Modelling routing information

Two concepts underpin how routing information, both RRI and PRI, is applied to RoadLink, RoadNode, PathLink or PathNode features – the concepts of directed nodes and directed links. These concepts are central to understanding and using RRI and PRI.

Each link feature has a node feature at each end. These nodes are called the link’s directed nodes; they indicate the start and end of the link in terms of the direction in which the link was created by Ordnance Survey.

The start node is indicated in the GML supply format by a minus sign (-) and the end node by a plus sign (+). The directed nodes do not indicate a direction of traffic flow.

RRI and PRI on link features

RRI and PRI can apply to either a specific location on a link, part of the link or the whole link.

If the RRI or PRI applies to a specific location on a link, that location is indicated by the value in the ‘distance from start’ attribute on the RoadLinkInformation or PathLinkInformation feature. The ‘start’ in question is the ‘start’ directed node.

If the RRI or PRI applies to part of the link, the information can be found in the subsectionDistance and subsectionPoint attributes of the RoadPartialLinkInformation or PathPartialLinkInformation features. The subsectionDistance attribute gives the distance, in metres, from the road or path link’s start directed node to which the routing information applies. There will be two subsection points that provide the coordinates of the location where the routing starts (coincident with the subsection distance) and the point from which the routing information no longer applies.

RRI and PRI on routes

Routes are collections of links where the specific direction of flow is controlled. If routing information applies to a given location on part of a route in a specific direction, for example, a restricted access section along one link in a one-way system, the location of that restricted access will be recorded in the ‘distance from start’ attribute for the RoadRouteInformation or PathRouteInformation feature. The routing information will also reference the TOID of the particular link within the route that the location occurs. As mentioned before, the start in question is the road or path link's start directed node.

If the direction of flow along a link is controlled, either for the entire link or in part, that link is considered to be a directed link. The RoadRouteInformation, PathRouteInformation, RoadRoutePartialInformation and PathRoutePartialInformation feature types have a ‘directed link’ attribute that stores the information about the direction of flow. This attribute has two values recorded against it:

• Orientation (direction of flow) – the value is assigned relative to the direction of the link’s creation indicated by the link’s directed nodes. If the flow is the same as the direction of creation, a plus (+) value is assigned. If the flow is directed in the opposite direction to the direction of creation, a minus (-) value will be assigned (see the following diagram).
• TOID – the identifier of the RoadLink or PathLink in question

If the control on flow applies to a route (more than one link), the TOID and orientation for each link is supplied. If the RRI or PRI applies to part of the route, the information can again be found in the subsectionDistance and subsectionPoint attributes of the RoadPartialRouteInformation or PathPartialRouteInformation features.

A RoadLink or PathLink feature refers to its two nodes to indicate which is the start node or end node, in terms of the direction in which the link was created. RoadRouteInformation and PathRouteInformation features apply an orientation to RoadLink and PathLink features, indicating whether they should be interpreted as being in the same direction as the direction of creation (start node to end node) or the reversed state (end node to start node).

Grade separation

The grade separation attribution identifies any RoadLink or PathLink features that may be above the level of another. If two road or path links have different grade separation values then it is not possible to navigate between them at that point.

The grade of separation is recorded as a value of between one, representing the first level above ground level, to a theoretical possible value of six – although three is the highest value that appears within the Great Britain road network at present. As ground level is considered to be the normal level, any part of the network at that level does not have a grade separation value. However, some application software may insert a value of zero when the ITN data is translated from the supplied GML format to the preferred storage format; customers should check their data holdings to see if this has been done.

The grade separation value is recorded on the directedNode attribute of a RoadLink or PathLink. In the following example based on the ITN Layer Urban Paths theme, two PathLink features forming a footbridge passing over another PathLink are shown against the OS MasterMap Topography Layer. The PathNode where the two paths cross (highlighted in red) is TOID osgb4000000546372819.

PathNode osgb4000000546372819 is the start directed node for PathLink osgb4000000091827364 and PathNode osgb4000000023091689 is the end directed node of PathLink osgb4000000091827364.

The start node osgb4000000546372819 also marks the point where a grade separation occurs on the PathLink. As they cross at different levels, the directed node osgb4000000546372819 has a grade separation of '1' where it is referenced by the links of the footbridge and no value where the node is referenced by the footpath, which is at ground level.

PathNode osgb4000000546372819 is also a directed node for the footpath PathLink features under the bridge. These links will have the PathNode referenced as a directed node but it may be either a start node or an end node to them. As these PathLink features are at the normal level, there will be no grade separation recorded in the references to the node.

Note: If a grade separation occurs at a road bridge, there may be information on height or width restrictions available in the RRI theme.

Working with route information

The following example illustrates how route information may be represented using GIS application software and how that information might be used to assess route options for road network users. 

Note: RRI symbols shown in the following maps are not part of the ITN product and are included for illustration only.

The shortest path to a delivery address for a freight company is represented by a proposed route, highlighted in red. However, this route doesn't take any routing restrictions into consideration, nor does it account for the type of van used by the freight company.

Displaying the routing information identifies the location of restrictions that may alter the route:

Recalculating the route based on these restrictions, and taking into consideration the type of vehicle that will be used to make the delivery, introduces a significant detour to the delivery address. The revised route, shown in blue, avoids a toll bridge with both weight and width restrictions.

Case Studies

A number of case studies on our web site also illustrate some of the many applications for route information. These case studies include:

Optimising waste collection routes in Northamptonshire – identifying the best routes for waste collection based on routing restrictions such as left and right turns, weight restrictions and timer barriers.

Providing efficient transportation services in Durham – Durham County Council's re-evaluation of its provision of transportation services.