[Talk-us] Rewriting route direction documentation to emphasize subrelations

[Minh Nguyen]

Back in 2013, a discussion on this list [1] led to a wiki article 
documenting how to indicate the cardinal direction along a route, such 
as the "north" in northbound I-75. [2] I've drafted a rewrite of this 
article at [3] and would like to get the community's feedback on it 
before touching the original, which has often come up in discussion. The 
rewrite attempts to clearly explain route directions for both domestic 
and international audiences. These days, overseas mappers are very 
active in navigation mapping, and route directions are also relevant to 
some non-English-speaking regions.

According to the original documentation, setting relation roles to 
cardinal directions is preferred over creating a separate relation for 
each direction and setting direction=* to the cardinal direction (and 
joining the relations in a superrelation). The rewrite reverses this 
guidance, placing more emphasis on subrelations and superrelations than 
relation roles.

The directional roles have had the advantage of being easier to 
introduce than superrelations, especially for routes that only 
occasionally run along divided highways. That was important early on, 
when route relations were first being built out. However, it has tended 
to result in routes that are only partly tagged with cardinal 
directions, requiring data consumers to infer the cardinal direction 
along ways with forward, backward, or unset roles. Additionally, editors 
and quality assurance tools have not added specialized support for these 
roles as they have for forward/backward, so existing coverage has been 
fragile.

Although subrelations and superrelations are initially more difficult to 
create, they are easier to maintain in the long run and are already 
well-established for major road routes as well as public transportation 
routes. I see them as part of a natural progression in OSM towards 
greater detail, coverage, and structure.

This would only be a rewrite of documentation. I'm not proposing an 
effort to systematically restructure existing route relations into 
subrelations and superrelations. However, this documentation would 
better support mappers who are interested in carrying out such 
improvements themselves. Thanks for your attention.

[1] 
https://lists.openstreetmap.org/pipermail/talk-us/2013-November/thread.html#12165
[2] 
https://wiki.openstreetmap.org/wiki/Highway_Directions_In_The_United_States
[3] https://wiki.openstreetmap.org/wiki/User:Minh_Nguyen/Route_directions

-- 
minh at nguyen.cincinnati.oh.us