Digimap OS MasterMap Guidance Notes

• Digimap OS MasterMap Guidance Notes
• OS MasterMap Basics
  • What is OS MasterMap?
  • Topography Layer
  • Integrated Transport Network (ITN)
  • Data Model and Structure
  • The TOID
  • Version Numbers and the Object Life Cycle
• Topography Layer Features
  • OS MasterMap Topography Layer Themes
  • Classification of OS MasterMap Topography Layer Features
  • Feature Attributes
  • Feature description attributes
• ITN Layer
  • ITN Roads Theme
  • ITN Roads Features
  • Road Routing Information (RRI) Theme
• Geography Markup Language (GML)
  • What is GML?

OS MasterMap Basics

What is OS MasterMap?

OS MasterMap is not a product, it is the framework on which future Ordnance Survey products will be based (the project was initially known as the Digital National Framework or DNF). The central concept of the OS MasterMap framework is the closer relationship between real world objects and features held in the OS spatial database. The Ordnance Survey currently produces four layers of MasterMap data; topography, address, imagery and ITN (roads) layers. Initially two OS MasterMap layers will be delivered by Digimap, the Topography and Integrated Transport Network (ITN) layers.

Topography Layer

In OS MasterMap, topographic features are representations of real-world objects, including buildings, roads, paths, railways, rivers, lakes, ponds, other structures (such as oil storage tanks and pylons), and land parcels. The data also includes non-topographic features such as administrative and electoral boundaries, cartographic text and symbols. All features are represented as points, lines, areas and text, each with attributes that provide additional information relating to that feature.

Integrated Transport Network (ITN)

OS MasterMap ITN features are representations of physical roads. These features also include the name or number of the road, the road classification and the nature of the road. The ITN layer also includes road routing information such as on-way streets, and junction information.

Data Model and Structure

The data model and structure are fundamentally different from that used in Land-Line. The topographic layer data are sourced from an enhanced version of the OS Topo96 large-scale data from which the Land-Line product is derived. However, the data have been restructured into a seamless database (in contrast to a tiled database as for Land-Line) of approximately 440 million objects. It is fundamentally different from the tile-based Land-Line data. Where possible and desirable, each discrete and identifiable real world object is represented by an OS MasterMap feature.

A significant number of enhancements have been made during the re-engineering of Land-Line to OS MasterMap. A significant departure from Land-Line is the creation of polygons to represent area objects, such as buildings. This adds value to the data and marks a significant improvement over the previously unstructured, unpolygonised data in Land-Line.

The TOID

Every OS MasterMap feature has a range of attributes characterising it, building in more descriptive detail and richer attribution than is present in the older product. One of these attributes is always a unique 16 digit integer – the Topographic Object Identifier or TOID. Every OS MasterMap feature has a unique identifier known as a TOID. This is a number that uniquely identifies that feature. TOIDs hold no intelligence; they are allocated sequentially as updates are applied to the database. The TOID will stay the same throughout the life of a feature. OS MasterMap TOIDs have several purposes:

• TOIDs are designed to allow the association of data pertaining to real-world objects, thus reducing the need for multiple parties to capture and maintain feature geometry and facilitating sharing of information between OS MasterMap users.
• TOIDs are also used to refer to one OS MasterMap feature from another. For instance, the boundary of an OS MasterMap area feature in topological polygon data is specified by a chain of line feature TOID references.
• TOIDs are used to inform the user of modified and departed features in change-only data update.
• Sets of TOIDs can be used as complex features: representing schools; factories; road junctions; or individual properties comprising a house, its garage and garden.

Version Numbers and the Object Life Cycle

In addition to the TOID, each feature has a version number. The version number is incremented each time there is change of any kind to the feature in the Ordnance Survey database. This means that a feature can be uniquely identified at any stage in what is known as its life cycle. Real-world objects have life cycles. For example, the life cycle of a building might be as follows: it is constructed, it may be extended one or more times, it may eventually be demolished. OS MasterMap features also have life cycles. For example: a new building feature will be created in the data; it may be modified once or more times (to record the changes made to the realm building), and eventually it will be deleted (when the building is demolished). Information about the stage of the life cycle of a feature is provided in the attributes attached to a feature.

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Topography Layer Features

Features are grouped into themes such as buildings, roads and land, to enable more flexible data selection by users. Themes should not be thought of as independent data layers, because one feature can be a member of more than one theme. Themes allow a user who is only interested in, say, building footprints, to select just this data.

OS MasterMap Topography Layer Themes

The themes currently available in the topographic layer are:

1. Administrative boundaries: The limits of responsibility and representation defined for electoral and administrative purposes.
2. Buildings: Roofed constructions, usually walled. Includes permanent roofed constructions that exceed 8.0 m2 in area (12 m2 in private gardens). Exceptions are made for smaller buildings in such a detached position that they form relatively important topographical features. Mobile homes, residential caravans and so on are not captured. Storage tanks may be classified as buildings.
3. Heritage and antiquities: Features and cartography which depict sites or constructions of historic interest. This theme is currently very limited in content.
4. Water: Features that; contain, delimit or relate to real-world objects containing water.
5. Land: Man-made and natural features that delimit and describe the surface cover other than communication routes and buildings. This also includes all features relating to man-made and natural slope and cliff. All General features are also placed in the land theme.
6. Rail: Features related to travel by railway or tramway. All features that have been identified as part of the rail network. Currently railway tunnels are not classified as railway and so will be present in the Structures theme.
7. Roads tracks and paths: Features related to transport by vehicles, cycles or pedestrians. This includes features that are road, made paths or text classified as road, track or path related.
8. Structures: Man-made constructions that are not buildings. These may or may not obstruct passage at ground level.
9. Terrain and height: All current height information included in OS MasterMap defining ground level. This is currently very limited.

Classification of OS MasterMap Topography Layer Features

OS MasterMap classifies features using feature type, feature description attributes, and feature code. This section is an overview of the function of each of these and the relationships between them.

There are eight types of OS MasterMap feature. For each feature type, a set of feature attributes is defined. The seven types are:

1. TopographicArea: All OS MasterMap area features.
2. TopographicLine: All OS MasterMap line features, except administrative boundary features. This includes features representing objects which are not physically present, such as inferred area feature boundaries.
3. BoundaryLine: Administrative boundary line features.
4. TopographicPoint: All OS MasterMap point features. This includes features representing objects which are not physically present, such as spot heights.
5. CartographicSymbol: All cartographic symbol features.
6. CartographicText: All cartographic text features.
7. DepartedFeature: In change-only update data supply only, this type is used to indicate features which have been deleted or have otherwise departed from the requested data since a given date.

Feature Attributes

Each feature type has a set of attributes associated with it, for example, the attributes held on a feature of type TopographicLine are:

• TOID: The unique 16-digit reference number of a feature. If leading zeroes are omitted, the number may appear as less than 16 digits.
• version: The version number of the feature, in the range 1 to 25565535.
• versionDate: The date on which this version of the feature became the current version.
• theme: A theme that the feature belongs to.
• featureCode: A five-digit integer.
• accuracyOfPosition: The accuracy of a horizontal position in metres at the 95% confidence level.
• changeHistory: Reason for change and change date.
• descriptiveGroup: The primary classification of the feature.
• descriptiveTerm: Further terms classifying the feature.
• heightAboveDatum: The height of the feature above the Ordnance Survey Datum Newlyn (ODN) vertical datum, in metres.
• heightAboveGroundLevel: The height of the feature above ground level, in metres.
• make: Attribute describing whether a feature in natural or man-made.
• physicalLevel: Indicates the physical level of a feature with reference to the normal cartographic surface level. Only four values are used at present:
  • -1 = Underground detail,
  • 49 = Obscured detail below the normal cartographic level,
  • 50 = Detail at the normal cartographic level,
  • 51 = Overhead detail above normal cartographic level
• physicalPresence: This attribute indicates the real-world presence of the object represented by the feature. For example, a value of obstructing indicates that the feature prevents pedestrian access, whereas edge/limit means that the feature represents a change of surface type and does not prevent access.
• polyline: An ordered set of points (see above), interpolated linearly to define the geometry of a line feature. See section in product specification on “Geometry and Topology” for details.
• referenceToFeature: Reference by TOID to a related feature. In topographic features, this is used to point from a heighted feature to an unheighted feature. In cartographic symbol features, it is used to point from the symbol feature to the topographic feature it refers to.

Feature description attributes

OS MasterMap features have up to five attributes which make up the description of the feature: descriptiveGroup, descriptiveTerm (optional), physicalLevel, physicalPresence (optional), and make (optional). A combination of these attributes can be used to determine the appropriate representation and symbology to use when displaying the data. The information conveyed by each of these attributes is as follows:

• descriptiveGroup. This is the primary classification attribute of a feature. It assigns each feature to one or more of 21 groups (see Table 1 below), most of which are categories of real-world topographic objects such as Path, Building, Natural Environment etc. In general, values of this attribute are not specific to particular feature types – features of any type can have any descriptiveGroup value. Due to limitations in the source data from which OS MasterMap was created, there are some exceptions to this principle. For instance, descriptiveGroup buildings or structure contains text describing or naming buildings and structures, while the associated topographic features have the descriptiveGroup Building, Glasshouse or Structures.
• descriptiveTerm (optional). This attribute, if present, gives further classification information about the feature. A feature may have multiple descriptive terms, but this is little used at present. Most features have zero or one descriptive term. A situation where multiple descriptive terms are used is in natural environment area features. These features can have one or more descriptive terms specifying the natural land cover types present in the area.
• physicalLevel. This attribute states whether the feature is underground, obscured below normal level, at normal level, or overhead.
• physicalPresence (optional). This attribute indicates the real-world presence of the object represented by the feature. For example, a value of obstructing indicates that the feature prevents pedestrian access, whereas edge/limit means that the feature represents a change of surface type and does not prevent access.
• make (optional). Indicates whether the feature is man-made or natural.

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ITN Layer

The OS MasterMap ITN Layer is currently composed of two themes – Roads and Road Routing Information. This may be expanded in the future to include rail, water, track and path themes.

ITN Roads Theme

The Roads theme consists of a fully topologically structured link and node network representing the driveable roads of Great Britain.

ITN Roads Features

The following features are included in the Roads theme:

1. RoadLink
2. RoadNode
3. InformationPoint
4. FerryLink
5. FerryNode

The product contains the following information:

1. Road classifications
2. Road names
3. Types of roads
4. Motorway junctions
5. Information potentially relevant to routing
6. References to the intersecting topographic polygons

Road Routing Information (RRI) Theme

RRI is information about a route that may affect a driver’s choice of route. It could be either restriction information such as a prohibited turn of one-way street, or advisory information such as the presence of a ford.

Within ITN, routing information is maintained as separate features from the base network information, but these features reference the underlying road network features to provide their location.

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Geography Markup Language (GML)

What is GML?

OS MasterMap is supplied in geography Markup Language (GML). GML is an international standard for storing and transporting geographical information developed by the OpenGIS Consortium (OGC). This is an open standard, based on XML, compatible with a host of web-based technologies and encourages interoperability. It has been chosen as the data supply method in place of traditional transfer formats for a variety of reasons. The main reasons may be summarised as follows:

• GML is a ‘dialect’ of the Extensible Markup Language (XML) and is not tied to any proprietary GIS or database software. OS MasterMap will, therefore, be more accessible.
• XML may be considered as the standard language of the Internet and GML has been designed for the transfer of spatial data over the Internet. The Geographic Information industry will be able to take advantage of a wide array of different technologies.
• GML is easy to manipulate, change and add to.
• GML is feature based and matches the object oriented structure of OS MasterMap.
• Exchange of data will be easier. Ordnance Survey MasterMap data are currently represented in GML v2.1.2.

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