The flowchart below shows the process followed whenever a real-world object represented as an OS MasterMap Topography Layer polygon feature appears, changes, or is removed from the physical environment (i.e. referred to as Inserts, Updates and Deletes). The rules are described in more detail in the following sub-sections, particularly the guidelines used to answer the question in the centre of the flowchart (i.e. 'Is it still the same real-world object?').
Polygon feature creation process in OS MasterMap Topography Layer. Creation of polygon features due to real-world change
When a new real-world object with an area (for example, a building or pond) comes into being, a new polygon feature is created in the data, with a new TOID and version number. Users with local holdings of OS MasterMap Topography Layer data will be informed of new features in their holding via Change-Only Update (COU).
Deletion of polygon features due to real-world change
When an object represented as a polygon feature no longer exists in the real world, the polygon feature is deleted from the database. A record is kept in the database to indicate that a feature with this TOID used to exist and when it was deleted. Users with local holdings of OS MasterMap data are informed of the deletion in their next COU.
Modification of polygon features due to real-world change
When an object represented as a polygon feature changes but is considered to be still the same real-world object, the corresponding modified feature is retained in the database. The version number is incremented and the date on which the new version became current is stored.
If, however, the real-world object has undergone change to such a degree that it is not considered to be the same object as before, the polygon feature representing it is deleted and one or more new features created.
Changes to geometry of polygon features
When a real-world polygon object expands or contracts, due to alteration to its bounding lines, it is considered to be the same real-world object, and as such retains its TOID. For example, the polygon feature representing the back garden of a property is retained, even if it is greatly reduced in size due to extension work done to the house. This is because its identity and association to a property remains unchanged despite extensive changes to its geometry.
If it is not clear whether the real-world object after modification is the same object or a new one, the following considerations are used as a guideline:
Is there topographic information to suggest the function of the resultant real-world object is the same as that of the original?
Is the resultant real-world object more than half the size and less than twice the size of the original?
Does the majority of the extent of the resultant real-world object lie within the bounds of the original?
Is the resultant real-world object the obvious logical successor to the original?
If the continuation of the feature cannot be justified on one or more of these grounds, the feature is deleted and replaced with a new feature.
Examples
A private house is extended. The building and garden features are retained.
A field changes shape and reduces in size due to the realignment of one of its boundary fences alongside a road. The field feature and the adjacent road features are retained.
Splitting of polygon features
When a real-world polygon object is split into two or more separate real-world objects, one of the features may be clearly recognisable as the original real-world object. If this is the case, then the feature is retained.
If it is not clear whether one of the resultant features represents the same real-world object as the original feature, then the following considerations are used as a guideline:
Is the function of one of the resultant real-world objects the same as the original?
Is one of the resultant real-world objects the obvious logical successor to the original?
Does one of the resultant real-world objects occupy more than half the area of the original?
If the continuation of the feature cannot be justified on one or more of these grounds, the original feature is deleted and replaced with new features.
Examples
A new housing development is completed within an agricultural field. Part of the field remains and continues to be used for agriculture. The feature representing the remainder of the field is recognisable as the original with the same function, therefore it is retained. New polygon features are created to represent the new housing development.
An agricultural field is subdivided into three approximately equal parts that continue to be in similar usage. Using the guidelines above, none of the fields can be considered the obvious successor to the original field: all have an area less than half of the original; therefore, the original feature is deleted and three new features are created.
A house is divided equally in two by an externally surveyable division. The original feature is deleted and new features are created. This is because neither of the resultant houses is the obvious successor to the original.
A large agricultural building is split into two by the addition of an externally surveyable division enclosing approximately 25% of the original area. The original feature is retained to represent the larger part, and a new feature is created to represent the smaller part.
Most of the large garden of a residential property is sold off for development. The garden feature is retained to represent the much-reduced garden.
Joining of polygon features
When two or more real-world polygon objects are merged by the removal of physical boundaries, it may be that one of the original real-world objects is clearly recognisable as subsuming the other. If that is the case, the feature representing the dominant real-world object is retained and the other feature is deleted.
If one of the original real-world objects is not clearly dominant, the following considerations are used as a guideline to determine whether a feature is retained:
Is the function of the resultant real-world object the same as one of the originals?
Can one of the original real-world objects be considered the obvious predecessor to the resultant real-world object?
Is the area of the resultant real-world object less than twice that of one of the original real-world objects?
If the continuation of the feature cannot be justified on one or more of these grounds, all the original features are deleted and replaced with new features.
Examples
Two fields, one of which is larger than the other, are merged into one, such that the resultant real- world object is recognisable as the larger field subsuming the smaller field. The feature representing the larger field is retained. The smaller field feature is deleted.
Three fields, which are broadly similar in size, are merged into one, such that none of the original fields are recognisable as the obvious predecessor of the resultant field. The original features are deleted, and a new feature is created to represent the field.
A pond within a field is filled in. The feature representing the pond is deleted and the field feature is retained.
Two semi-detached cottages of equal size are combined into one dwelling, with no alteration to the external geometry of the building. Both of the original features are deleted, and a new feature is created.
A large greenhouse lies within a parcel of land only marginally larger than itself. The greenhouse is demolished. The feature representing the greenhouse is deleted, and the feature representing the land parcel is deleted as it has increased significantly and can no longer be considered as the same object.
Change of polygon feature classification
When a real-world object represented by a polygon feature changes such that the nature of the feature changes, the feature is retained, unless changes to its geometry indicate deletion of the feature under the guidelines above.
Examples
An area of agricultural land is wholly planted with trees; there are no changes to its bounding features. The descriptive group of the feature changes but its geometry is unchanged. The feature is retained.
An area of woodland is felled, and the area now consists of rough grass and scrub. The feature is retained.
A barn is converted into a private dwelling. There is no change to the nature of the building (it is still a building) and the feature is retained.
Modification of polygon features due to error correction
When a polygon feature is changed solely to correct errors either in geometry or other attributes, the feature is retained. If the feature has been moved to correct an error and simultaneously modified for real-world change; for example, when natural movement of a physical feature occurs, such as a riverbank or foreshore, then the feature modification rules above are followed.
Examples
A line feature representing an old fence is found to have an error in its position and is corrected. The line feature and the polygon features bounded by it are retained. The version numbers of the features are incremented.
The feature representing an area of road has been assigned an incorrect descriptive group. The feature is reclassified and retained. The feature version number is incremented.
An area of non-coniferous trees has been incorrectly assigned the descriptive term 'coniferous trees' by photogrammetric revision techniques. The feature is reclassified and retained. The feature version number is incremented.
A building foundation captured as a feature with a descriptive group of ‘unclassified’ is completed, and the feature is reclassified to a descriptive group of ‘building’. The feature is retained. The feature version number is incremented.
A feature is no longer included within Ordnance Survey’s capture specification. The feature is not retained.
