[talk-au] Mapping interchanges levels or turn restrictions

[Roy Rankin]

With the resolution and newness of Nearmap images Freeway/Motorway 
interchanges are being mapped in detail rather than schematically. This 
brings up the issue on how to best do this.

Picture this case. A freeway goes north-south and a dual lane east-west 
road goes over the freeway with a connection to the freeway. West bound 
traffic on the road going onto the freeway heading south must cross the 
east bound traffic on the road. Both these ways are oneway.

I see two possible ways to map the lane(s) turning south and the lane(s) 
going east.

1> The ways cross with a common node and a no_right_turn or 
only_straight_on restriction. (both lane(s) must be broken into two ways 
at the common node as required by  relations)
2> The ways cross without a common node but are on different layers. 
(Although physically using the same pavement where they cross). With 
freeway over/under road and 4 on/off ramps, 4 layers are  required.

To me the second method is the simplest although I usually think of 
different layers as having different elevations.

The reason for this e-mail is I see no discussion of this in the OSM 
documentation, but OSM will be easier to maintain if similar situations 
tend to be mapped in similar ways. The following anecdote describes my 
experience today.

Today I came across some issues with the turn restrictions on the 
interchange between Falcon Street and the Gore Freeway in North Sydney. 
 
(http://www.openstreetmap.org/?lat=-33.82907&lon=151.21494&zoom=17&layers=0B00FTF 
) The mapper did not realise that ways with turn restrictions must break 
at the via node. Unfortunately the current tested JOSM does not handle 
turn restriction relations properly when a way is split and much effort 
is required to fix up the relations after a way split. As each way had 
quite a few turn restrictions I realized that after splitting the ways 
it would require some time to sort out the relations. I thus used method 
2 above and deleted all the relations.

Regards,
Roy Rankin