The underlying topology derives from established road network geometry. The nodes are numbered and the edges are made up by "measured links". This way of measuring also derives from geometry. The measured links, sensitive to their own geometry, serve as basis for the vast majority of location referencing methods.
A location referencing system may either be 'direct' or 'indirect'. The term 'direct' indicates that the location referencing system is founded on coordinates in a known datum. The term 'indirect' suggests that the location referencing system is founded on a linear measurement along a system of edges and nodes. When the measured links and the numbered nodes are put into system, any road feature and its extension in the network may be located or positioned. This is called a Location Referencing System (LRS). By supervising the locations and extensions of the edges representing the road element, several perceptions levels may be created:
- Simplified, one edge per road
- Regular, one edge per carriageway
- Detailed, one edge per dual carriageway, ramp, lane etc.
Geometry, Topology and the Linear Referencing System (LRS)
In this context, topology refers to information on road network construction, i.e. how to travel from one location to another within the road network. In topology, nodes and links can be described via the Spatial Attribute, deriving from geometry. A road owner will for most purposes require one or more LRS. The LRS are associated with topology via the Location Attribute.
The Reference Link
A reference link represents a road distance interval which is currently in existence, has ceased to be in existence or has yet to be realized. A reference link coincides with the edge and has the edge geometry and length. Overall, we do not distinguish between a reference link and an edge. A reference link attribute used for describing the valid time period (spanning from the commencement date to the termination date) is a part of a specific reference link and is thus not treated as a separate feature. The reference link attribute is integrated into a reference link and does not hold a separate identity or geometry.
A node represents those road intersections and road discontinuations which are either currently in existence, having yet to exist or have ceased to be in existence. A node holds a valid time period and a position.