[Tagging] Equivalence relation

Peter Wendorff wendorff at uni-paderborn.de
Tue Jan 11 12:45:19 GMT 2011


Am 11.01.2011 04:26, schrieb Steve Bennett:
>  On 11/01/2011 12:20 AM, Richard Mann wrote:
>> I put adjacent=yes on the highway=cycleway, so the user of the
>> cycleway=track tag on the main road can ignore ways with adjacent=yes
>> on them. The user who'd prefer to use highway=cycleway ways doesn't
>> know that the cycleway=track is a duplicate, but routers only have to
>> give a slight preference for highway=cycleway over cycleway=track to
>> use the "right" one (and even if they use the "wrong" one, it doesn't
>> much matter anyway).
> IMHO, this is being extremely optimistic about the powers of a router. 
> So you're saying that a router could see the "adjacent=yes", then 
> locate a nearby cycleway which is "adjacent" in some way, and conclude 
> that the two refer to the same thing?
I think you think too complicated here.
In much cases Steve could be right.
The routing engine has not to consider explicitely the adjacent-tag.
The slightly preference of the separate tagged way over the tag, 
compared with the usually equal lenght of both alternatives will push 
the routing engine to prefer the cycleway explicitly tagged.

But: While routing engines will work well this way, renderers could fail.
A renderer has to check if there in fact is the separated cycleway to be 
"allowed" to drop the tag for rendering. (If it's the target to render 
cycleways).

regards
Peter



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