OpenHistoricalMap/Tags/Relation/multilinestring

multilinestring
Description
A discontiguous series of unclosed ways that semantically represents a single physical object.
Group: properties
Members
(empty)
Status: in use
multilinestring relation Relations: Members: Prevalent roles: More details at taginfo More details at taginfo
Tools for this tag
overpass-turbo QLever

A relation tagged type=multilinestring is a discontiguous series of unclosed ways that semantically represents a single physical object.

Beyond the niche uses in OpenStreetMap, some OpenHistoricalMap contributors use multilinestring relations to represent a linear highway=* feature as it evolves over time, particularly as its highway classification changes. Compared to mapping multiple overlapping ways, this method has the advantage of populating the relation list in JOSM, making it easier to navigate among the stages in the physical roadway's evolution. However, it has the disadvantage of requiring more elements overall. This additional complexity leads to poor software support and frequent modeling errors.

How to map

Remove all the tags from the ways. Add the ways to a multilinestring relation that has the same tags that were on the ways.

Examples

Let's consider a road that is modeled using the basic method of stacking multiple overlapping ways:

1800–1900	Way 1 highway=primary
1900–2000	Way 1′ highway=secondary

You can factor out the stack into multilinestring relations:

	Way 1 (member of 2)
1800–1900	Multilinestring 1 (1 member) highway=primary
1900–2000	Multilinestring 1′ (1 member) highway=secondary

Since a multilinestring relation's attributes must be consistent from end to end, it is limited by any change that occurs along the feature. For example, if a road traverses a bridge, you would ordinarily split the roadway at the bridge's endpoints in order to add bridge=yes to the bridge segment. You would do this for each roadway in the stack:

1800–1900	Way 1 highway=primary	Way 2 highway=primary bridge=yes	Way 3 highway=primary
1900–2000	Way 1′ highway=secondary	Way 2′ highway=secondary bridge=yes	Way 3′ highway=secondary

As an alternative, you can maintain just one set of ways but overload it with multiple sets of multilinestring relations:

	Way 1 (member of 2)	Way 2 (member of 2)	Way 3 (member of 2)
1800–1900	Multilinestring 1 (1 member) highway=primary	Multilinestring 2 (1 member) highway=primary bridge=yes	Multilinestring 3 (1 member) highway=primary
1900–2000	Multilinestring 1′ (1 member) highway=secondary	Multilinestring 2′ (1 member) highway=secondary bridge=yes	Multilinestring 3′ (1 member) highway=secondary

If the road is a member of a designated route, the route needs to be modeled as a route relation. Route relations can efficiently group stacked ways as long as they exist in different time periods:

1800–1900	Way 1 (member of 2) highway=primary	Way 2 (member of 2) highway=primary bridge=yes	Way 3 (member of 2) highway=primary
1900–2000	Way 1′ (member of 2) highway=secondary	Way 2′ (member of 2) highway=secondary bridge=yes	Way 3′ (member of 2) highway=secondary
1900–1950	Route 1 (6 members) ref=A
1950–2000	Route 1′ (6 members) ref=B

When using multilinestrings, the multilinestrings need to exist parallel to the route relations. Do not make the multilinestring relations members of the route relations:

	Way 1 (member of 4)	Way 2 (member of 4)	Way 3 (member of 4)
1800–1900	Multilinestring 1 (1 member) highway=primary	Multilinestring 2 (1 member) highway=primary bridge=yes	Multilinestring 3 (1 member) highway=primary
1900–2000	Multilinestring 1′ (1 member) highway=secondary	Multilinestring 2′ (1 member) highway=secondary bridge=yes	Multilinestring 3′ (1 member) highway=secondary
1900–1950	Route 1 (3 members) ref=A
1950–2000	Route 1′ (3 members) ref=B

Software support

OpenHistoricalMap vector tiles render multilinestrings that have certain tags like highway=*.

Nominatim does not index multilinestrings, so turning a roadway into a multilinestring makes it unsearchable. Future routers are unlikely to support routing over a multilinestring. No editor or validator supports multilinestrings.

Alternatives

For linear features like roadways and railways, the alternative is to stack multiple ways that have similar attributes. To minimize the size of the stack, use other types of relations can group the feature's constituent elements by certain attributes, independently of the physical infrastructure:

• type=route represents a route designation
• type=street represents a street's identity (i.e., its name)
• type=route route=railway represents a railway's identity
• type=enforcement enforcement=maxspeed represents a speed limit