Shape and Connectivity
The Overture Transportation theme captures the physical shape and connectivity of the transportation network through the interaction of the theme's two feature types: Segments and Connectors.
The Connector feature type
Point geometry which describes a location where a physical
connection between two or more segments occurs (or may occur in the
Connectors have no properties apart from geometry and standard Overture feature properties. All other aspects of the Transportation theme are modeled on Segments.
The Segment feature type carries
LineString geometry which describes the physical shape of a section of
the transportation network. A segment may represent an entity with a
tangible real-world existence, such as a paved road; or it may represent
an intangible entity, such as a ferry route, which has a well-known
shape but no observable presence in the real world.
Two or more segments are physically connected at a given connector if
connectors property contains a reference the
The connector geometry's coordinates should preferably be contained within the segment geometry's coordinates, in which case the connector coordinates define the point of physical connection. This constraint will always be met by official Overture data releases. Where this is not possible, the point of physical connection is the closest point to the connector coordinates which intersects the segment geometry.
Conversely, two segments are not physically connected if their
connectors properties do not reference a shared connector, even if
their geometries overlap or even share a coordinate in common.
Travel from a point on one segment to another point on a second, physically connected, segment is allowed unless limited by an explicit restriction such as an access or turn restriction.
All segments in official Overture Transportation data releases have a minimum of two connectors, one at each end of the geometry, even if those endpoint connectors are not attached to any other segment. This is not a mandatory minimum (and is not enforced by the schema). It is done to allow new segments to connect into the existing network without needing to change the properties of existing segments.
Start, end, and orientation
The first coordinate in a segment's geometry is the "start" of the segment and the last coordinate is the "end". A segment is "oriented" away from the start and toward the end. The examples below show two segment geometries with identical coordinates, oriented in opposite directions.
Travel along a segment's geometry can follow one of two possible
forward heading proceeds
toward the end of the segment; while the
backward heading proceeds
back toward the start of the segment.
🚧 We are developing a segment-level directionality concept similar to lane directionality to indicate what travel headings are allowed or prohibited along the segment. This effort is ongoing, so please check back soon.
Segment features have a
subType property whose value gives more
specific information about the segment's role within the transportation
subType property may be one of
water but only
road is currently well defined. A
road segment models any kind of
road, street, or trail, including a dedicated path for walking, cycling,
and like activities. For more information about
road segments, see
the page on Roads.
Segment geometry is two-dimensional. In the real, 3D, world, however the entities represented by segments can be above or below each other, as may happen with tunnels, bridges, overpasses, and stacked multi-level highway interchanges. To accurately render "top-down" 2D maps, it is important to know the relative stacking order, or Z-order, of segments.
Segment Z-order is given by the
level property. A
level value of
0 indicates visual level, with positive numbers indicating above
visual level, negative numbers indicating below visual level, and in
general, a lesser number indicating a lower position in the stacking
order than a greater number.
Note that two segments with different
level values may be physically
level is an approximation for rendering and is not
meant be a precise indication of elevation at different points along the
segment. Connectors do not have a
The term "segmentation" describes the process of converting upstream source data into Overture Transportation shape and connectivity data modeled as segments and connectors.
A primary goal of Overture's segmentation process is to promote stability of segment shape across Overture data releases. For example, if a certain real-world stretch of Main Street is represented by a single segment with particular geometry in release 1, we will strive to avoid slicing the exact same geometry up into two, three, or four segments in release 2.
Note that aiming for segment shape stability categorically does not mean that Overture aims for a stable transportation dataset. On the contrary, we aim to continuously improve data accuracy and coverage, and expect the transportation network dataset to constantly evolve and grow as a result. Our goal is simply to minimize unnecessary, semantically meaningless, changes in how the geometry is sliced into segments across data releases.
Several features of the Transportation theme schema were designed to allow the segmentation process to achieve its segment stability goal. These features include:
GERS ID stability
The reason we pursue shape stability is to improve our ability to assess whether two segments from different points in time (or from different upstream data sources) represent the same real-world entity. Our success at this assessment directly feeds into the stability and precision of Global Entity Reference System (GERS) IDs we assign to segments. In turn, higher GERS ID stability and precision makes Transportation theme data more useful for conflation.
A key feature of the Overture Transportation schema which enables shape stability is the ability of segments to support connectors at interior positions along their geometry, not only at their endpoints. The ability to add internal connectors prevents the segmentation process from having to blindly follow every split or join introduced in upstream source data.
For example, imagine a square city block bordered by road on all four sides has been mapped in the source data, but a back alley dividing the block along the east-west axis has not. If the back alley is subsequently mapped in the source data, the Overture segmentation process can connect to the transportation network without having to subdivide any existing segments by simply introducing internal connectors on the north-south road segments bordering the block to the east and west. As a result, the GERS IDs of the north-south road segments remain as they were and no data needs to be re-conflated.
Note that in the above example, an official Overture data release would insert coordinates in the middle of the north-south segments, if they did not already exist, because Overture data releases will always ensure that every segment's geometry includes all of its connectors. From a computer's perspective, this is a very minor alteration of the segment's shape.
Many segment properties may include a linear reference so that they apply only to a part of the segment geometry. We refer to these linearly-referenced property values as being geometrically scoped and discuss geometric scoping at greater length in the page on Scoped Properties.
Geometric scoping allows the segmentation algorithm to avoid introducing segment splits simply because a certain property has different values along different parts of the geometry. Like interior connectors, geometrically-scoped properties enable the segmentation process to make decisions that promote shape stability, ultimately resulting in more precise and stable GERS IDs and less churn in conflated data.
Although it is technically possible to use the Overture schema to express a segment forming a connected loop, such loops are considered invalid and will never be produced by the segmentation algorithm.
An illegal loop where one of a segment connects to the other end can be corrected by splitting the segment and introducing a second connector to maintain physical connectivity. An illegal self-crossing loop of degree N can be corrected by splitting the segment into N pieces.