[Tagging] Clarification on the role link in route relations

[Minh Nguyen]

Vào lúc 14:27 2022-01-08, Dave F via Tagging đã viết:
> On 08/01/2022 21:02, Minh Nguyen wrote:
>> This is a high enough number that it can't be dismissed as a mere 
>> tagging mistake, yet it falls far short of the 766,473 
>> highway=motorway_link ways, let alone other highway=*_link ways. [8]
> 
> I think there maybe a misunderstanding of the difference between route 
> relations & ways upon which route relations are placed.
> 
> The link role of a route relation can be placed on *any* way. It's 
> commonly used as an indicator for 'this is the way to go to get to the 
> route' and not just connections between different routes.
> In the UK National Cycle Routes have signs for this with the ref number 
> in brackets. (Roads also have this scheme, but IMO roads determined by 
> their reference numbers are not valid route relations)

Can you provide an example of what this looks like? How does it differ 
from what destination:ref or destination:ref:to would be tagged on? 
(destination:ref:to is for where a trailblazer sign says "To ABC 123" 
with an arrow, as opposed to just "ABC 123" with an arrow.)

The current documentation for "link" explicitly states that it's for 
link road connections between different road routes, making no mention 
of recreational routes. I think it's unlikely that U.S. mappers, at 
least, would support comprehensively using the "link" role for anything 
that so much as mentions a road route, considering that there are more 
maintainable representations of that information. There are countless 
"To ABC 123" signs that are many miles away from the route itself, with 
nothing in between mentioning the route. Here are some examples tagged 
with destination:ref:to:

https://www.openstreetmap.org/way/19051231
https://www.openstreetmap.org/way/48889597
https://www.openstreetmap.org/way/60108744

Including them as "link"s in the route relation would turn the route 
relation into a very messy geometry quite unlike the original concept of 
a route. But it sounds like the National Cycle Route signs are used in a 
different manner.

>> Meanwhile, the inclusion of "link" members causes problems for any 
>> editor, QA tool, or data consumer that lacks support for this role 
>> when determining the linear progression of a route.
> 
> If they 'lack support' for the route role, how can it  causes problems' 
> for them?
> The OSM database is 'as is'. If there's a problem in data consuming 
> software using the database then you need to contact the software's 
> developers to let them know about /their/ problem.
> 
>> For example, iD has a known issue...
> 
> iD editor has many known inaccuracies. It's very frustrating having to 
> fix incorrect data added by it, especially it's validation system

My point is that the downsides seem to outweigh the upsides with regard 
to road route relations, considering their very low usage on them. If 
they make more sense for other kinds of route relations, that's the kind 
of insight I was looking for when starting this discussion. Ultimately, 
the wiki should document relation roles consistently without misleading 
mappers into using them for things they weren't intended for.

-- 
minh at nguyen.cincinnati.oh.us