DataA router relies on data to be able to find a route.
OpenStreetMap DataThe data that is collected by the OpenStreetMap project consists of nodes, ways and relations.
- A node is a point that has a latitude and longitude and attributes that describe what type of point it is (part of a way or a place of interest for example).
- A way is a collection of nodes that when joined together define something (for example a road, a railway, a boundary, a building, a lake etc). The ways also have attributes that define them (speed limits, type of road and restrictions for example).
- A relation is a collection of items (usually ways) that are related to each other for some reason (highways that make up a route for example).
Router DataThe information that is needed by a routing algorithm is only a subset of the information that is collected by the OpenStreetMap project. For routing what is required is information about the location of roads (or other highways), the connections between the highways and the properties of those highways.
- Location of highways (nodes)
- The locations of things is provided by the nodes from the OpenStreetMap data. The nodes are the only things that have coordinates in OpenStreetMap and everything else is made up by reference to them. Not all of the nodes are useful, only the ones that are part of highways. The location of the nodes is stored but none of the other attributes are currently used by the router.
- Location of highways (ways)
- The location of the highways is defined in the OpenStreetMap data by the ways. Only the highway ways are useful and the other ways are discarded. What remains is lists of nodes that join together to form a section of highway. This is further split into segments which are individual parts of a way connected by two nodes.
- Properties of highways (tags)
- The ways that belong to highways are extracted from the data in the previous step and for each way the useful information for routing is stored. For the router the useful information is the type of highway, the speed limit, the allowed types of transport and other restrictions (one-way, minimum height, maximum weight etc).
- Connections between highways
- The connections between highways are defined in the OpenStreetMap data by ways that share nodes. Since the ways may join in the middle and not just the ends it is the segments defined above that are not part of the OpenStreetMap data that are most important.
Interpreting Data TagsThe tags are the information that is attached to the nodes and ways in OpenStreetMap. The router needs to interpret these tags and use them when deciding what type of traffic can use a highway (for example).
There are no well defined rules in OpenStreetMap about tagging, but there is guidance on the OpenStreetMap Wiki "Map_Features" page. This describes a set of recommended tags but these are not universally used so it is up to each application how to interpret them.
The tagging rules that the router uses are very important in controlling how the router works. With Routino the data tags can be modified when the data is imported to allow customisation of the information used for routing.
Problems With OpenStreetMap DataThe route that can be found is only as good as the data that is available. This is not intended as a criticism of the OpenStreetMap data; it is generally good.
There are some problems that are well known and which affect the router. For example highways might be missing because nobody has mapped them. A highway may be wrongly tagged with incorrect properties, or a highway might be missing important tags for routing (e.g. speed limits). There can also be problems with highways that should join but don't because they do not share nodes.
A lot of these problems can be found using the interactive data visualiser that uses the same Routino routing database.