[OSM-talk] Mapping everything as areas

[Jean-Marc Liotier]

Morten Kjeldgaard wrote:
> On 25/11/2009, at 14.11, Jean-Marc Liotier wrote:
 >
> The "map-drawing approach" is valuable in OSM because it allows us to
> indicate residential areas parks, etc. However, in addition, OSM has
> a graph-based approach for a description of the network of roads 
> which makes it *uniquely* valuable. Graphs prefectly represents the 
> road map and can be used for many applications, routing is an example
> that many people use daily.

"Graph" is the word I was looking for... Thanks for introducing it to
the debate. Indeed the graph (nodes+edges) is the simplest way
to model a network. In modeling, the simplest way is likely to be the best.

But if we map everything as an area, do we lose the ability to perform 
graph calculations ? Can't an area be considered as a set of edges 
connecting all nodes inside it ?

 > In my view it would make much more sense to work on a more expressive
 > (perhaps BNF based?) tagging scheme. This would enable a gradual
 > enhancement of the map, where the new tagging syntax could live
 > along-side the old.

 From my OpenStreetMap novice point of view, modeling ordered sub-ways 
inside a highway, each with its own set of tags would go a long way 
toward removing the need to model ways as areas. Special cases would 
remain, but if for a given way I can define the order of sidewalk, 
bicycle lane, bus lane, car lanes, separators and whatever else, each 
with speed limit, width and various other tags, I barely see the need 
for area mapping of ways.

Is there any problem with this approach ? It would introduce hierarchy 
and ordering, but it would reuse all the existing tags and remain 
compatible with the existing scheme. Notice that introducing hierarchy 
and ordering fits the existing OpenStreetMap XML schema quite naturally: 
all that would be needed is to nest a <way> inside a <way> - except that 
the nested <way> would have no <nd> but only tags.

> Conversely, there isn't much you can do with graphs that can't be 
> done with areas, and since the "map-drawing" approach has great 
> appeal to people enjoying beautiful and detailed maps, the pressure 
> for deprecating the graph-based approach in favour of the map-drawing
> approach will be ever increasing.
> 
> We need to resist that. Let's not throw out the baby with the bath- 
> water!

I now realize that there is a big risk of diluting the model. So maybe 
finding a way to make the model more expressive without changing its 
focus from graph to areas is a better way to address the need without 
losing what we have.