[OSM-talk] Divided roads proposal

[Morten Kjeldgaard]

On 04/12/2009, at 06.49, Steve Bennett wrote:

> I don't know how to convince you that I'm not proposing changing the  
> way major roads and dual carriageways are mapped. This is about  
> minor divisions in minor roads.
>
> Let me ask you: how do you think that a road with a painted traffic  
> island down the middle should be mapped:
> 1) As a single road with no special tagging
> 2) As a single road with a tag indicating the division
> 3) As two separate ways, with ways connecting them every time  
> there's a gap in the painted division.

This discussion is reminiscent of other discussions just like it.

There are two orthogonal approaches to mapping in the OSM. One is the  
"drawing" approach, which is the most intuitive, since it reminds of  
the way maps have been drawn with paper and pencil for hundreds of  
years. The drawing approach is favoured by newbies and people who  
mostly care about having a beautiful, detailed map to look at. The  
other approach is the "tagging" appoach, where details about the  
landscape are mapped onto a line passing through it.

The "tagging" approach to mapping puts emphasis on a graph  
representation of the surrounding world which for example can be used  
for routing, area- and distance determination, and may types of  
statistical and database applications, situations where the "drawing  
method" doesn't work. Therefore, the "tagging" method has some digital  
properties that are incridibly useful and powerful, and it is these  
properties that is the innovative secret behind the incredible success  
of OSM.

The regrettable fact is that more and more mappers don't realize the  
incredible power of the tagging approach. Drawing dual carriage roads  
as two separate ways is appealing in many ways since "that's how it  
looks". However, it creates problems in more complicated situations,  
for example if intersections with other roads, with cycle- and  
pedenstrian paths are involved, because the connectivity very easily  
gets screwed up. Keeping the connectivity correct forces you to draw  
things that aren't representitive of the real world, and suddenly, the  
drawing isn't "pretty". The result is, such as I've seen lately, that  
people start to say "why should we care about routing at all?"

One example:  an intersection of  two crossing dual carriageways will  
result in four nodes. If the intersection is regulated by a traffc  
light, you will need to specify four traffic lights, but to OSM this  
will appear as four intersections, and passing through the  
intersection, your GPS will think you have to pass two intersections  
instead of one. So, here, while drawing dual carriageways as two ways  
"looks right", it is wrong from a topological point of view.

There's nothing you can do with drawing that can't be done with  
tagging, and there's in principle nothing being done with tagging that  
can't be rendered beautifully on the final map. It only depends on the  
richness of the tagging language and the sophistication of the  
renderers.

It is very important that OSM keeps its head straight and doesn't  
succumb to the increasing pressure of making the map a 2D-drawing of  
the world.

Cheers,
Morten