[Tagging] Micro Mapping, was Race track

[Steve Bennett]

On Fri, Feb 5, 2010 at 7:13 AM, Roy Wallace <waldo000000 at gmail.com> wrote:
> On Thu, Feb 4, 2010 at 8:06 PM, John Smith <deltafoxtrot256 at gmail.com> wrote:
>>
>> Imagine a mechanism in your favourite editor when you can drag the
>> "width" of the node outwards to match the width of the road, this then
>> gets stored against the node information for the way.
>
> Ah ok. Hmm, I'd prefer that the OSM way is the centerline of the
> feature. This would mean:
> 1) the OSM way is consistently meaningful in itself, and matches current usage
> 2) you only have one width value per node, which might simplify editing.

I think I'm inclined to agree. Any renderer that supports all these
tags won't be particularly inconvenienced by the fact that the line
isn't down the centreline, and any renderer that doesn't will do a
better job of rendering it.

That said, it would be fairly rare for the central divider to be far
from the geometric centre of the road for a non-divided highway,
wouldn't it? Are there really that many asymmetric roads, with say one
lane on one direction, and two in the others (as in the diagram)?

The biggest problem I see with John's proposal (as I understand it) is
that it adds lane information not onto nodes, and not onto ways, but
onto the *relationship* between ways and nodes. I could be
misunderstanding it, as I'm not really familiar with the underlying
data structure. And in any case, lane information is intuitively not a
property of a node, but of a section between two nodes. This proposal
would appear to make it very easy to make long sections of road where
the number of lanes is changing - not particularly useful.  In fact,
I'm not sure it's even useful to be able to code that.

> But it seems like you're suggesting that the OSM way instead should represent:
> a) if a oneway feature: the centerline
> b) if a twoway feature: the divider between traffic travelling in each direction

It does seem a bit inconsistent. A three lane one-way road will have
the way running down the centre of the middle lane. A three lane
two-way road will have it running between two lanes. Or, to put it
differently, for a given way with lane tags, where a renderer should
render the road will depend on the property of the oneway=* tag. Which
is extremely counterintuitive.

Steve