# Topology Model

The Topology Model describes road topology, such as nodes at intersections and segments connecting the nodes, corresponding polylines, and road-level attributes.

The Topology Model is implemented by the Road Topology & Geometry Layer. In this layer, each segment is a polyline connecting exactly two nodes. Segments may optionally have additional shape points connected by polylines. Nodes and shape points have latitude and longitude positioning.

The "Segment-Node" topology uses the HERE "topology segment" concept: each node represents an intersection (or dead-end). Hence, the base topology of the road network is a simplified segment and node network, as in, simpler than a link-based network, that models only intersection-to-intersection connectivity. Polyline geometry adds shape points to represent curved roads, but shape points are optional. The HERE Map Content Topology Model is different from the model in the HERE infotainment map in the sense that infotainment links are aggregated in this new model to be truly intersection to intersection.

This topology-focused approach minimizes changes in the model over time. Segments and nodes are modified only when road connectivity actually changes through the addition of new intersections, not for any attribution changes. Segments and nodes are allocated unique identifiers that are stable across map versions, unless a topological change occurs. When a change occurs, only the modified elements have new references assigned.

## Nodes

A **node** is a point with latitude and longitude position. A node also has a "Z-level" representing its relative elevation with respect to other nodes at the same latitude/longitude. Only one node can be defined at any given latitude/longitude/Z-level. If there are two or more points at a given latitude/longitude, one will have Z-level = 0; other Z-levels range from -5 to +5. Larger Z-levels indicate higher relative elevations, and negative Z-levels represent lower relative elevations. A node must be attached to at least one segment. HERE Map Content does not publish any bivalent Nodes. However, there are some exceptional cases where, due to suppression of segments having non-publishable geometry, bivalent nodes are introduced.

## Segments

A **segment** is a directed polyline connecting two nodes with optional intermediate shape points. The logical orientation of a segment is defined as being from its starting node to its ending node. This orientation is not related to the direction of travel on the segment. However, directional attribution such as direction of travel or speed limit reference segments relative to this logical orientation. A given node may be both a start and end node for different segments that adjoin it.

A segment may optionally contain shape points along its length to help refine its representation of curved roads. Like nodes, shape points have a latitude and longitude, as well as a Z-level; The Z-level is always 0 unless there are multiple points at the same location. A shape point always belongs to exactly one segment.

For example, consider the simple road topology in the first figure below. This topology would be represented as segments, nodes, and shape points as shown in the second figure below, in which segments are depicted as lines, nodes are squares, and shape points are circles.