Glossary

accuracy

The closeness of the results of observations, computations or estimates to the true values or the values accepted as being true. Accuracy relates to the exactness of the result and is the exactness of the operation by which the result is obtained.

administrative area

A blanket term used by Ordnance Survey to refer to all public administrative areas, specifically local government management and electoral areas.

administrative unit

A single administrative area.

area

A spatial extent is defined by circumscribing lines that form a closed perimeter that does not intersect itself.

attribute

An attribute is a property of an entity, usually used to refer to a non-spatial qualification of a spatially referenced entity. For example, a name or descriptive code indicating what an entity represents or how it should be portrayed.

boundary

Boundaries define the areas of the various national, local government and some European authorities.

code

An alphanumeric attribute code used in digital map data to describe each feature in terms either of the object surveyed or its representation on the map (or both).

coding

Allocation of a feature code to a feature being created from constituent construction data: points and/or segments; with optional linking to an existing feature of the same feature code.

coordinates

Pairs of numbers expressing horizontal distances along original axes. Alternatively, triplets of numbers measuring horizontal and vertical distances. Row and column numbers of pixels from raw imagery are not considered coordinates for the purpose of the standard.

currency

An expression of the up-to-dateness of data.

data structure

The defined logical arrangement of data as used by a system for data management; a representation of a data model in computer form.

distinctive name

A text feature consisting of a text string(s) which form(s) a proper name.

entity

Something about which data is stored in a databank or database, for example, boundary and name. The data may consist of relationships, attributes, positional and shape information and so on. Often synonymous with feature.

explicit

Data that is directly represented in digital form. For example, the relationship between two objects is explicit if recorded by such means as pointers and does not have to be deduced by further analysis of the data.

feature

An item of detail within a map that can be a point and/or symbol, text or line.

geographical information system (GIS)

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 application software.

layer

A subset of digital map data selected on a basis other than position. For example, one layer might consist of all features relating to counties and another to wards. Also known as a level.

level

A level corresponds to a single type of administrative unit (for example, a ward or a district) and is conceptual in form. See also layer.

line

A series of connected coordinated points forming a simple feature with homogeneous attribution.

line feature

The spatial abstraction of an object in one dimension. Lines may intersect with other lines. They are defined as a series of two or more coordinate pairs and may be curved or straight. Curved lines consist of a series of very short straight-line segments. As an object abstraction, a line has no width.

link or edge

Links are the representation of line features. They are made up of one or more consecutive

non-intersecting link segments with common attributes between two terminating nodes. Links have no connection with other links except at the start or end, via common (shared) terminating nodes (points). All links contain their terminating coordinates. Links may form the boundaries of polygons and may be shared between polygons.

name

The proper name or label of an object (real world) or feature (object abstraction). The descriptive name might consist of one or more text strings or be an attribute of the object or object abstraction.

National Grid

A unique referencing system that can be applied to all Ordnance Survey maps of Great Britain (GB) at all scales. It is used by Ordnance Survey on all post-war mapping to provide an unambiguous spatial reference in Great Britain for any place or entity whatever the map scale. The National Grid is defined by the OSGB36 spheroid.

object

A collection of entities which form a higher-level entity within a specific data model.

object (real world)

A recognisable discrete part of the real world.

operative date

The date the order was made; not necessarily the day it becomes ‘live and in use’.

origin

The zero point in a system of rectangular coordinates.

polygon

Polygons are a representation of areas. A polygon is defined as a closed line or perimeter completely enclosing a contiguous space and is made up of one or more links. At least one node occurs on the perimeter of a polygon where the bounding link completes the enclosure of the area. There may be many nodes connecting the bounding links of a polygon. Links may be shared between polygons. Polygons may wholly contain other polygons; or be contained within other polygons.

positional accuracy

The degree to which the coordinates define a point’s true position in the world, directly related to the spheroid and/or projection on which the coordinate system is based.

record

A set of related data fields grouped for processing.

rectangular coordinates

Also known as X-Y coordinates and as eastings and northings. These are two-dimensional coordinates that measure the position of any point relative to an arbitrary origin on a plane surface, for example, a map projection.

relative accuracy

The measure of the internal consistency of the positional measurements in a dataset. For many local area purposes (for example, records of utility plant), relative accuracy is more important than absolute accuracy. In this case, accurate measurement of offsets from fixed points is required rather than knowledge of the true position in space.

resolution

A measure of the ability to detect quantities. High resolution implies a high degree of discrimination but has no implication as to accuracy. For example, in a collection of data in which the coordinates are rounded to the nearest metre, resolution is 1 metre, but the accuracy may be ± 5 metres or worse.

segment

A chord defined by two consecutive coordinates in a line string.

Statutory Instrument

An order made by a Minister under delegated power from Parliament. Contains changes to boundary information, the alignment, type or relationship to a named area (amalgamation) or a change of name to an area are made by order.

transfer format

The format used to transfer data between computer systems. In general usage, this can refer not only to the organisation of data, but also to the associated information, such as attribute codes, which are required in order to successfully complete the transfer.

vector

A straight line joining two data points.