Address Matching
A geocoding process which matches the street address of property to its location. This usually involves the matching of two database files, one containing the addresses of interest, the other a list of addresses and their co-ordinates. Address matching is central to many applications in direct marketing.

Automated Mapping/Facilities Management. This is a specific application of GIS to the management and production of maps of plant such as cables, pipes, valves etc. It is currently the most widely used application of GIS, and particularly relevant to local authorities and utilities.

American Standard Code for Information Interchange. A standard set of codes which represent alphanumeric characters
stored as a single byte value. For example, using the ASCII code, a byte containing the value 69 would represent the letter E. Because of its simple nature, ASCII text is one of the best ways of transferring information between different programs and platforms.

An item of text, a numeric value or an image that is a characteristic of a particular spatial entity


A zone of user-specified distance around a point, line or area. The generation of buffers to establish the proximity
of features is one of the most common forms of GIS analysis. For example, it may be used to find all areas of industry less than 5km from a reservoir.


Computer Aided Design. Software programs for the design, drafting and presentation of graphics. Originally designed for manufacturing drawing, now also widely used for mapping.

A data set containing information related to land ownership and rights. This usually takes the form of maps and
descriptions of uniquely identifiable land parcels. For each parcel, legal information such as ownership, easements and mortgages are recorded.

The basic element within a grid or raster data set.

The centre point of a polygon, often used to attach attribute information to an area such as a census ward. The centroid may be mathematically derived (such as the centre of gravity) or may be user defined. It must always be placed inside the polygon.

CO-ordinate Geometry. Algorithms for handling basic two and three dimensional vector entities built into all surveying, mapping and GIS software.

Numbers representing the position of a point relative to an origin. Cartesian co- ordinates express the location in two or three dimensions as the perpendicular distances from two or three orthogonal axes.


Data Model
A generalised, user-defined view of data representing the real world.

Conversion or encoding of existing maps from an analogue form (paper) into digital information, usually in the form of Cartesian co-ordinates. This may be via a digitising table or tablet with a hand-held cursor, or via a scanner.

Digital eXchange Format A data format defined by Autodesk originally for the transfer of data between CAD systems. Due to its simplicity, it is now widely used in the transfer of data between GIS, despite a number of limitations.


A list of spatial entities held in computer form, such as properties or streets, which allows for rapid search and query. The gazetteer often forms the core of larger GIS-based applications.

The element in a database used to identify the location of a particular record, for example a postcode. The process
of geocoding is similar to that of address matching, in that a data file is compared against a file of Geocode and their associated co-ordinates.

Geodetic Datum
A set of parameters defining co-ordinate systems for all or parts of the earth. These datums have been refined
and revised over time. NAD 27 is the North American datum for 1927, for example. ED50 is the European datum for 1950, and WGS84 is the World Geodetic System for 1984.

Global Positioning Systems. A position-finding system which uses radio receivers to pick up signals from four or more
special satellites (there are 24 in orbit) and compute WGS co- ordinates for the receiver. Accuracy depends on the sophistication of processing and the time available for reception. Real-time navigation using GPS on aircraft and ships can be to better than 10m. Processed data from several hours observation can provide relative positions accurate to centimetres.

Graphical User Interface .A method of interaction with a computer which uses pictorial buttons (icons) and command lists controlled by a mouse. It is generally regarded as simpler and easier to learn than command line interfaces, where commands have to be typed. Examples include MS WINDOWS for PCs, Open Look or MOTIF for workstations and System 7 for Macintosh.

Grid Data
A data structure composed of square cells of equal size arranged in columns and rows.


Land Information System. A subset of the geographic information industry that is dedicated to the management, analysis and presentation of information relating to land, including ownership and legal rights. Often an automated development of the Cadastre.

A spatial reference system for the Earth’s surface. Latitude is an angular measurement N or S of the equator, longitude is an angular measurement E or W of the meridian at Greenwich, UK.


A series of program commands or instructions which are stored in a file and can be recalled when necessary. Macros are commonly used to customise high-end GIS toolkits for individual applications.

Map Projection
A mathematical model used to convert three dimensional reality into two dimensions for representation on a map, or within a two dimensional GIS database. All map projections have particular strengths, some preserve shape, other preserve distance, area and direction. All projections have limitations, however, of which you should be aware.

Map Scale
The measure of reduction between the representation and the reality, be it a map or a spatial database. Scale is usually represented as a representative fraction of distance e.g. 1:50,000, one unit of distance on the map representing fifty thousand units in reality. The nominal scale of a spatial data set has considerable influence over the possible application of the data set, and you should always be aware of any such implications. For example, it would not be sensible to compare the shape of a road represented in a 1:625 000 scale data set with one of 1:1250. Theoretically, a dataset does not have a scale (unlike a map) but the terms Scale is usually used as a metaphor for resolution and content.

Remote sensing in two or more spectral bands.


A model representing the interconnected elements through which some form of resource can be transmitted or will flow. In GIS this is represented as a series of nodes connected by arcs, each or which has attributes representing flow characteristics e.g. a road or pipeline network.

A basic spatial entity within the vector data model which represents the beginning or end of a segment. Also, a node may be formed when a number of segments join. For example a node might be represented in a road network as a highway intersection.


Operating System
A series of computer programs which control the operation of the computer itself. Application programs such as GIS software run under an operating system. Examples of operating systems include MS Windows 95, MS Windows NT, UNIX, VMS, DOS and OS/2.


A hardware component which is connected to a computer to perform specialist functions. Common GIS peripherals include plotters, digitising tables, and printers. When selecting GIS software it is important to ensure that it is compatible with any existing peripherals you use.

A picture element of a raster image as displayed on a screen or raster plot.

A spatial entity that represents the simplest geographical element. Represented in the vector data model as a single x,y co-ordinate, and in the raster as a single cell. The point may have associated attributes which describe the element it is representing; the telephone number of a public call box, for example.

A representation of an enclosed region defined by an arc or a series of arcs that make up its boundary. Polygons may have attributes describing the region they represent, such as the population of a census ward.


A data structure composed of a grid of cells. Groups of cells represent geographical features; the value in the cell represents the attribute of the feature.

Relational Database
A database which structures data in the form of tables. Each table contains information relevant to a particular feature, and is linked to other tables by a common value. For example, two attribute tables could be linked to a spatial data table via a Geocode, such as the postcode.

Remote Sensing
The science of acquiring information about the earth using instruments which are remote to the earth’s surface, usually from aircraft or satellites. Instruments may use visible light, infrared or radar to obtain data. Remote sensing offers the ability to observe and collect data for large areas relatively quickly, and is an important source of data for GIS.

The resolution of a digital dataset expresses the size of the smallest object which can be depicted. The term is most commonly associated with the raster data model where the resolution of a raster or grid is equal to the size of the cell in the real world. For example, the resolution of a remotely-sensed image may be 10m (each cell representing 10mx10m on the ground). Increased resolution leads to larger storage requirements, increased processing and higher costs for a given area.


A data capture technique which digitises information from paper or film hard copy into digital raster data. The process is rapid, but the resulting raster data set only has colour, grey scale or black and white attributes associated with it, and may not have the intelligence necessary for GIS analysis. In effect, the result of scanning is a raster image of the original source material. Segment One of the basic spatial entities, and a basis for spatial models. Formed from a set of ordered co-ordinates (vertices) that represents the shape of a geographic object. An arc begins and ends in a node.

Spatial Analysis
Spatial analysis is the process of applying analytical techniques to geographically-referenced data sets to extract or generate new geographical information. Spatial analysis may be used to model complex geographical interactions, and is useful for investigating site suitability and predicting future events. Although the overall analytical technique may be complex, it is usually a combination of simple techniques applied in the appropriate order.

Structured Query Language. A language developed by IBM in the 1970s for defining and manipulating relational databases. It has since become the industry standard, and is often used to enable GIS toolkits to access the data held in existing corporate databases.


Thematic Map
A map which communicates a single theme or subject. For example, a population density map and political boundary map are both thematic maps. This contrasts with a topographical map which is a general purpose map containing landscape features such as rivers, roads, landmarks and elevation.

The relationships in spatial terms between connected or adjacent geographical objects. Topology is used to apply intelligence to data held in the vector data model. For example, topological information stored for an arc might include the polygon to its left and right, and the nodes to which it is connected.


Vector Data
A data model based on the representation of geographical object by Cartesian co- ordinates, commonly used to represent linear features. Each feature is represented by a series of co- ordinates which define its shape, and which can have linked information. More sophisticated vector data models include topology.