On 28/11/2009 14:01, John F. Eldredge wrote: > So, ground level is level 0? I had wondered about that, as the > scanty documentation that I have seen didn't make that point clear.
well, it is the *default* level and the levels are relative. As with all things OSM, as there is no rigid spec, whether it is *ground* level is disputable. Some might say that in this case if the railway is on an embankment and the path does not change level, the railway should be +1 and the path 0 even though marked as a tunnel. I really don't think it matters if the levels are correct relatively speaking, and generally you'll find bridges mostly at level 1 and tunnels mostly at level -1. Actually, I personally think it should not be necessary to tag levels at all except in ambiguous cases. Bridges always go over and tunnels under, by definition. Only where there's a bridge over a bridge and you need to resolve the ambiguity should it really be necessary to say. And even without a bridge explicitly marked, rivers always run below roads by definition in all but a handful of special cases where an aqueduct would need to be explicitly marked. However, the widely accepted convention is that we do use levels, so forget my own opinion there. Though tagging for the renderers is frowned on, one thing that helps the renderers because it is algorithmically hard to do neatly otherwise, is to have all ways meeting at a node at the same level - so break a slip road off a motorway half way up the ramp if the grade separated junction it leads to is at level 1, and don't run a bridge straight into a non-bridge junction. Shouldn't be necessary, but it gives much cleaner results. David _______________________________________________ talk mailing list talk@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk
