| Glossary term | Definition |
|---|---|
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.
American Standard Code for Information Interchange (ASCII)
A 7-bit code for encoding a standard character set.
area
A spatial extent 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.
attribute code
An alphanumeric identifier for an attribute type.
boundary
Boundaries define the areas of the various national and local government authorities and some European authorities.
chain
A closed loop of links bounding a polygon.
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.
contour
A line connecting points of equal elevation.
coordinate pair
A coordinate pair is an easting and a northing.
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.
data format
A specification that defines the order in which data is stored or a description of the way data is held in a file or record.
data model
An abstraction of the real world that incorporates only those properties thought to be relevant to the application or applications at hand. The data model would normally define specific groups of entities and their attributes, and the relationship between these entities. A data model is independent of a computer system and its associated data structures. A map is one example of an analogue data model.
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.
eastings
See rectangular coordinates.
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.
Extensible Markup Language (XML)
This is a markup language written in a textual data format designed to encode documents and data structures for transfer over the Internet. It was developed by the World Wide Web Consortium (W3C). XML schemas express shared vocabularies and allow machines to carry out rules made by people. They provide a means for defining the structure, content and semantics of XML documents.
Extensible Stylesheet Language Transformations (XSLT)
This is a language for transforming XML documents into objects that can be presented in a format that is more easily read by the user, such as HTML for web pages or plain text.
feature
An item of detail within a map that can be a point and/or symbol, text or line.
feature identifier
A unique code to identify an individual feature. A specified part of a record containing a unit of data, such as the date of digitising. The unit of data may be a data element or a data item.
feature record
The logical information, both spatial and attribute, describing a feature or entity.
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 applications software.
Geography Markup Language (GML)
GML was developed by the Open Geospatial Consortium (OGC), a global organisation of developers and users that aims to maximise the benefit of geographic information. GML is a spatially enabled dialect of XML schema.
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.
line segment
A vector connecting two coordinated points.
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.
map scale
The ratio between the extent of a feature on the map and its extent on the ground, normally expressed as a representative fraction, such as 1:1250 or 1:10 000.
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.
Northings
See rectangular coordinates.
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.
origin
The zero point in a system of rectangular coordinates.
point and line data
A form of vector data designed for map production in which all map features are designated as points, lines or text. Point and line data does not carry the topological relationships between features.
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.
polygon boundary
The link(s) which enclose a polygon, projected into the horizontal plane. A chain.
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).
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.
shapefile
This is a data format developed by Esri to describe features such as points, lines and polygons to enable spatial analysis. A shapefile consists of several files designed to hold information essential for the transfer of this data between software products which are capable of reading shapefiles.
spatial data
Data that includes a reference to a two or three dimensional position in space as one of its attributes. It is used as a synonym for geometric data.
spot height
A point on the Earth’s surface for which the height, above a reference datum, is known and which has been fixed by observation.
String
A set of items which can be arranged into a sequence according to a rule. A sequence of coordinate pairs or triplets making up a line or a link.
structured data
Data within which collections of features (of any type) form objects. Topographically structured data also contains topological information, defining the relationships between features and objects.
Topography
The study of the physical features of the Earth. A topographic map’s principal purpose is to portray and identify the features of the Earth.
Topology
The study of the properties of a geometric figure that are not dependant on position, such as connectivity and relationships between lines, nodes and polygons.
Vector
A straight line joining two data points.
vector data
Positional data in the form of coordinates of the ends of line segments, points, text positions and so on.