[OSM-talk] The segments vs ways vs superways question again...

[Jochen Topf]

On Wed, Jan 03, 2007 at 02:13:56PM +0100, bvh wrote:
> I keep asking : what specific problems are you solving? and I am getting
> very superficial answers from people who are making proposals that
> would introduce a lot of work or even risk to set back the project.

Ok, here are a few problems I have encountered. Some of them I have
mentioned already. I don't say they can't be solved with the current data
model, but at some point you have to ask yourself whether the fixes you
can make are only hacks and whether a different data model couldn't
solve the problem in a better way.

And before I get to the list another important thought: Whatever we do,
it has to be backwards compatible to the current data and reasonably
easy to switch the software over. Otherwise it is not going to happen.

Currently we can't do:

- Areas with holes in them. Current hack around this: Define inner area
  with higher layer. Disjoint areas: Say several parking lots around a
  convention center that we want to tag together.

- Road with tram line down the middle. Current hack around this: Define
  two ways on top of each other (which makes them basically uneditable)
  or use different slightly off-centered nodes which leads to bad
  rendering.

- Motorway. Current hack: Two ways, one for each direction. But now you
  can't render the motorway correctly on all scales. On lower scales the
  two ways will overlap, on larger scales they might be too far apart.
  Now think what will happen if part of that motorway goes over a bridge.
  You can only model this as two bridges by giving each of the motorway
  directions a layer=1 attribute. If you want one bridge what are you
  going to do?
  On a even lower scale (say a national map) you might want to draw the
  motorway network without proper motorway links, just draw the interchanges
  as a single dot. You can't do this without hacks because you haven't
  modelled the motorway correctly.

- Automatic reordering or re-directioning of segments/ways is not
  generally possible because there might be oneways or other features in
  there which depend on the direction. This is not a problem with the
  data model but with the way it is currently used. With the current
  software it is obviously difficult for users to do this correctly.
  I think this is in large part due to the issue with streets beeing
  part way oneways and partly not, making it awkward to handle. Similar
  for streets beeing part-way divided and part-way not. To make matters
  worse we can't know which tags are dependent on the direction of a
  segment and which are not, because everybody can invent tags himself.

- Tagging longer collection of ways together: Lets say you have a
  tourist route which consists of lots of different roads and together
  they are the "Route du vin" or whatever. Currently you have to tag all
  ways in this route with the extra "route=tourist" and "name=Route du
  vin". Ups. Can't do that. "name=" is already taken for the individual
  parts. Same goes for bus routes for instance.

- Large rivers: On the one hand you want to model them as a way (for
  instance to make them routable for ship routes and to be able to draw
  the flow direction). On the other hand they are more like an area
  because they tend to get narrower and wieder, open up to the sea etc.
  Currently these aspects can both be modelled but as two distinct
  features. There is no way to combine them. A similar thing happens
  with nodes and areas like a nodes with a place name and the area this
  place covers or a parking lot symbol and the area that parking lot
  covers. They really should be connected in some way which allows
  renderers to do the right thing at all scales, i.e. render as point on
  on scale, as area on another.

Ok, thats enought for now. :-)

Jochen
-- 
Jochen Topf  jochen at remote.org  http://www.remote.org/jochen/  +49-721-388298