Re: [OSM-talk] How to mark a footpath that goes under a bridge

[John F. Eldredge]

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.

-- 
John F. Eldredge -- j...@jfeldredge.com
Reserve your right to think, for even to think wrongly is better than not to 
think at all. -- Hypatia of Alexandria

-Original Message-
From: David Earl da...@frankieandshadow.com
Date: Sat, 28 Nov 2009 13:56:23 
To: j...@jfeldredge.com; talk-boun...@openstreetmap.org
Subject: Re: [OSM-talk] How to mark a footpath that goes under a bridge

On 28/11/2009 13:52, John F. Eldredge wrote:
 I am in the process of learning how to use JOSM to transform a GPS
 trace into a way, and have a question about how to mark a footpath
 that passes under a highway bridge.  As I understand the conventions,
 placing a node at this crossing point would imply that they connect
 to each other, which is not the case.  Should the ways simply cross,
 relying on the layer tag to mark which one is above the other?  The
 existing highway data, probably derived from a TIGER import, does not
 indicate bridges as opposed to regular roadways.

The should cross, not connect, but the higher way one should be split
along the length of the bridge and marked layer=1, bridge=yes (unless
the footway is more a tunnel under the line, in which case instead split
the footway and mark the sub-railway section as layer=-1, tunnel=yes)

(This isn't JOSM specific BTW)

David
___
talk mailing list
talk@openstreetmap.org
http://lists.openstreetmap.org/listinfo/talk

Re: [OSM-talk] How to mark a footpath that goes under a bridge

[David Earl]

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