On Wed, Jul 29, 2009 at 12:26 PM, Ben Supnik <bsup...@xsquawkbox.net> wrote:
> Hi Y'all, > > I am trying to build an algorithm to produce correctly layered, > correctly connected road networks from OSM ways + the layer tag. > > There are a few "easy" cases: > > - When two ways cross laterally but have different layers and have no > node at the intersection, clearly they do not connect in space, and the > layer of each of the four partial ways at the intersection point is clear. > > - When two (and only two) ways end at a common node, and the ways have > different layers, it's probably a safe bet to say that this is where the > layers transition and the two ways should connect. (E.g. a way with > layer=1 and a way with layer=0 form a highway that has a bridge...this > junction is where the bridge "hits the ground". > > There are two cases I am wondering about though: > > - Two ways cross and have different layers, but a common node exists at > the crossing point. > > - Three ways come together, two of one layer, and one of another layer. > > An example of this last case might be a residential road that is about > to "bridge" over a highway, but the bridge ends at a T junction to > another residential road, e.g. > > ----*===== > | > > (Sorry for the ASCII drawing...) > > These two cases seem tricky to me because there may be some cases where > ways of different layer need to be brought together and some where they > need to be kept separate. > > Does anyone have thoughts (or has anyone implemented) a heuristic to > decide when two ways need to be put into the same layer at a given node? > > Thanks! > Ben > The topology rules are simple--if the ways share a node, then they are connected and it is possible to navigate from any of the connected ways to another (subject to turn restrictions, etc.) The layer tag is primarily a hint for renderers for proper display of vertically separated features. If the data is otherwise, then it is an error and should be corrected. Karl
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