Tracking changes to real-world objects

The flowchart in figure 1 illustrates the process for tracking a real-world object when it appears, changes or is removed from the physical environment. These real-world objects are represented in ITN as features.

Figure1: Feature tracking in ITN

Life cycle rules for line features

Line features are maintained by topological data-structuring rules. If an ITN line feature intersects with another line, the line is broken at that intersection. As a result, a single linear real-world object, such as a road or path, is often represented by several ITN line features. No real-world object should ever consist of a partial line feature.

As there is no recorded relationship between ITN line features and individual real-world objects, any 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 change to the real-world object represented by the feature. It is not possible to predict which of the resultant line features will retain the original TOID. The exception to this principle is when the reason for change is the correction of an error rather than real-world change; in this case, features are retained whenever possible.

Life cycle rules for general point features

Point feature life cycles are relatively simple compared to those of lines or polygons, since they cannot change in size or split into multiple features.

Creation of point features

When a new real-world object appears, a new point feature is created to represent it. If, however, the object is a replacement for a previous real-world object in the same position, the original feature is retained.

Deletion of point features

When a real-world object no longer exists, the point feature that represents it in our data holding is removed. We will keep a record to indicate that the feature, with the original TOID, used to exist.

Modification of point features due to real-world change

Due to the nature of the real-world objects represented as point features in ITN data, such as rising bollards, gates or fords, it is unlikely that one will be modified without changing its identity.

As a result, any modification to a point feature as a result of real-world change will result in the deletion of the original feature and creation of a new feature, unless there is a clear reason to associate the resultant real-world object with the original. This principle applies to both changes to the geometry of the feature and to the descriptiveGroup or descriptiveTerm attributes.

Modification of point features due to error correction

When the attribution of a point feature is found to be incorrect or the feature is moved to correct an error in the positional accuracy, the original feature is retained, appropriately modified.

Examples of ITN life cycle rules (for roads)

Each feature in either the Roads Network theme or the Road Routing Information (RRI) theme belongs to one of 12 different feature types. Five feature types have a geometric element, either actual in terms of being represented by a line or a point or implied in that there is a linear connection (for example, FerryLink).

The remaining seven types provide information about the geometric feature types and are related to them by references within their attribution.

The following examples highlight where, due to the nature of the real-world object they represent or the way the data is structured, a different approach to the general line and point life cycle rules is adopted.

Road features

Road features generally persist as named or numbered roads. Only on rare occasions will Road features completely cease to exist in the real world. The Road life cycle rules are:

• The addition or subtraction of component RoadLink features, or other change of attribution, will result in the Road feature persisting with an incremented version number.
• If RoadLink features referenced by a Road feature are altered (but not deleted) there is no change to the Road feature.
• If a new name is applied to a Road feature or part of a Road feature, a new feature will be created.

RoadLink features

RoadLink features do not have a definitive relationship to the real world. For some users they represent simply a connection between two points whose geometry is irrelevant; for other users they represent a specific section of a road that no longer exists if a degree of change occurs. The RoadLink life cycle rules are:

• Where a RoadLink feature is shortened or lengthened, it will persist – the version will increment.
• A RoadLink feature that has its alignment changed (for example, highway improvements) will persist where possible – the version number will increment.
• Where any change of attributes alone has occurred, the feature will persist – the version will increment.

A change of road name or number alone will not result in update to RoadLink features because this information is stored on the Road feature.

Changes to RoadLink topology are subject to the following life cycle rules:

• Where a RoadLink feature is split through structuring of another RoadLink feature or addition of a RoadNode feature, one of the resultant features will retain the original TOID and have its version incremented. The smaller section(s) will be created as new RoadLink feature(s).
• Where a RoadLink feature is merged by the removal of either a joining RoadLink feature or a RoadNode feature, one of the original RoadLink features will persist with an incremented version.

RoadNode features

The RoadNode life cycle rules are:

• A RoadNode feature that is moved but represents broadly the same intersection of RoadLink features will persist with the version incremented.

Changes to RoadNode topology are subject to the following life cycle rules:

• A RoadNode feature that has additional RoadLink features joined to it will persist with an incremented version.
• A RoadNode feature that has RoadLink features removed from it will persist, with an incremented version, provided it is still required to represent either the intersection of two or more RoadLink features, a change or end of road name, or the end of a RoadLink feature.

InformationPoint  features

InformationPoint features are found at motorway junctions and as these junctions rarely disappear in the real world, InformationPoint features rarely cease to exist. The InformationPoint life cycle rules are:

• A changed motorway junction number will result in a new feature being created.
• If the intersecting Road features change, the InformationPoint feature will be retained with an incremented version.

FerryLink and FerryNode features

FerryLink and FerryNode features have a simple relationship to the real world; the first represents a ferry route and the second represents the start or end of a ferry route. In general they are simply created or deleted.

• If a previously deleted FerryLink feature or FerryNode feature is subsequently reinstated, it will be captured as a new feature.

Changes to FerryLink and FerryNode topology are subject to the following life cycle rules:

• A FerryNode feature that has a FerryTerminal feature or FerryLink feature linked to it will persist with an incremented version.
• A FerryNode feature that has FerryTerminal feature or FerryLink feature removed from it will persist provided that it is still required to represent the end of one or more FerryLink features. If the FerryNode feature persists, the version will increment.

FerryTerminal features

FerryTerminal features represent the interchange between networks. They generally persist unless the interchange they represent no longer exists.

Changes FerryTerminal topology are subject to the following life cycle rules:

• Where a FerryTerminal feature has reference to network attributes changed to reflect additional or reduced references, it will be retained with an incremented version.

Routing information feature

In general, changes to road route information features will result in the deletion of any existing feature and the creation of a new one except in the following circumstances:

• Changes to an existing feature that does not involve addition or subtraction of RoadLink references for that feature will result in the feature being retained with an incremented version number.
• Any real-world change to routing information features that involve addition or removal of RoadLink feature references will retain the feature only if the first and last RoadLink features are the same as the original feature. Otherwise, a feature will be deleted and a new one created.
• Changes to the Road features referenced by a routing information feature may result in the deletion of the original feature and the capture of a new one because the identifiers of the network features have changed.