[Talk-us] Relations, cycle routes, shapefiles

[Paul Johnson]

On 02/03/2011 02:25 PM, PJ Houser wrote:
> Hi all,
> 
> I have some basic questions:
> 
> 1) Why are relations preferred for bike routes?

Relations are preferred for /all/ routes, actually.  This is because the
attributes of said route span unique ways.  For example, only parts of
Marine Drive in Portland are part of the 40 Mile Loop cycleway, and
across town, most but not all of the Springwater Corridor is part of 40.
 Likewise, many routes may use the same way, but don't share attributes
with other routes on the same way.  An example of this would be US 30
and I 84, which frequently share ways, but don't share ways for the
entire length, and have radically different attributes associated with
each route.  Or, say, Martin Luther King, Junior Blvd in Portland:  It's
both a road route (OR 99E) and a bus route
(TriMet 6).  Likewise, tagging route attributes on ways causes tagging
collisions (as we're experiencing with road routes now): underlying ways
often have refs that belong to them (like bridge numbers) but not the
route itself.

Then there's rendering.  Road routes aren't particularly of interest if
you're generating a bus map from OSM, for example.  Likewise, foot
routes are useless to truck drivers (You wouldn't expect a big rig on
Metro's "4T" trail, would you?).  Using relations for route data makes
custom renders much easier.

> 2) In the database, how do relations apply to ways? The attributes
> associated with a relation - how are they tied to ways? 

Relations are essentially "tags" on a way, as most editors view them.

> Will routing software use the attributes in a relation to determine if a way is suitable?

Depends on the routing engine.

> 3) In Portland, Oregon, we have an interconnected series of unnamed bike
> boulevards - how should we split these into relations? There are
> different tiers of bike boulevards (low traffic, middle traffic, bike
> lane, cycletrack, traffic calming devices but no bike lane, etc). Should
> we make a relation out of all connecting and similarly tiered ways? It'd
> be easier to just add attributes to the ways, but the OSM wiki seems
> quite clear on bike routes as relations.

Bike boulevards are on the same network as each other (well, the
Portland ones, are, at any rate; note I'm not referring to official
routes for state highways like 99 or federal highways like 205 or 84
since you're talking about city bike boulevards).  You're confusing
attributes that belong to the way (cycleway=lane, cycleway=track) with
those that belong to the route (network=lcn).

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