[OSM-talk] [tagging] RFC :left/:right (asymmetrical roadside features)

[David Earl]

On 17/02/2009 10:36, Andy Allan wrote:
> On Mon, Feb 16, 2009 at 7:11 PM, Norbert Hoffmann
> <nhoffmann at spamfence.net> wrote:
>> Andy Allan wrote:
>>
>>> And every time using :left and :right comes up, we all have a big
>>> discussion about it and then nobody pays any attention and it comes up
>>> again a few months later.
>> Perhaps this is because the concept "left"<>"right" is so simple - and the
>> aversion against editors, that are not totally key-ignorant is not so easy
>> to understand.
> 
> And nobody pays attention. The main problem is that two-way roads have
> no inherent, real-world, "direction" - neither side of the road is the
> right or the left. Or rather, both sides of the road are the right or
> the left, depending on which way you are facing. The only place that
> right and left has any intrinsic sense is on one-way roads, which *do*
> have an inherent direction (and signs to that effect).
> 
> Let's say you have a church beside a road. If it's a oneway street, it
> makes some kind of sense to say "it's halfway along the road, over on
> the left".
> 
> Let's say you have a church beside a two-way road. If I said "it's
> halfway along the road, over on the right", you still wouldn't know
> which side of the road it is on.
> 
> Now the problem is that most people at the moment in OpenStreetMap are
> tech-heads, and are so used to mental constructs and abstractions like
> every road having a completely arbitrary intrinsic direction - but
> that doesn't mean it's a great idea. Editor support is less important
> - and far easier to fix - than explaining to all the people who don't
> even realise that all roads have a direction in openstreetmap - and
> except for oneway roads, I have no idea which ways are pointing in
> which directions, and it shouldn't be important unless it *has* to be
> important.

Real People often talk about "the church on the left when you're heading 
towards somewhereville", so it's not *that* alien a concept. And in 
terms of the other concepts you have to understand to edit the map, it's 
hardly a big one. And you do already have to know about it - for one way 
streets and also for rivers and also for boundaries where exactly this 
left/right issue arises.

So I think you're overstating the problem with this, and the reason it 
isn't widely adopted is because there has been no consensus in the past, 
not because it is fundamentally hard.

There's only really two ways to deal with this geometrical relationship: 
relative or absolute. left/right is relative and suffers from lack of a 
"natural" direction to base it on; north/south/east/west is absolute, so 
is independent of any reversals done in the editor, but suffers badly on 
roads which turn more than 90 - 180 degrees - so you'd have to split 
them, which is just as arbitrary a rule as using the direction, though 
probably rarer.

Since for any N people discussing something in OSM there always seem to 
be N+1 opinions, the only way this is likely to be resolved is if people 
just do it (in their preferred way) and see if one of them wins. It's 
more likely to win if the renderers act on it.

Incidentally, :left/:right (or :north, etc) have a problem with 
languages on names. So if the name on the left (north) is different, 
using name:left (name:north) would have to be dealt with as a special 
case as name:x is usually used with x as a language.

David