Architecture
Content Layers
The HD Map layer group implements content layering: partitioning map attributes that change on different timelines into separate sets of files to help manage bandwidth requirements.
| Layer | Contents | Use Cases |
|---|---|---|
| Topology | Graph of roads, represented as Links and Nodes along with basic routing attributes such as Functional Class and Speed. | Basic routing |
| Simplified Geometry | Rasterized polyline representations of road center lines. | Basic map matching |
| Full Geometry | Full B-spline representation of road center lines. | Advanced visualization and extraction of geometric features |
| Attributes | Full road attributes, including link navigability, condition, TMC codes and signs. | Advanced routing |
| Lane Model | Lane topology and B -spline geometry. | Advanced routing and vehicle localization |
Geographic Partitioning
To enable efficient incremental update and delivery, each HD Map content layer is divided into geo-spatial tile files; each file contains the data relevant to a specific tile.
HD Map tiles are currently published at tile level 14, corresponding to non-square tiles approximately 2 x 2 km, varying depending on latitude. Below are sample level 14 tiles from Chicago and Manila illustrating the distortion differences at different latitudes on a Mercator projected map.
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These tiles are identified using the HEREtile ID encoding scheme. This scheme follows a quad tree convention which is further transformed into single 32-bit integer identifiers, encoding the tile's level (size) and (X, Y) coordinates on the world-map. Full details of HEREtile partitioning and ID encoding can be found in HEREtile Tiling Scheme.
Generally, every map feature with a geographic extent (e.g. road link geometry) is defined only once, in its unique "master tile". A feature's master tile is defined by the tile in which its "starting point" lies. Features can be referenced from other tiles with the reference including both an object ID (e.g. link ID) and the feature's master tile ID.
Delivery
HD Map content is distributed through the HERE DataStore publication service. The API for data delivery and change notification is described in the separate DataStore API Developer's Guide.
Version Control
The HERE HD Map layer group is published as version-controlled snapshots. A single global version number is assigned to the entire map across all layers and tiles. Every map update results in a new increment to the overall map version number. Map layers and tiles can then be retrieved by version with internal consistency across all published content layers and tiles.
Data Definition Language
The Protocol Buffers format is used to encode the HD Map tile data. Protocol Buffers (commonly known as "Protobuf") is an efficient programming language - a neutral, platform-neutral, extensible mechanism for data encoding.
