[OSM-talk] Crossing ways

Jon Bright jon at siliconcircus.com
Sat Aug 11 17:49:32 BST 2007


Hi David,

David Earl wrote:
> 
> Here's the typical way to do it.
> 
>                   |
>     --------------x------------
>                   |
>     --------------x------------
>                   |
>                   |
> 

I considered this, but wasn't sure.  There are two problems that I see. 
  The first is, as you mention, that the turning restrictions at the 
junction can't be represented.  The second is that, at least for some 
junctions, the resulting distance between the minor road and the 
opposite carriageway is incorrect.

In my case, the junction actually ends up more like

         a
----<----------------<---
           \  /
            \/
---->------+--------->---
          \ |
           \|
            |c


For somebody driving from c to a, if I represent the junction as you 
suggest, the routing algorithm will count


15m
--+
   | 15m
   |
   +
   |

=30m

If I represent the junction as one actually drives over it, the routing 
algorithm will count

  \  21.2m
   \
    +
    |

Granted, it's only a difference of 8.8m.  For larger junctions,
it would be larger.  Given a long route, it wouldn't take too much to 
get a 1km difference....

--
Jon




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