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Last updated
Last updated
OS MasterMap Topography Layer is grouped into feature themes. Within each theme there will be features deemed to belong to that theme. A feature can belong to more than one theme.
Each feature will have one or more versions of itself which can be tracked as a means of demonstrating change over time. New versions of features will replace existing features through COU, and features that no longer exist can be deleted as a result.
This page introduces the concepts of themes, features, and attributes applicable to OS MasterMap Topography Layer.
A theme is a set of features that have been grouped together for the convenience of users and to provide a high-level means of dividing the data on the layer in a logical and user-friendly fashion. A feature can be a member of any number of themes but must belong to at least one theme as a minimum requirement.
A theme is created by applying rules based on the attributes of features. A theme rule can depend on the common values contained in the feature attributes. A feature is considered a member of each theme to which it passes a theme rule.
Themes are not part of the classification system of features for OS MasterMap. Because of this, a new theme can be created, for the convenience of our users, without in any way affecting either the existing themes or the classification of OS MasterMap features. An overview of each theme for OS MasterMap Topography Layer is given on the product themes page.
In this Overview, the term 'real-world object' is used to describe a geographic entity that can be captured and represented in the data. A real-world object is represented by a feature in OS MasterMap data.
A complete list of the real-world objects and their feature representations is given in the OS MasterMap Real World Object Catalogue, which is available from the Related Links section at the bottom of the OS MasterMap Topography Layer Product Support page on the OS website.
Each feature has one of three geometrical structures – a point, a line, or a polygon. A line feature will have a start and end node that reflects the start and end of the real-world object it represents. Where the start and end node are coincident, the feature created is structured as a polygon. Lines and polygons represent both the location and the geometry of the real-world object. Points do not necessarily represent the exact geometry of the real-world object, just the centroid of its location. Text features are used to provide additional information and context about real-world objects represented by point, line, or polygon features. They are represented as a point, which indicates the location where the text should be displayed.
Polygon features fit with each other topologically within structuring layers. This means that polygons sit adjacent to each other like pieces of a jigsaw puzzle, rather than on top of each other. In the image below, it can be seen that buildings (orange squared-off polygons), if ‘removed’, leaves their footprints in a land feature (green polygon); the land feature does not exist below the buildings.
Each feature comes with an extensive set of attributes that provide information about the feature, for example, its identity, its relationship to other features, its geometry, and the kind of real-world object it purports to represent. Each type of feature has a different set of attributes.
There are two types of attribute information:
Attributes that provide data about the real-world object the feature represents, such as its area or its nature.
Attributes that provide additional information about the feature, such as its lifecycle and quality.
Three of the most important pieces of additional information for the vector layers – the TOID, version, and version date – are discussed in more detail in change management and data association.
OS MasterMap Topography Layer provides attribution that can be searched for and queried within a GIS. Attribution makes it possible to select parcels of land with common features, for example, a polygon attributed as having coniferous tree cover.
