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Ordnance Survey - Glossary: mapping terminology and acronyms Category: Electronics and Engineering > mapping Date & country: 24/09/2007, UK Words: 414

Descriptive Name
A subtype of Name; positioned by a coordinate pair.

Descriptive Term
An optional attribute providing further description of a feature e.g. cliff, scrub, standard gauge track.

DGPS
See Differential GPS.

Differential GPS
A technique for reducing the error in GPS-derived positions by using additional data from a reference GPS receiver at a known position. The most common form of DGPS involves determining the combined effects of navigation message ephemeris and satellite clock errors (including the effects of SA) at a reference station and transmitting pseudorange corrections in real time to a users receiver, which applies the corrections in the process of determining its position.

Digital
Data or voltages consisting of discrete steps or levels. Data which is expressed as numbers (digits) in computer readable form is said to be digital.

Digital (Map) Archive
Archival map data stored in digital format.

Digital Audio Tape
A storage medium that is increasingly used for data storage. A DAT cartridge is slightly larger than a credit card and contains magnetic tape that can hold from 700MB to 2.3GB of data.

Digital Chart of the World
A vector dataset based on 1:1 000 000 scale air navigation charts produced by the United States Defense Mapping Agency.

Digital Elevation Model
A 3D representation of the height and shape topography of the Earth's surface. A DEM is formed by a regular grid of height values and can be overlayed with other data to create DTM.

Digital Identifier
An identifier that is primarily intended to provide unique and unambiguous feature identification for the purposes of exchanging feature based information between computer systems, or associating data within a computer system.

Digital Map
A term used by Ordnance Survey to describe a particular tile of digital map data.

Digital Terrain Model
A 3D representation of the Earth's surface. It's construction includes a height model (i.e. a DEM) and overlaid with map data relating to features on the surface (i.e. Map Data or Aerial Photograph).

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

Dilution of Precision
A dimensionless number that accounts for the contribution of relative satellite geometry to errors in position determination. Generally the wider the spacing between the satellites being tracked by a GPS receiver, the smaller the position error. The most common quantification of DOP is through the position dilution of precision (PDOP) parameter. Other DOPs include the geometric dilution of precision (GDOP), horizontal dilution of precision (HDOP) and vertical dilution of precision (VDOP).

DIP
See Document Image Processing.

Dirichlet Tesselations
See Thiessen Polygons.

Disk Mirroring
A security feature that protects against hard disk failure by copying data from one hard disk to another. The latter operates automatically should the first fail.

Distinctive Name
A name given to a feature or place to distinguish it from other features or places of a similar nature e.g. River Thames, Park Lane Methodist Church, Leeds or New Forest.

Distributed Component Object Model
The conceptual model which underlies Microsoft's ActiveX technology. The component model describes how components (which may be on different machines) may interact with each other.

DNF
See Digital National Framework.

DNS
See Domain Name System.

Document Image Processing
This involves capturing a digital image of paper-based information using scanners and DIP software. At this stage you can also incorporate OCR software for automatic data capture.

Domain (Application Context)
The body of knowledge defining the range and scope of an application in terms of elements, rules and behaviours. More specifically as applied by the Ordnance Surveys a domain is a body of data that shares a common creation and maintenance regime.

Domain (System Context)
A class of systems that have similar requirements and capabilities.

Domain Name System
Allows the TCP/IP addresses, basically just a series of numbers, to be organised as meaningful addresses e.g. www.ordnancesurvey.co.uk

DOP
See Dilution of Precision.

Doppler Effect
The shift in the frequency of a received radio signal due to the relative motion of the transmitter and receiver.

Dots per Inch
A measurement of input device resolution.

Down-Sampling
The reduction in resolution of an image, necessitating a loss in detail.

Dpi
See Dots per Inch.

DTM
See Digital Terrain Model.

E-Delivery
The delivery of digital products and services to customers by electronic means, primarily by use of Internet technology.

E-Mail
The exchange of text messages and computer files over a communications network.

Easement
The right, priviledge or liberty given to a person or group to use land belonging to another for a specific and definite purpose e.g. the right given to an electricity company to bring electricity transmission lines across a private property.

Eastings
See Rectangular Coordinates.

Edge
See Link.

Encryption
Using a set of either public or public/private keys to encrypt and decrypt data, it ensures that information is unreadable by anyone other than the intended recipient.

Entity
Something about which data is stored in a databank or database e.g. building or tree. The data may consist of relationships, attributes, positional and shape information and so on. Often synonymous with Feature.

Entity Type
A grouping of entities with common characteristics.

EOR
See Extent of the Realm.

Ephemeris
A description of the path of a celestial body indexed by time (from the Latin word ephemeris meaning diary) The navigation message from each GPS satellite includes a predicted ephemeris for the orbit of that satellite valid for the current hour. The ephemeris is repeated every 30 seconds and is in the form of a set of 16 Keplerian-type parameters with corrections that account for the perturbations to the orbit caused by the earth's gravitational field and other forces.

Extensible Mark-up Language
A markup language used on the Internet to describe contents of documents, datasets and other entities. HTML is a subset of XML.

Extent of the Realm
The Territorial Waters Jurisdiction Act 1878 and the Territorial Waters Order in Council 1964 confirm that the extent of the realm of Great Britain as used by Ordnance Survey is properly shown to the limit of mean low water (mean low water springs in Scotland) for the time being (except where extended by Parliament). The external bounding lines of Land-Line data is the extent of the realm.

Extranets
Company to company intranets that allow access only to authorised users.

Face
A surface bounded by a closed sequence of edges. Faces are contiguous and fill the spatial extent of the dataset and do not overlap.

Feature
A digital representation of a real world entity or an abstraction of the real world. It has a spatial domain (attribute), a temporal domain, or a spatial/temporal domain as one of its attributes. Examples of features include almost everything that can be placed in time and space, including desks, buildings, cities, trees, forest stands, ecosystems, delivery vehicles, snow removal routes, oil wells, oil pipelines, oil spill and so on. Features are usually managed in groups as feature collections. In the context of the Ordnance Surveys, a feature may not be mobile.

Feature Code
A numeric attribute used in digital map data to describe each feature in terms either of the object surveyed or its representation on the map (or both). A feature code is equivalent to a layer in DXF.

Feature Identifier
A unique code to identify an individual feature.

Feature Serial Number
A number used as a feature identifier usually allocated on a sequetial basis e.g. The order in which features are digitised.

Feature Serving
A service which allows any application (Ordnance Survey, licensed partner or third party) to access the OS MasterMap database direct, online and in real-time. The applications will be able to make data requests and be supplied with the resultant features in GML. It is a purely machine-to-machine service (application to database) and replaces local holdings of data with real-time, online access to a remote holding (i.e. the OS MasterMap database) This service will be made available via a Web feature server. see also Map Serving.

Feature Type
A fundamental or primary classification of a feature. Examples are more precise examples building, archway, coniferous tree, benchmark.

Field
A specified part of a record containing a unit of data.

File
An organised collection of related records. The records on a file may be related by a specific purpose, format or data source and the records may or may not be arranged in sequence. A file may be made up of records, fields, words, bytes, characters or bits.

File Set
The set of files required to describe a single data supply in a given format. Where the format enables all the data to be contained in a single file, the file set will be the same as the file.

File Transfer Protocol
A protocol which allows a user on one computer to transfer files to and from another computer over a TCP/IP network (e.g. Internet). This will be the method by which OS MasterMap data is made available for online supply.

Firewall
Used as a security measure to protect intranets from the traffic that passes in and out of them. A combination of hardware and software that manages authorisation of uses trying to access the intranet. The firewall bars unauthorised data packets from entering or leaving a network. Firewall software specifies which data packets are authorised. It typically resides on routers or dedicated servers.

Flatbed Scanner
Any scanning device that incorporates a flat transparent plate on which original images are placed for scanning. The scanning process is linear rather than rotational.

Footprint
The ground area occupied by an object e.g. a building.

Foreshore
The foreshore is taken to be the area of land between mean high water (MHW) or mean high water springs MHWS) in Scotland and the extent of the realm.

Format
The process of 'initialising' a floppy disk so that it can be used to store data.

Format
The specified arrangement of data e.g. The layout of a printed document, the arrangement of the parts of a computer instruction, the arrangement of data in a record. Different programs store data in different formats.

Free End
The end of a line feature which does not intersect or connect with any other line feature i.e. The point defining the free end does not share a coordinate pair with any other feature. Note: It may be coincident with a point feature e.g a post box on the end of a wall.

FSN
See Feature Serial Number.

FTP
See File Transfer Protocol.

Fuzzy Logic
A means of quantifying uncertainty. Allows us to represent statements such as 'John is very tall' without having to define hard limits for 'tall'. Words like 'very', 'fairly' and 'quite' can be represented in mathematics.

Fuzzy Search
A search which encompasses words that either look or sound very similar e.g. A fuzzy search for Smith might include Smythe or Smit.

Gazetteer
A list of spatial entities held in computer or printed form, such as properties or streets, which allows for rapid search or query.

Geocode
The element in a database used to identify the location of a particular record e.g. 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 coordinates.

Geodata
Information that identifies the geographical location and characteristics of natural or man-made features and boundaries of the earth. Geodata represent abstractions of real world entities such as roads, buildings, vehicles, lakes, forests and countries.

Geodemographic Data
Statistical population data, or demographic data, with a spatial reference. For example, census information based upon enumeration districts. This is a type of map data.

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

Geographic Information
Information about objects or phenomena that are associated with a location relative to the surface of the earth. A special case of spatial information.

Geographical Information System
A system for capturing, storing, checking, integrating, analysing and displaying data that is spatially referenced to the earth. This is normally considered to involve a spatially referenced computer database and appropriate applications software.

Geography Mark-up Language
The subject of a recommendation paper from OpenGIS, which describes a markup language based on XML, used to store and transfer geographic data over the Internet (specifically geographic data which conforms to the OpenGIS simple features specification).

Geoid
An imaginary shape for the earth defined by mean sea level and its imagined continuation under the continents at the same level of gravitational potential.

Geoidal Height
The height of a point on the geoid above on ellipsoidal reference surface.

Geometric Data
Data about position within an absolute or relative coordinate system.

Geospatial Data
Another term used to describe Map Data but commonly isn't directly associated with a map e.g. An address has a spatial reference associated with it but not in map coordinates.

GI
See Geographic Information.

GIS
See Geographical Information System.

Global Positioning System
A system of coordinating a ground position in three dimensions using radio transmissions from a pattern of US defence satellites.

GML
See Geography Mark-up Language.

GPS
See Global Positioning System.

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.

Greyscale
A continuous tone image comprising black, white and grey data only.

Grid
The planimetric frame of reference e.g. The National Grid.

Grid Goordinates
A plane-rectangular coordinate system based on and mathematically adjusted to a map projection in order that geographic positions (latitudes and longitudes) may be readily transformed into plane coordinates and the computations relating to them made by the ordinary methods of plane surveying.

Ground Height
The height where the building wall intersects the ground.

Ground Station
A facility capable of receiving signals from earth observation satellites such as LANDSAT, SPOT, ERS, JERS AND MOS.

GUI
See Graphical User Interface.

Hard Disk
A device containing one or more inflexible disks coated with material in which data can be recorded magnetically, together with their read/write heads, head positioning mechanism and the spindle motor in a sealed case. This offers high capacity data storage. Most hard disks are permanently sited in a computer.

History
A record of change over time for features or objects e.g. a boundary change. In the context of geospatial data, the storage (and potentially the supply) of deleted features and superseded versions of features.

Holdings
See Agreement.

Home Page
The first page of a Web site, whether on Internet or an intranet.

HTML
See HyperText Markup Language.

HTTP
See HyperText Transfer Protocol.

Hydrographic Surveying
The measurement and description of the physical features offshore and adjoining coastal areas with special reference to their use for the purpose of navigation.

Hypertext Link
These allow the user to jump from one page/Web site/document to another by generating a browser request simply by clicking on the link. These links are often depicted in blue, underlined text.

HyperText Markup Language
An open format for sharing electronic text documents with hypertext extensions via the Internet/Intranet.

HyperText Transfer Protocol
The simple request/response protocol that runs on top of TCP/IP that allows Web browsers to access files on any Web server.