remote sensing can also help scientists to identify the extent of change in ground conditions over the seasons, such as seasonal soil moisture changes. This is particularly useful in mapping areas that witness frequent changes in land cover, like agricultural lands, tidal areas and marshlands.

Stereoscopy (three dimensional view), which is important for such purposes as topographic and forest canopy studies, is possible through remotely sensed images. Stereoscopy is made possible by the overlap of adjacent images (neighbouring images which cover common areas). This can be achieved through the overlap of orbit paths, or the tilting of the sensor to get a sideways view of adjacent areas. Images taken by the French satellite SPOT, for example, are especially popular with land resource scientists using remote sensing techniques, because they combine fairly high resolution (maximum 10 metres) with some overlap.

Visible light (the colours sensed by our eyes) constitutes only a narrow band (wavelength range) within the vast electromagnetic spectrum. One distinct advantage of remote sensing is that it offers the opportunity to use many invisible bands outside the spectrum of visible light. There are many natural phenomena which could only be sensed from distance by using invisible bands in the electromagnetic spectrum. For example, satellites can identify geothermal reserves close to the earth’s surface by sensing the heat emanating from such spots. The procedure is similar, in principle, to the use of infrared (night-vision) security cameras to detect intruders in the dark. The results of sensing invisible phenomena can be developed for human use by assigning false colours to the picture (or false grey shades in black-and-white prints).

In landuse applications, several bands of the electromagnetic spectrum are often used to sense the same place at once. This is useful because each band is especially sensitive to a given phenomenon: showing vegetation boundaries in one case, soil-moisture differences in another, and so on. Therefore, by using multiband images, more information can be obtained as a result of having more colour combinations available for analysis.

Sensors can be divided into two groups: passive and active. Passive sensing resembles the way our eyes operate, by detecting the radiation reflected by the sensed objects from the sun (or another source of illumination). Active sensing is achieved in the reverse manner: the energy is transmitted by the sensor itself, bounces back on impact with the object, giving a ‘backscatter’ echo which is registered by the sensor. In remote sensing, microwave radar (Radio Detection And Ranging) operates by active sensing.

Because of Radar's ability to penetrate clouds, water, snow and (thin) vegetation, a radar image offers the possibility of obtaining a view of the examined phenomena through such obstacles. The technique is especially suitable for surveying areas with frequent cloud cover or under grass.

Radar is also very useful because it accentuates surface roughness on remote sensing images. This occurs because relative height increases when the wavelength of the sensing radiation becomes longer. Relative height also increases when the angle of incidence (the degree of camera tilt from vertical position) is widened. To the soil scientist, this facility can be very useful when studying such phenomena as stoniness where relative height must be accentuated in order to improve detection. An image of a stony soil surface, taken by a side-looking radar sensor, can show a distinctly mottled texture.

Before any remote sensing image can be made available for mapping, it must be cleared of errors caused by geometric displacement or atmospheric interference. The common sources of error in satellite imagery are:

• orbit and flight errors,
• earth curvature,
• earth rotation and
• rough terrain
• atmospheric interference (cloud, fog and smog).

However, image correction is not generally recommended in soil and vegetation inventories, because it may give false data (as image corrections usually involve pixel-value manipulations).

Images marred by significant interference from atmospheric phenomena (such as clouds) should be avoided. Atmospheric errors are especially difficult to correct, because guess work is often inevitable. Similarly, images marred by 'noise' (communication errors, which occur during the transmission of images from satellites to the receiving stations) should also be avoided.

Digital correction of image errors is made possible by computerised image processing techniques. Remote sensing images are normally kept in digitised form (recorded in binary code). Correcting the geometric errors of individual images is usually undertaken by giving the computer the correct coordinates of a number of control points (points whose position is known exactly) on each image. The image is then automatically stretched (rescaled and rotated) to produce a geographically-correct image map.

GIS bibliography

Books

Aronoff, S. 1989. Geographic information systems: A management perspective. Ottawa, Canada: WDL Publications.

Bernhardsen, T. 1992. Geographic Information Systems. Arendal, Norway: Viak IT.

Burrough, P.A. and R. A. McDonnell. 1998. Principles of Geographical Information Systems. Oxford: Oxford University Press.

Huxhold, William E. 1991. An Introduction to Urban Geographic Information Systems. New York: Oxford University Press.

Larsson, R. 1991. Land Registration and Cadastral Systems: Tools for Land Information Management. Harlow, UK: Longman.

Martin, D. 1996. Geographic Information Systems: Socioeconomic Applications. London: Routledge.

McDonnell, R. and K. Kemp. 1995. International GIS Dictionary. Cambridge, UK: GeoInformation International.

Star, J., and J. Estes. 1990. Geographic information systems: An introduction. Englewood Cliffs: Prentice Hall.

Journals and magazines

• International Journal of Geographical Information Science: Taylor & Francis Ltd, Rankine Road, Basingstoke, Hants RG24 8PR, UK (Telephone +44 1256 813000, Fax +44 1256 479438).
• Business Geographics: Adams Business Media, 308 Cambridge Science Park, Cambridge CB4 4ZD, UK (Telephone +44 1223 423020, Fax +44 1223 425787)
• GIS World: Adams Business Media, 308 Cambridge Science Park, Cambridge CB4 4ZD, UK (Telephone +44 1223 423020, Fax +44 1223 425787)
• GIS Europe: Adams Business Media, 308 Cambridge Science Park, Cambridge CB4 4ZD, UK (Telephone +44 1223 423020, Fax +44 1223 425787)
• Mapping Awareness: Adams Business Media, 308 Cambridge Science Park, Cambridge CB4 4ZD, UK (Telephone +44 1223 423020, Fax +44 1223 425787)

Glossary

Accuracy: is the degree to which information on a map or in a digital database matches control values. Accuracy is not the same as precision. See Precision.

Address geocoding: See Geocoding.

Address matching: A mechanism for relating two files using (street or postal) address as the relate item (the item in common).

Algorithm: A (computer) procedure used to solve a mathematical or computational problem, or to address a data processing issue. Algorithms usually consist of a set of rules written in a computer language.

American National Standards Institute (ANSI): ANSI standards have been established for many elements of GIS and computer systems. See BSI.

American Standard Code for Information Interchange (ASCII): A widely used industry standard code for exchanging alphanumeric codes, over computer networks.

Analogue: A continuously varying electronic signal (contrast with binary or digital). The term is also used to describe traditional paper mapping products and aerial photographs.

Application programme interface (API): Computer software designed to access services from programmes across a network.

Application: The technical name for a computer programme.

Arc: A line connecting a set of points that can form one side of a polygon. In ARC/INFO, the vertices at either endpoint of an arc are called nodes. In a topological GIS system (see Topology), arcs are linked to nodes (arc-node topology) and to polygons (polygon-arc topology).

Area: A fundamental unit of geographical information, defined by a continuous, closed boundary. Also known as polygon. Examples include fields, counties, lakes, local authority boundaries, school districts and census enumeration districts.

Aspect: The geographical direction toward which a slope faces, measured in degrees from north, in a clockwise direction.

Attribute data: Descriptive information about features or elements of a database, listed as numbers, characters or images. For a database feature like census districts, attributes might include demographic facts such as population, average income, gender and age. In ARC/INFO, an attribute table is an INFO or other database file containing rows and columns, associated with a class of geographical features. Each row represents a geographical feature, and each column represents one attribute variable.

Automated cartography: The process of drawing maps with the aid of computer-based display devices such as plotters and visual display units (monitors).

Automated Mapping / Facilities Management (AM/FM): The systems used by public and private utilities, local governments, and other organisations that manage very large capital equipment, stock, or facilities. AM/FM most commonly describes databases designed to manage networks of road, pipe, or cable assets.

Azimuth: The horizontal direction of a vector, measured clockwise in degrees of rotation from the positive y-axis. For example, degrees on a compass measured from north.

Backup: A copy of a file, or a set of files, saved on a separate device or computer, for safekeeping, in case the original data were lost or damaged.

Band separate: An image format that stores each band of data in a separate file. The original data are usually collected by multispectral scanners, onboard satellites or aeroplanes.

Band: A single range of multispectral data for an interval within the electromagnetic spectrum (such as light or infrared energy). Satellite imagery such as Landsat and SPOT provide multispectral images of the Earth, containing several bands.

Bandwidth: A measure of the volume of data that can flow through a communications link (cable). Also known as throughput.

Base map: A map containing geographical features used for locational (map grid) reference. Property boundaries are commonly used as base maps because they are accurately referenced. Base Maps provide the background on which other data layers are overlayed and analysed.

Basic Land and Property Unit (BLPU): A reference number used in the UK address gazetteer system known as BS7666. See Gazetteer and UPRN.

Baud rate: The speed of data transmission between computer and other devices, measured in bits per second.

Benchmark Tests: Procedures for comparing the performance of competing hardware and software. Specific benchmark tests can be developed by GIS managers to test new equipment or software cope under conditions close to those which will be encountered in day-to-day use. See Sensitivity analysis.

Binary code and binary files: Digital information and commands stored and used by hardware and software, as sets of on-off (1-0) signals. Most systems of binary encoding in GIS are proprietary to particular hardware and software vendors. Binary data are usually the most compact means of storing information. However, binary files are not easy to transfer between computer systems which use different binary configurations. Binary files are therefore usually translated into ASCII (text) form for transfer between computers. See American Standard Code for Information Interchange (ASCII).

Binary Large Object (BLOB): The data type of a column in an RDBMS (Relational Database Management System) which can store large images or textual data.

Bit: The smallest unit of information that a computer can store and process digitally. A bit has two possible values, 1 or 0, which correspond to yes and no (or on and off). See byte.

Boolean operator: A keyword that specifies how to combine simple logical expressions into complex expressions. Boolean operators negate a predicate (NOT), specify a combination of predicates (AND), or specify a list of alternative predicates (OR). For example: slope > 100 AND vegetation = forest.

Breakline: A linear feature that defines the areal extent and controls the surface interpolation of a digital elevation model. The term is used as a command in ARC/INFO’s TIN (Triangular Irregular Network) module. Terrain features containing shorelines are often clipped as breakline features, otherwise sea-level surfaces could be erroneously interpolated as hills or valleys. See Digital Terrain Model (DTM).

British Standards Institute (BSI): The UK equivalent of the American National Standards Institute (ANSI).

Buffer: Enclosed polygon created around points, lines or areas at an equal distance in all directions. The results represent areas at set distances from the original object. For example, the creation of buffer zones around a polluted industrial site may represent the varying extent of pollution measured at specified distances from the source of the contamination. Buffers are therefore useful for proximity analysis and environmental impact assessment.

Bug: An error in a computer programme that causes it to crash or malfunction.

Bundled GIS: A reference to the way software and hardware are sold together or separately. Some GIS vendors offer a bundled package of hardware and software, at a discount negotiated with the software developers and hardware manufacturers.

Byte: A computer-memory and data storage unit composed of contiguous bits, usually 8. File sizes are measured in bytes or megabytes (one million bytes). Bytes contain values of 0 to 255 and a collection of bytes (often 4 or 8 bytes) represents real numbers and integers larger than 255.

Cadastre: A cadastral survey involves the mapping, tracing and recording of private and public land resources. The term cadastre is French in origin, meaning a record of the ownership, extent and market value of a property, for tax and legal purposes.

Cartesian coordinate system: A two-dimensional, planar coordinate system in which x and y represent distances from a point of origin, and where each point on the plane is defined by an x,y coordinate. Locations in the coordinate system can be established using any unit of measurement such as meters or yards. Relative measures of distance, area, and direction are constant throughout a Cartesian coordinate plane.

Cartography: The geographical discipline concerned with map preparation and communicating geographical information.

CD-ROM: Common abbreviation for Compact Disk-Read Only Memory. A CD-ROM disk can hold about 650 megabytes of information, as text, maps, tables or images.

Cell: The basic element of spatial information in the raster (grid) description of spatial features. See Pixel.

Centroid: The centre of a polygon. In the case of an irregularly shaped polygon, the centroid is derived mathematically and is roughly the equivalent of its centre of gravity. Centroids are often used in GIS to index the polygon within which they are located (ie, the polygon’s attributes are referenced to its centroid).

Computer Graphics Metafile (CGM): A graphic image exchange standard.

Chain: A sequence of coordinates defining a complex line or boundary.

Character: An alphanumerical (alphabetical or numerical) value that represents a single unit of data

Choropleth map: A map in which areas of different value are separated by clearly defined boundaries. The value of the underlying data (eg, soil moisture) are represented by colour or shading densities, and the map legend acts as a look-up key to explain the values shown on the map.

Client/server architecture: A software that handles requests from multiple end users (clients) on the network.

Clip: A polygon which defines the boundaries of features in a map, by cutting the lines off its edge. Clips are used to restrict the extent of data processing or querying done in GIS. See Breakline.

Column: The vertical dimension of a table, representing the variables of the geographical features listed in the rows.

Command line interface: The software window which allows the GIS operator to type in commands at a prompt, rather than use a Windows-based system. Command line instructions are faster to execute, but typing errors can cause delays. Most modern GIS are Windows-based, but would also allow the user to work from a command line interface.

Command: A computer instruction, issued by the user to perform a specific function.

Computer Aided Design (CAD): CAD systems are used to create maps and plans and are closely related to GIS systems, as many GIS systems, such as Intergraph and AutoCAD, have their origin in CAD software.

Concurrency management: A database management procedure for preserving a system’s security against accidental or deliberate data loss, which supports simultaneous access by multiple users. This type of access regime allows only one user at a time to change the content of the database (ie, have write-access), while other users will have read-only access. The next user wanting write-access to the database will have to wait until the first person has completed their transaction. All database changes would be logged, showing the time of transaction and the name of the operator.

Conditional operator: An SQL (Structured Query Language) keyword specifying how to compare values. Conditional operators are used to query a database. Examples include = , >, LIKE and CONTAINING.

Connectivity: A topological property relating to how geographical features relate to one another spatially. The topological identification of connected lines are established by recording the ‘from’ and ‘to’ nodes for each line. Lines that share a common node are said to be topologically linked. Connectivity is useful in network analysis.

Contiguity: The topological definition of adjacent polygons, by recording the left and right polygons of each edge of the enclosed polygon.

Continuous data: A surface for which each location has a specified or derivable value, such as DEM’s (Digital Elevation Models).

Contour: A line on a surface model representing points of equal height value.

Contouring: The interpolation of contour lines from height data.

Contour interval: The difference in surface values between successive contours.

Conversion: The process of transforming data derived from existing records and maps to a digital database, or from one digital form to another.

Coordinate Geometry (COGO): A data input method similar to the way land surveyors enter their survey data. Input begins at a point, moves along a given bearing for a set distance, and continues in the same fashion until the geographical feature (such as a pond) is completely outlined. Unlike the error-prone process of tablet digitisation, COGO data entry generally establish more accurate locations and boundaries.

Coordinate: A set of numbers that designate a location in a given geographical reference system, such as x,y in a two-dimensional coordinate system or x,y,z in three-dimensional terrain models.

Coordinate system: A reference system used to measure horizontal and vertical distances on a planimetric (two-dimensional) map. A common coordinate system (such as the Ordnance Survey’s National Grid) is used to reference the x,y location of geographical objects.

Coverage: The name used by ARC/INFO to denote a map or layer of geographical features (stored as arcs, nodes, polygons, and label points). A coverage represents a single theme such as vegetation type, population density or property boundaries.

Dangle: A line with one end (or both) that does not connect to any other feature in the map. A dangle occurs when the end of the line undershoots or overshoots the map feature to which it is meant to be attached.

Data Access Language (DAL): The standard adopted by Apple computers to facilitate access to relational databases.

Data integrity: Generally speaking, the term data integrity is used to refer to the relevance of the data kept in a database. For example, the presence of characters in a column that is meant to hold only numbers indicates poor data integrity.

Data model: A user-defined, abstract representation of data describing the behaviour of the geographical entities represented by the data. For example, the terrain of a geographical location can be represented with an x,y,z data model known as Digital Terrain Model (DTM), where x and y represent the horizontal plane and z represents spot heights at the respective x,y coordinates.

Data set: A collection of logically related information arranged in a specified manner.

Data type: The quantitative or alphanumeric characteristics of variables that define the type of data used. Examples include character, integer and floating point (numbers with decimals).

DataBase Management System (DBMS): Computer software used to enter, store, organise, retrieve and query information in a database.

Database: A collection of organised information, usually stored on magnetic tape or disk. A GIS database includes data about the location and the attributes of geographical features that have been coded as points, lines, polygons or grid cells (pixels).

Datum: The singular for data. However, the term datum is often used in GIS circles to refer to geodetic datum. See Geodetic datum.

Digital Chart of the World (DCW): A widely available 1:1,000,000 digital base map of the world, containing vector data digitised by the US Defense Mapping Agency.

Digital Elevation Model (DEM): See Digital Terrain Model (DTM).

Digital Terrain Model (DTM): A database with x,y,z coordinates, used to represent landform digitally. Also known as Digital Elevation Model. See Data model.

Digital: See Binary code and binary files.

Digitisation: The process of converting analogue map data into digital codes stored and processed by computers. Digitising involves tracing map features into a computer using a digitising tablet or mouse.

Directory: A set of data files, stored on computer disk. Operating systems use directories and subdirectories to organise data. The location of a directory is specified with a pathname, for example C:\LIS\PROPERTIES\. Under Windows-based systems, directories are visually represented with a folder icon.

Discrete data: Geographical features containing well-defined represented by points, lines or polygons boundaries. Unlike raster (grid cell) GIS, vector GIS represent geographical features as discrete (sharply defined) features.

Dots Per Inch (DPI): Measure of the resolution of graphic displays and printers, representing the number of pixels per inch.

Downstream: The direction along the routes of flow, used in hydrological modelling.

Drape: A perspective visualisation of ground features superimposed on a Digital Elevation/Terrain Model. See Digital Terrain Model (DTM).

Drawing Exchange Format (DXF): File exchange format developed by Autodesk. DXF files are ASCII (text) records of the objects in a drawing file. DXF is widely used for exchanging map files.

Dynamic segmentation: In ARC/INFO, the term refers to analysing events on network routes, based on ‘event tables’ for which distance and impedance attributes are listed.

Edge Matching errors: Sliver polygons often occur when neighbouring map tiles are misaligned as they are laid edge-to-edge. These misalignment errors occur when features run across the boundaries of their respective map sheets. The errors often originate during digitisation, generalisation of map features, or map projection. Warping (rubber-sheet correction) and the automatic elimination of (user-defined) tiny sliver polygons are two major methods of mitigating edge-matching errors. See Rubber sheeting.

Element: Basic geographical unit of information, such as a point/node, line/arc, area/polygon or pixel/cell. Also known as map entity.

Ellipsoid: See Geodetic datum.

Embedded SQL: SQL statements that are embedded in a programming language (such as AML: ARC/INFO Macro Language). See Structured Query Language (SQL).

Encapsulated PostScript (EPS): See Postscript.

Environment: In computing jargon, the system’s configuration which defines the various display, editing, and data manipulation processes handled by GIS.

Ethernet: A local-area network (LAN) protocol used for high-speed communication between linked up computers.

Event: An additional geographical feature occurring on or along a linear feature, such as a route.

Extended character set: Extended character sets support additional languages which require double-byte characters, such as Arabic or Greek.

Facet: A tile or subset database which contains information about one sub-area of the overall digital map. Facets are an effective way of dividing a continuous map into units which can be worked with separately.

Field: A common synonym for column in a database.

File Transfer Protocol (FTP): A common protocol for copying or moving data across a computer network or the Internet.

File: A collection of related data (textual or graphical) in a computer. Files are the basic units managed by the computer's operating system.

Filter: A grid of weighted pixel values, used to remove superfluous (noise) features, or to generalise spatial patterns from a raster data set.

Format: The method in which data are organised and stored in a computer, for transmission between computers or between a computer and a device. Most GIS have proprietary formats used to store and process geographical information.

Gazetteer: A work of geographical reference that supplies place names and location information. In the UK, an address standard known as BS7666 has been adopted since 1984, based on two sets of identifiers known as BLPU (Basic Land and Property Unit) and UPRN (Unique Property and Reference Number). See BLPU and URPN.

Generalisation: In its basic form, the process of removing vertices from a line or polygon, according to a pre-specified tolerance level or algorithm.

Geocoding: The process of identifying or designating the coordinates of a geographical object, given its address. Also known as address geocoding.

Geodesy: The study of the size and shape of the Earth, and the determination of exact longitude and latitude positions on it.

Geodetic Datum: A three dimensional (ellipsoidal) model used to represent the shape of the Earth in a specified region. A geodetic datum is the basis for the geographical coordinate system adopted by a country. For example, the National Grid of the United Kingdom is based on a geodetic datum representing the shape of the Earth in the UK. A national geodetic datum gives a more accurate representation of the shape of Earth at local level, because global models are quite generalised.

Geodetic framework: A spatial framework of points whose positions have been precisely determined on the surface of the Earth. The UK’s National Grid is the British geodetic framework. Also known as geodetic network. See Geodetic datum.

Geographical Information System (GIS): A computer-based system for capturing, storing, analysing and displaying locational data.

Geographical Resource Analysis Support System (GRASS): A public-domain raster GIS developed by the US Army Corp of Engineers Research Laboratory (USACERL).

Georeferencing: Establishing the location of a given geographical object, according to an agreed system of map coordinates such as the National Grid.

Global Positioning System (GPS): A system of Earth-orbiting satellites, each providing precise time and position information which enables GPS receiving devices to compute positions on the Earth. Signals must be received from at least three satellites in order to establish the receiver’s position in latitude and longitude, or according to a national coordinate system provided by the equipment. Measurements from a fourth satellite are required to calculate altitude (height) position.

Graphical User Interface (GUI): A monitor, menu-based method of controlling how a user interacts with a computer. Instead of issuing commands at a prompt, the user performs the required tasks by using a mouse, or selecting a menu item. See Command line interface.

Graphics display terminal: A computer unit used to view and manipulate graphic information.

Gravity model: An analytical technique used in geographical research to analyse the geographical pattern of economic behaviour. The underlying assumption of the model is that the influence of populations on one another is inversely proportional to the distance between them. The term gravitational was ‘borrowed’ from Newton’s theory of gravitation.

Grey scales: Levels of brightness used in displaying information on monochrome display devices, or on non-colour printers.

Grid cell: A unit that represents a single position on an array of equally sized (square) cells arranged in rows and columns. Each grid cell is referenced by its geographical x,y location. Also known as pixel in raster GIS.

Hard-copy: A print.

Hardware: The physical components of a computer system, such as the computer and its attached devices: digitisers, plotters, printers, etc.

Heuristic: A computational method that uses trial and error to approximate a solution, for computationally complex problems.

Host: A computer to which other guest computers are connected. The host computer usually handles complex or time-intensive computing tasks. The guest computers pass requests to the host whenever its services are required.

Hub: A node in a network that can be used to channel goods from origins to destinations. Hubs are used at strategic locations in a network to reduce transportation costs.

Image analysis: The processing and interpretation of raster information that are held in digital form (such as satellite images). An image is usually stored as values which represent the intensity of reflected light, or other values in the electromagnetic spectrum.

Image resampling: See Rectification.

Impedance: The amount of resistance (friction or cost) required to traverse a line (in a network) from its origin node to its destination node. Resistance may be measured as travel distance, time or other obstacles (such as road conditions). An optimum path in a network is the path of least resistance.

Index coverage/layer: The polygon coverage that is used as an index (reference layer) for the structure of other layers of geographical features overlaid onto the index layer.

INFO database: The basic DBMS used by ARC/INFO to store and manipulate feature attribute tables.

INFORMIX: A corporate relational DBMS.

Infrastructure: A reference to the basic utility structures that support a local economy, such as roads, electricity pylons, water and drain pipes, etc. GIS are used to hold information about these structures, and map out their location.

INGRES: A corporate relational DBMS.

Input device: A hardware component for data entry, such as a digitiser, keyboard, scanner, mouse, disk drive, etc.

Institute of Electrical and Electronics Engineers (IEEE). An important US organisation, which has played a major role in setting standards for many engineering and GIS related applications.

Integer: A whole number (a number without decimals).

Interactive Graphics Design Software (IGDS): An Intergraph GIS file format.

Inter-Application communication (IAC): The capability of different computer software systems to communicate with one another. With IAC, several computer programmes can execute commands simultaneously, share data, and make requests of each other.

Interface: A hardware and software link that connects two computer systems, or a computer and its peripherals.

Internet: An international consortium of Wide Area Networks (WAN’s) that operate using a standard set of communication protocols. The Internet grew out of a US Defense Advanced Research Projects Agency (DARPA) research project in the early 1970s. Private and public networks have joined the Internet since then.

Internet GIS: Internet-based GIS is a growing area of software development and application. The technology enables Internet users to see and query a GIS map, using a Web browser such as Netscape Navigator or Internet Explorer. Major GIS developers now provide special Internet Server software that allows a GIS (such as ARC/INFO or Intergraph) to be browsed over the Internet by remote users.

Interpolation: Estimating the value of a point, from measurements made at surrounding points. A basic example is the estimation of z (height) values of a surface model, using the known z values of surrounding points.

Intersection: The topological integration of two spatial data sets within the area common to both data sets.

Isoline: A line on a surface connecting points of equal value.

Isopleth map: A map showing the distribution of data as lines connecting points of equal value.

Item indexing: A means of accelerating logical queries by creating an index for key terms in the database table. The benefits of this approach are similar to finding information quickly in a book by consulting its index.

Item: A column of information in an attribute table.

Land Information System (LIS): A GIS-based system concerned with capturing, storing, analysing and retrieving land and cadastral information. See Cadastre.

Land line: Property boundary maps, in digital form, created by the Ordnance Survey.

Land parcel: A piece of land representing a legal estate. See Cadastre.

Latitude: Angular distance, expressed in degrees, along a parallel north or south of the Equator. Also known as parallel.

Latitude-longitude: A global coordinate system used to measure locations on the Earth's surface. Latitude and longitude are angles measured from the Earth's centre to locations on the Earth's surface. Latitude measures angles in a north-south direction. Longitude measures angles in an east-west direction.

Lattice: A surface representation with a rectangular array of grid points spaced at constant sampling intervals in the x and y directions.

Layer: A thematic plane of GIS features containing geographically and logically related data (such as vegetation types). Layers are the basic components of overlay operations in GIS.

Least-cost path: The path between two points which has the lowest travel cost, where cost is a function of time, distance, or other impedance factors.

Legend: The part of the drawn map explaining the meaning of the symbols and colours used to encode the geographical elements. Also known as map key.

Line: A basic geographical element, defined by two or more points with known x,y coordinates. Examples include motorways, streams and cable paths.

Local Area Network (LAN): A network arrangement for connecting computers, within one site. The networked computers on a LAN can share data and peripheral devices, such as magnetic storage devices, printers and plotters.

Logical operator: One of several words, including AND, OR, NOT and XOR, used to build complex logical expressions in a query. Also known as Boolean operator.

Longitude: The angular distance east or west from a standard meridian (such as the Greenwich line) expressed in degrees.

Look-up table: A set of data values that can be accessed by a computer programme to convert data from one form to another (for example from numerical values to colours or symbols).

Macro: A script (text) file containing a sequence of commands that can be executed as one command. Macros can be built to perform frequently used or complex operations.

Many-to-one relate: A relational database arrangement in which many records in a table are related by a single record in another table.

Map extent: The rectangular limits of a map, in terms of monitor or printer coordinates.

Map Generalisation: The process of reducing detail on a map as a consequence of reducing the map scale. See Generalisation.

Map projection: A geometric model that transforms the locations of features on the Earth's surface to corresponding locations on a two-dimensional surface. It is impossible to project a spheroid perfectly onto a plane; but some projections can preserve shape, while others preserve area, distance, or direction.

Map scale: The relationship between distance on a map and the corresponding distance on the Earth's surface. Map scale is often expressed as a fraction of distance, such as 1:50,000 (where one unit of distance on the map represents 50,000 of the same unit of distance on Earth). Map scale can also be expressed as a statement of equivalence using different units, for example, 1 metre = 3 kilometres.

Map: An abstract representation of geographical features using recognisable symbols. Maps often highlight (generalise) features, depending on what they are intended to illustrate. For example, a road atlas may not show navigable canals, and a motorway map may not show all side roads.

Meridian: A line running vertically from the north pole to the south pole. The Prime Meridian runs through Greenwich, England, where measures of longitude are negative to the west and positive to the east of Greenwich. See Latitude-longitude.

Metadata: Data that describe the nature of a data set, in terms of accuracy, extent, copyright and restrictions. There is growing interest in metadata, because of increasing interest in data quality.

Metes and Bounds: A method of land surveying in which the boundary lines of land parcels are entered in terms of relative bearing and distance (from one station to the next). See Coordinate Geometry (COGO).

Model: An abstract representation of reality used to analyse, predict or simulate a process, using a set of rules and procedures.

Module: A distinct piece of software that can be called by other modules in the programme. Modules are designed to reduce the bulk of large software systems, and reduce running times.

Mouse: A hand-controlled computer device for interacting with a computer terminal or entering data.

Multispectral scanner (MSS): A scanning system on some satellites used for imaging the earth. An MSS image will have data recorded by the scanner from several wavebands of the electromagnetic spectrum.

National Transfer Format (NTF): British Standard BS 7567, which defines an exchange format for the transfer of vector data. NTF is the main format used by the Ordinance Survey.

Network: A system of connected geographical features, such as roads, pipes, or cables. A network structure enables the calculation of optimal routes or the simulation of flow through rivers or pipes.

Network analysis: A set of techniques to study the properties of networks in order to simulate flows of traffic or water. Network analysis is often employed to calculate shortest (or lowest cost) paths between two points in a network.

Node: The location at which two or more lines connect. Nodes can carry information about the topology of a network or adjacent polygons.

Normalisation: A conceptual database task designed to avoid data redundancy. See Redundancy.

Null value: The equivalent of saying that there is no value specified. However, null is not the same as blank or zero.

Object Linking and Embedding (OLE): A Microsoft standard, which allows objects from one application to be embedded within another (for example, taking a Map Object and embedding it into a Windows application like Excel).

One-to-many: A relate in which one record in a table is related to many records in another table, within a relational DBMS.

Online access: Direct access to data that does not involve file transfer. In practice, this means real-time interaction between a server and end user, or between a computer an a device (such as a printer) on the network.

Open Database Communication (ODBC). A Microsoft API (Application Programme Interface) which enables communication between different database systems.

Open GIS Consortium (OGC): A consortium of GIS developers, academics and government organisations, set up to define and encourage open systems standards for the GIS industry. The consortium’s main objective is to facilitate OGIS (Open Geodata Interoperability Specification) between GIS software and data formats.

OPEN LOOK: A GUI (Graphical User Interface) for the X Windows system developed by Sun Microsystems. The interface is used mainly to emulate UNIX-based work on PC’s. See X Windows.

Open Software Foundation (OSF): An international consortium which promotes the UNIX operating system, and defines its standards.

Operating system (OS): Computer software designed to allow communication between the computer and the user, and between the computer and the devices attached to it. The operating system controls the organisation of files and directories, the flow of data between the computer and other hardware and the execution of commands and software. DOS (Disk Operating System) and Windows are the two most popular operating systems for PC’s. Other operating systems include VM and MVS (for IBM mainframes), VMS (for DEC machines), UNIX (including AIX, Ultrix, and UNICOS), OS/2 (for PC’s), and Apple OS.

Optical Character Recognition (OCR): The automatic recognition of text, using scanning devices.

Optical disk: A storage technology that uses optical media to store data. Optical disks are slower than magnetic disks, but store more data and cost less per unit of stored data. Optical disks are often used to store very large amounts of data.

ORACLE: A popular, corporate relational DBMS.

Ordnance Survey (OS): The UK’s main public mapping agency.

Overlay: Another name for a layer or coverage. More specifically, an overlay refers to an additional layer superimposed on the base layer. See Layer.

Overlaying: The process of stacking data layers on top of each other, so that geographical position in one theme layer can be analysed in relation to other themes for the same position.

Panning: Moving the viewable portion of a displayed map around the display unit, in order to display map areas which lie outside the viewing window.

Parcel: A basic cadastral (property) unit. See Cadastre.

Path: A continuous set of network links and nodes.

Pathfinding: The process of finding a path between an origin and destination, on a network.

Peripheral device: A hardware device not part of the main computer unit, such as digitisers, printers, zip drives and modems.

Photogrammetry: The use of stereo pairs of aerial photographs (or satellite images) to produce geometrically correct maps.

Pixel: A shortened word for picture element, the basic grid-cell unit in raster image files (images). See Grid cell.

Platform: A synonym for computer, especially powerful computers capable of complex processing.

Plotter: A printing device used to print out maps with high precision, especially for engineering purposes.

Point: A geographical object, with a single x,y coordinate, used to designate such features as schools, wells, trees, etc.

Postscript: An industry-standard page-description language developed by Adobe, and supported by laser printers. Also available as encapsulated graphic form.

Precision: A measure of the tendency of digitised geographical objects to cluster, measured by root mean square error (RMSE) or the standard deviation (SD). Note that high precision does not necessarily indicate high accuracy. The term precision is also used to refer to the number of significant digits used to store numbers and map coordinate values. See Accuracy

Projection: See Map projection.

Protocol: A set of rules or standards which control communication between computers and between computers and peripheral hardware.

Query: A statistical or logical expression used to highlight or analyse a set of data. See Logical operator.

Raster: A regular grid of cells covering an area, usually in the form of a scanned image (such as a satellite image or scanner-captured aerial photograph).

Raster-vector conversion: The process of converting a raster image into a vector map.

RDBMS: See Relational Database Management System.

Real number: A numbers with decimal digits. See Integer.

Rectification: The process of geometrically correcting an image, according to a set of real-world coordinates. Rectification usually involves the rotation and re-scaling of the original image. The process is also known as image resampling.

Redundancy: The (unnecessary) duplication of data in a database.

Reference ellipsoid: See Geodetic datum.

Relate key: The common attribute (column) used to relate two databases.

Relate: An operation that establishes a connection between corresponding records in two tables, in a relational database.

Relational DataBase Management System (RDBMS): A database management system based on using several tables, connected through one or more common columns in the tables to be linked. Therefore, the data in a relational database must be structured as collections of tables that are logically associated to each other by shared attributes. A relational database arrangement allows separate tables to be linked as and when the need arises, thus eliminating the necessity of storing all attribute data in one very large table.

Remote sensing: Acquiring information about an object at a distance, using aerial photography, radar, and satellite imagery.

Resampling: See Rectification, Rubber sheeting.

Resolution: The size of the smallest feature that can be detected or mapped on a scanned or digitised image (or monitor display).The larger the map scale of the image, the higher the resolution. Also known as visual definition.

Root mean square error (RMSE): See Precision.

Route: A continuous set of links in a network.

Rubber sheeting: A computer-based geometric process used to edge match mis-aligned image or map tiles. The term is also used to refer to the warping or resampling of images. See Rectification.

Run Length Encoding (RLE): A simple data compression technique for storing raster image files.

Scale: The relationship between the size of an object on map or image, and its size in the real world.

Scanner: A data capture device for converting images or vector shapes into digital form automatically. Some scanners are also able to convert raster data into vector data.

Scanning: The process of converting images into digital form automatically, using a scanner. It is usually part of the digitising process.

Sensitivity analysis: A set of procedures used in a prototype GIS project to estimate the level of accuracy which can be achieved by a GIS, relative to a set of test objectives. See Benchmark tests.

Shape file: A proprietary ARC/INFO file format.

Single precision: A level of accuracy based on the number of significant digits that can be stored for each coordinate. Single-precision numbers store up to 7 significant digits for each data entry.

Sliver polygon: A small overlap area, or gap, along the borders of polygons, which results from errors in overlaying or edge-matching several map tiles.

Slope: A measure of change in surface value over distance, expressed in degrees or as a percentage.

Snapping: The process of getting two geographical features, which are meant to be linked (such as a node and a line), to touch one another.

Spatial analysis/modelling: The modelling and interpreting of spatial (geographical) objects in a GIS. There are several types of spatial analysis: raster analysis, linear analysis, surface analysis, topological and contiguity analysis.

Spatial data: Data about the location or the geometric characteristics of geographical features. Postcode is an example of spatial data.

Spatial query: See Map query.

Spike: An overshot line, or an anomalous data point, created erroneously during digitisation.

Spline: A mathematically calculated curve to smooth an abrupt change in the direction of a continuous line.

SPOT: A common abbreviation for Systeme Probatoire de l’Observation de la Terre, a reconnaissance satellite system with high (10-30 metre) resolution launched by France in 1986.

String: A series of alphanumeric characters.

Structured Query Language (SQL): A computing language for defining and manipulating data from a relational database, which has become an industry standard for querying in most DBMS.

Surface model: See Digital Elevation/Terrain Model.

SYBASE: A commercial relational DBMS.

Symbol: A graphic pattern used to represent a feature in GIS and cartography.

Table: A set of data elements organised into rows and columns, within a DBMS.

TCP/IP: The common acronym for Transmission Control Protocol / Internet Protocol, which are the communication protocols that enable computers to send and receive data across a network (including the Internet).

Terminal: A computer unit acting as part of a network that is served by a more powerful (server) computer.

Terrain analysis: See Digital Terrain Model (DTM), Spatial analysis/modelling.

Tesselation. The process of splitting a map region into several (matching) tiles.

The Object Management Group (OMG): A computing industry group interested in promoting object-oriented interoperability among different computing systems. OMG has also developed the specifications required to make object-oriented interoperability work, known as Common Object Request Broker Architecture (CORBA).

Thematic map: A map displaying information relating to specific geographical themes, such as soil types, population density, etc.

Thiessen polygons: The boundaries of Thiessen polygons are defined by the perpendicular bisectors of the lines between all neighbouring points. Thiessen polygons are generated from a set of irregular points, and are designed to interpolate a continuous surface from an incomplete set of spatial data.

TIFF: The common abbreviation for Tagged Interchange File Format, an industry standard graphics data format.

Tile: A subset of a GIS database or map that contains information about one subset (usually, one sub-area) of the entire map, designed to save access and processing time.

Topographic map: A terrain (relief) map, often in the form of a contour map.

Topology: The spatial relationship between geographical objects. Topological properties include order, connectivity and neighbourhood (adjacency). Topological relationships are built from simple elements into complex elements: points (simplest elements), lines (sets of connected points), polygons (areas enclosed by connected lines), and routes (sets of route sections). Topology is an important prerequisite for sophisticated geographical analysis, such as network analysis and contiguity analysis.

Transformation: The process of converting coordinates from one coordinate system to another, which usually entails changing the orientation, scale or projection of a mapped area.

Translation: The process of converting data or commands from one GIS format to another, for example from DXF to shapefile.

Triangulated Irregular Network (TIN): A surface representation derived from irregularly spaced sample points and features, interpolated by Thiessen polygons. See Thiessen polygons.

Tuple: A row in a relational database.

Turnkey system: A self-initiating hardware and software system configuration.

Undershoot: A digitised line that does not intersect with another node or line, because it is too short. See Dangle.

Union: An overlay of two data layers, where all features from both layers are retained.

Unique Property Reference Number (UPRN): A subset of the of BLPU (Basic Land and Property Unit) reference system, used in the address gazetteer adopted in the UK since 1984 (known as BS7666). See Gazetteer, Basic Land and Property Unit (BLPU).

Universal Transverse Mercator (UTM): Global coordinate System which provides locational descriptions accurate to 1 metre (except the north and south poles).

Unix: A general-purpose, multi-user computer operating system.

USGS: United States Geological Survey, one of the US’ main mapping agencies.

USGS digital elevation models: Surface models produced by the Survey Branch of the United States Department of the Interior.

Vector: A coordinate x,y based data structure commonly used to represent linear geographical features. In vector format, spatial objects are defined by points and lines. See Discrete data.

Warping: See Rectification.

Wide area network (WAN): Computer network that connects computers at remote sites (not in the same location), usually through telephone links.

Wide-Area Information Server (WAIS): A networked client/server software system that provides sophisticated file indexing based on all text entries in each file. Searching can include Boolean expressions.

Workspace: A GIS directory containing path references to the set of maps and tables that a user is working on.

World Wide Web: A global computer-based network, developed by the European Laboratory for Particle Physics (CERN) in Switzerland. See Internet.

X Windows/Motif: A system that allows applications to be shared among different workstations and terminals (irrespective of the operating system used on the individual computers). Also known as OSF (after the Open Systems Foundation).

Zooming: Enlarging a visual display on a computer monitor.

Z-value: Usually a reference to the value of surface height in a Digital Elevation/Terrain Model at a particular x,y location.

Acronyms

ACSM: American Congress on Surveying and Mapping

AGI: Association for Geographic Information

AI: Artificial/Amplified Intelligence

Am/FM: Automatic Mapping / Facilities Management

AML: Arc Macro Language

ANSI: American National Standards Institute

ASCII: American Standard Code for Information Interchange

AV: ArcView (ESRI software)

AVL: Automatic Vehicle Location

BIOS: Basic Input / Output System

BLOB: Binary Large Object

BLPU: Basic Land and Property Unit

BM: Bench Mark

BMP: BitMaP

BSI: British Standards Institute

CAD: Computer Aided Drafting, Design, or Delivery

CAL: Computer Aided Learning

CAM: Computer Aided Mapping/Manufacturing

CASE: Computer Aided Software Engineering

CCT: Computer Compatible Tape

CD: Compact Disk

CD-ROM: Compact Disk-Read Only Memory

CGI: Common Gateway Interface

CLIENT: A software application which interacts with a server computer

COBOL: COmmon Business Oriented Language

COGO: COordinate GeOmetry

COORDS: x,y coordinates (or x,y,z for three dimensional models)

CPU: Central Processing Unit

DB: DataBase

DBMS: Database Management System

DCM: Digital Cartographic Model

DDE: Dynamic Data Exchange

DEI: Data Extraction Interface

DEM: Digital Elevation Model

DIR: DIRectory

DLG: Digital Line Graph

DLL: Dynamically Linked Library

DLM: Digital Landscape Model

DN: Digital Number

DOS: Disk Operating System

DPI: Dot Per Inch

DTM: Digital Terrain Model

ED: Enumeration District

EIA: Environmental Impact Assessment

EIS: Environmental Impact Statement

EISA: Extended Industry Standard Architecture

EOS: Earth Observation Satellite

EPS: Encapsulated PostScript

ES: Expert System

ESRI: Environmental Systems Research Institute (developers of ARC/INFO)

EXE: EXEcutable file

FORTRAN: FORmula TRANslation (programming language)

FTP: File Transfer Protocol

GB: GigaByte

GIF: Graphics Interchange Format

GIS: Geographical Information Systems

GPS: Global Positioning System

GUI: Graphical User Interface

HMLR: Her Majesty’s Land Registry

HP: Hewlett-Packard

HTML: Hyper Text Markup Language

HTTP: Hyper Text Transport Protocol

HW: HardWare

I/O: Input/Output

IE: Internet Explorer (Microsoft browser)

IEEE: Institute of Electrical and Electronics Engineers

IMS: Information Management System

IP: Internet Protocol

ISDN: Integrated Services Digital Network

ISO: International Standards Institute

ISPRS: International Society of Photogrammetry and Remote Sensing

KB: Kilo Byte

KCGIS: Kingston Centre for GIS

LAN: Local Area Network

LIS: Land Information System

LUT: Look Up Table

MB: Mega Byte

MHz: Mega Hertz

MIS: Management Information System

MO: MapObject

MODEM: MOdulator-DEMolulator

MOIMS: MapObjects Internet Map Server

NA: Network Analysis

NG: National Grid

NC: Network Computer

NDVI: Normalised Difference Vegetation Index

NET: NETwork

NIC: Network Interface Card

NFS: Network File System

NLIS: National Land Information Service

NOAA: National Oceanographic and Atmospheric Administration

NSF: (US) National Science Foundation

NT: New Technology (in Windows NT)

NTF: (UK) National Transfer Format

OCR: Optical Character Recognition

ODBC: Open DataBase Connectivity

OGIS: Open Geodata Interoperability Specification

OLE: Object Linking and Embedding

OLTP: OnLine Transaction Processing

OO: Object Oriented

OODBMS: Object Oriented Data Base Management System

OS: Operating System, Ordnance Survey.

PC: Personal Computer

PERL: Practical Report and Extraction Language (in UNIX)

PIXEL: PIcture Element

PS: PostScript

RAM: Random Access Memory

RDBMS: Relational DataBase Management System

RGS: Royal Geographical Society

RIMS: Road Information Management System

RGB: Red, Green, Blue

RISC: Reduced Instruction Set Computing

SAS: Statistical Analysis System

SCSI: Small Computer System Interface

SDE: Spatial Database Engine

SIMS: Survey Information Management System

SIS: Spatial Information System

SPSS: Statistical Package for the Social Sciences

SQL: Structured Query Language

SVGA: Super Video Graphics Adapter

SW: SoftWare

TCP/IP: Transport Control Protocol / Internet Protocol

TELNET: TELetype NETwork

TIFF: Tagged Image File Format

TIGER: Topologically Integrated Geographic Encoding and Referencing

TM: Thematic Map(ping)

UI: User Interface

UPRN: Unique Property Reference Number

URL: Uniform Resource Locator

UTM: Universal Transverse Mercator

VB: Visual Basic

VGA: Video Graphics Adapter

VR: Virtual Reality

VRML: Virtual Reality Modelling Language

WAN: Wide Area Network

WWW: World Wide Web