[OSM-talk] Left and Right?

[Chris Morley]

Karl Newman wrote:
> On Sun, Aug 24, 2008 at 9:53 AM, Rory McCann <rory at technomancy.org 
>     Gervase Markham wrote:
>      > What's current tagging best practice with things which are to the
>     left
>      > or the right of a way (e.g. bus stops)?
>      >
>      > A nearly-approved proposal for a canal-side object has been
>     objected to
>      > by someone who thinks that the tag should be on a node which is
>     part of
>      > the canal rather than next to it, with left/right indicated as
>     part of
>      > the tag key name.
>      > http://wiki.openstreetmap.org/index.php/Proposed_features/Mooring
>      >
>      > Do we do that for any other tags? Do we have highway:left=bus_stop?
> 
>     Personally I add the node to left of the way, not as part of the way. I
>     believe the OSM theory is that the way represents the middle of the
>     road. So things like mini-roundabounds and traffic lights are part of
>     the way (ie road), but a bus stop is off to the side of the road.
> 
>     A similar thinking is obvious in the Karlsruhe House Address Scheme
>     (http://wiki.openstreetmap.org/index.php/Proposed_features/House_numbers/Karlsruhe_Schema),
>     since the buildings that are numbered are not physically in the middle
>     of the road, they are added as nodes to the left or right of the way.
> 
> Yes, and that method is not topological, which makes it very difficult 
> to associate that feature (bus stop, house number, whatever) with the 
> way that it's actually located on. It should either be a node that is 
> part of the way, or have a relation to connect the node with the way.

I think that the topological aspect of OSM's data structure is important 
and well worth maintaining in nodes and ways as well as in relations. We 
are not just drawing pictures, we are also recording relationships. This 
is why the representation of a mooring - a stretch of canal where boats 
tie up - as a separate way not connected to the canal seems wrong to me. 
In this case and with bus stops or house numbers, if you convey which 
side it is on by having a separate node or way displaced an arbitary 
short distance to one side, then you lose this side information at lower 
scales, when it may still be important to a user. With a topological 
description it is still available.

Left/right are sometimes criticised as being dangerous because they can 
be "accidentally" reversed. It is editing programs that do all the 
reversing and it would be better if they provided better support. It 
would not be difficult to have a scheme with automatic reversal of tags 
(on the way or its nodes) containing "left" or "right" or a few others 
(like "oneway"), together with a more intelligent warning for the user 
in other cases.

Chris