Topology reference types for road attributes

Note: This topic pertains to Features and attributes and refers extensively to nodes and links (topology segments). For essential definitions and background, see Map foundations: topology and geometry.

Regardless of the values of attributes, all road-level attribution is associated with the road network via a consistent referencing system. This system expresses how attribute values relate to the link and node structure of the topology layer and its geometry. As discussed in the Road Model topic, the Road Centerline Model extends the basic Road Model in key ways — including the referencing of attributes at points and in ranges defined parametrically within links. This referencing model incorporates the following:

• Topology feature identity - how to identify the specific link(s) and node(s) an attribute relates to.
• Location - how to specify a particular point or range along the link an attribute relates to.
• Orientation - how to to specify the particular direction along a link, or traversal through an intersection, the attribute relates to.

The orientation of these attributes is a nuanced topic and deserves some further detail: In general, attributes may be oriented to indicate that they affect a subset of the possible road traffic on a link. Attributes generally affect road users traveling in a specific direction, potentially in specific lanes that support that travel. Hence, orientation could also be used to associate road-level attributes with a subset of lanes without requiring the use of the lane-level topology and attribution layers.

The orientation of link attributes is specified relative to the arbitrary digitization direction of the link topology, from its start to end node. This orientation can be forward (from start to end), backwards (from end to start) or both (bidirectional).

The orientation of node attributes is specified relative to a set of "from" and "to" adjoining links that define a specific traversal through a node intersection. For efficiency, multiple traversals can be specified simultaneously by multiple "from" and "to" links, or by omitting either/or entirely for "all". See the examples below.

When an attribute on a link is valid for a collection of subsequent links, it is a Link Strand type of attribute. The orientation of Link Strand attributes is defined by the order of the child links in the strand. The strand runs from the first link to the last. Each link referenced in a strand also includes a "re-orientation" value that indicates whether the link's inherent digitization direction needs to be re-oriented to align with the overall orientation of the strand. This information can also be derived by comparing the end and start nodes of adjoining links in a strand.

Road-level attributes can reference nodes and links in the Road Centerline Model in the following ways:

• Link Oriented Parametric Range
• Link Oriented Point
• Node Intersection
• Node Intersection Traversal
• Node Intersection Entry
• Node Intersection Exit
• Link Strand

Link Oriented Parametric Range

Attributes can be applied to a range along a link, defined as:

• Start location of the range, expressed as a parametric distance from the start of the link. By definition, a "parametric distance" on a single link is greater than or equal to 0, and less than or equal to 1. It represents the distance along the length of the link geometry. For example, a parametric point at position 0.5 is halfway between the start and end of the physical road represented by a link.
• End location of range, expressed as a parametric distance from the end of the link.
• Note that the start and end locations must sum to less than 1, so that the start location is closer to the link start than the end location, and the length of the range is non-zero.
• This approach was chosen so that the common "whole link" range can be compactly expressed as 0-0.

The directionality of range can be:

• Forward: same direction as link orientation
• Backward: opposite direction to link orientation
• Both: both directions or non-directional
• Unknown

In the figure below, and the other figures in this topic, "S" and "E" indicate the start and end of the link orientation.

Link Oriented Point

An attribute can be applied to a single point along a link, defined as a fractional distance (0-1) from the link's start.

The directionality of the point can be:

• Forward: same direction as link orientation
• Backward: opposite direction to link orientation
• Both: both directions or non-directional
• Unknown

Node Intersection

An attribute can be applied to an intersection node, regardless of the paths in or out of the node.

Node Intersection Traversal

An attribute can be applied to an intersection node, relative to a specific direction of traffic flow, defined as follows:

• Entry link, with a direction of travel that must include traffic flow toward the node
• Exit link, with a direction of travel that must include traffic flow away from the node

Node Intersection Entry

An attribute can applied to an intersection node, relative to all traffic feeding into the node, defined as follows:

• Entry link, with a direction of travel that must flow toward the node.
• This attribute applies to all links supporting traffic flow away from the node.

Node Intersection Exit

An attribute can applied to an intersection node, relative to all traffic flowing away from the node, defined as follows:

• Exit link, with a direction of travel that must include flow away from the node.
• This attribute applies to all links supporting traffic flow toward the node.

Link Strand

An attribute can be applied to a link strand (an ordered list of two or more contiguous links), defined as follows:

• An ordered list of continuous links
• The first and last links in the list can be partially included (via a "distance from start" indication)
• The directions of travel on the links must include traffic flow through the entire ordered list