As introduced in the "Map foundations: topology and geometry" topic, the basic Road Model uses nodes (geolocated points), and links connecting nodes, to define navigable topology representing road networks in the real world.
In addition to nodes at intersections, bivalent nodes connect exactly two links between intersections. Since link-level attributes apply to entire links in the basic Road Model, bivalent nodes are useful for supporting changes in attributes (such as a change in speed limit) between intersections. All nodes are geo-located using latitude, longitude, and relative heights in 3D space.
Links are topology segments that start, end, and meet at nodes — even if the roads they represent have curves. To approximate the curves of actual roads, geolocated "shape points" can optionally be placed along road links between intersections. A single link can contain multiple shape points to form a polyline. In the example below, the squares are nodes; the circles are shape points; and the lines are links.
Attributes such as Road Speed Limit and Direction of Travel can be assigned to links to capture real world driving considerations. These and additional attributes of road topology support assorted use cases such as routing: calculating safe, legal, and efficient routes to destinations selected by users via point addresses, place names, and so forth. Additional attributes such as road names, route numbers, assorted lane attributes, and special conditions can enrich the display of maps and support route guidance with turn-by-turn instructions:
Addresses are a property of the Road Name attribute.
Functional Class is an attribute assigned to all navigable links to indicate the importance of the link for calculating routes on a scale from 1 to 5: FC 1 for major routes like freeways down to FC 5 for small surface streets.
Elevation, Slope, and Curvature help self-driving vehicles navigate safely and are valuable for ADAS and fleet applications.
Conditions can be used for many things based on types of vehicles, weather, time of day, special use lanes, and more. Examples: Access Restrictions, School Speed Limit, Restricted Driving Maneuver, turn restrictions, carpool and HOV lanes, etc.
Most links can be used for navigation and route finding by one or more specified types of traffic, depending on the conditional attributes applied to those links. A link is navigable if pedestrians or one or more of the following types of vehicles can use it: autos, buses, emergency vehicles, motorcycles, pedestrians, taxis, trucks, ferries, and HOV (electric and hybrid) vehicles. Subsets of the Access Characteristics attribute include carpools, deliveries, and through traffic among others.
Attributes that affect display characteristics as well as routing include Controlled Access, Multiply Digitized, Ramp, POI (point of interest) Access, Road Name, and many others. Road Name typically includes a base name, directional prefix and suffix, street type, and route type when applicable. The names of navigable links are applied in local languages and character sets.
The Direction of Travel attribute is relative to a start and end reference on a link, as explained in the "All links" section below. Specifically, Direction of Travel is based on the "from Reference" and "to Reference," or "both." The Speed Limit attribute can be different for each valid Direction of Travel on two-way roads.
Administrative boundaries, land-use polygons, and other cartographic features are not themselves navigable, but they can be on the same geometry as navigable links, or can be on stand-alone links if the boundary is not a road. It’s not unusual that a city park is bounded on all sides by streets.
Some non-navigable links form open polylines to represent map features like rivers and rail lines. Other non-navigable links form closed polygons to represent cartographic features such as lakes, buildings, lane use areas (e.g. parks), and administrative areas. These non-navigable links enrich the display of maps on a screen. When a road seems like it might be a water boundary (e.g., the Golden Gate Bridge as the boundary between the Pacific Ocean water feature and San Francisco Bay water feature), a separate water-only link serves as that boundary.
The names of places and cartographic features typically include a base name and feature type. Example: Millennium Park (City Park, land use). These names are applied in local languages and character sets.
Some properties and attributes apply to all links, navigable or not. Examples:
All links have an orientation: They start at a Reference and end at a non-Reference. Orientation affects attributes that are sensitive to direction, side, or end-of-link situations.
Administrative information includes things like city, county, state, country, kreis, bundesland, province, and so forth. The left and right side of every link includes this administrative information at the lowest level up to higher levels — typically 3-5 levels, depending on the country. Postal codes can also be different on the left/right sides of a link. Administrative information and postal codes are critical to selecting destinations for navigation purposes. For example, "200 Main Street" might correspond to more than one place in Los Angeles County. Knowing the specific city in the county, and the specific postal code, are necessary to isolate a single place in a big county.
Bridges and Tunnels can apply to non-navigable railroads as well as navigable roads, affecting map display.
Road Centerline Model
The Road Centerline Model extends the basic Road Model with a few of the properties of the Lane Model: road surfaces defined in part by centerlines (known as surface paths) based on NURBS (non-linear uniform rational B-splines), and the parametric assignment of attributes to subsections of links without using bivalent nodes, but without direct references to lanes or inclusion of lane data. For more information, see the "Lane geometry" section in the Lane Model topic.