Line features in OS MasterMap Topography Layer are not as persistent in the same way as polygon and point features. This is because line features are maintained by what are called topological structuring rules. If a line feature is intersected by another line, it is broken at the intersection. This means that a single linear real-world object is often represented by several line features – no real-world object should ever be made up with a partial line feature. There is no concept in OS MasterMap Topography Layer of a line feature that is made up of multiple line geometry elements.
An illustration of this rule is shown in the diagram below. A fence cuts a field into two real-world objects. A new fence is built at right angles to the original to further divide one half of the field. Although the old fence has not changed at all, it will be split into two separate line features.
As there is no recorded relationship between OS MasterMap Topography Layer line features and discrete real-world objects, a change to a line feature may result in the deletion or significant modification of that feature and the creation of new line features. This change is not necessarily caused by a real-world change to the linear object. In the example above, the original line feature is retained as one of the resultant line features; the other line feature is new. The user cannot predict which of the resultant line features will bear the original TOID. The major exception to this is that when the reason for change is a correction of error rather than real-world change, then features are retained whenever possible.
The more rapidly changing lifecycle means that associating user data with OS MasterMap Topography Layer line features by TOID references needs to be considered very carefully, as there will be greater overheads in terms of managing change. In most cases, it will be more practical to associate with points and polygons, rather than lines.
Inferred links are a particular type of line feature that do not actually exist in the real world. An inferred link is a line that Ordnance Survey has introduced into the data to make some types of polygons into more manageable units. There are two main uses:
Network closing links are frequently found where road polygons meet at junctions. If the roads were not split in this way, the Road theme would contain numerous very large polygons that would not be particularly useful in terms of being able to derive data or attach meaningful attribution to them. Roads with comparatively few junctions, such as motorways, are also split where another feature crosses them, such as a road bridge or footbridge.
Polygon closing links are used to make more manageable, or logical, polygons. The types of link are shown in the figure below. One example would be the creation of a link to separate an open-plan garden around a pair of semi-detached houses into two distinct entities, reflecting that there are two properties there in the real world. It must be stressed that these polygon closing links do not constitute the legal boundary of any property any more than a physical line feature does. These links are clearly identified in the ‘descriptive group’ attribution and could be filtered out in most GIS if a user wishes not to display them.
When a new linear real-world object comes into being, a new line feature is created to represent it.
When a real-world object is no longer present in the real world, the corresponding line feature is deleted from the Ordnance Survey main holding. A record is kept in the database to indicate that a feature with this TOID used to exist. Users with local holdings of OS MasterMap Topography Layer data are informed of the deletion in their next COU.
As noted above, a line feature may be modified due to changes to the real-world object, or due to changes in adjacent real-world objects. The original feature may be retained if a portion of its geometry remains and one or more new features may be created to reflect the change. If the classification attributes of a line change, then it will usually be retained, and the version number incremented. Occasionally, a line feature may be replaced with a seemingly identical line feature that is considered a new feature. For example, where a line is created to represent a newly erected fence placed along the alignment of an existing line boundary between a garden and the pavement.
When a line feature is changed solely to correct a surveying or cartographic error, the feature is retained, unless the resulting topological changes with adjacent features make this inappropriate.
