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

Mike Harris mikh43 at googlemail.com
Tue Feb 17 12:50:14 GMT 2009

While sympathetic to the underlying need being discussed in this thread, I
suspect there is a further problem. Although a way has an intrinsic sense in
OSM, this is fairly volatile! All it needs is someone to reverse a way - and
this can happen rather easily, say, when combining two ways with the same
tags but different senses (yes - there is a warning but it's all too easy to
click through). Reversing the ways then, of course, reverses the 'left' and
'right' descriptors with their differing tags! This leads me to wonder
whether an absolute sense (north, south, etc.) is still better even though
it might require that a way is divided a bit. Most ways do have a 'general'
compass direction for long segments even if this is often more human-obvious
than machine-obvious. The main exceptions are likely to be short residential
streets on housing developments etc. - 'circles' etc. - but these are less
likely to require unilateral tagging.

Btw, I have encountered the same problem with canals. Some mappers describe
towpaths as being 'left' or 'right'. Personally, I prefer to map the towpath
as a separate way alongside the canal - with the added advantage that this
allows me to tag the towpath, e.g. with access rights, surface condition,
barriers, reference numbers, route relations, etc.

Perhaps this would also be a better approach for e.g. cycleways alongside
motor roads? Although, I have to admit that it doesn’t solve the problem of
unilateral naming.

Mike Harris

-----Original Message-----
From: David Earl [mailto:david at frankieandshadow.com] 
Sent: 17 February 2009 11:10
To: Andy Allan
Cc: Norbert Hoffmann; talk at openstreetmap.org
Subject: Re: [OSM-talk] [tagging] RFC :left/:right (asymmetrical roadside

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

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.


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