[Talk-us] script for adding layer=1 to bridges

dion_dock at comcast.net dion_dock at comcast.net
Thu Jan 28 19:34:53 GMT 2010

----- "David ``Smith''" <vidthekid at gmail.com> wrote: 
> On Wed, Jan 27, 2010 at 9:01 AM, Bill Ricker <bill.n1vux at gmail.com> wrote: 
> > On Wed, Jan 27, 2010 at 2:13 AM, Frederik Ramm <frederik at remote.org> wrote: 
> >> > I've noticed that a lot of bridges don't include a layer= tag. I 
> >> > suspect this is because they render OK in mapnik...but not so well with 
> >> > osmarenderer. (Consider the railroad 
> >> > in 
> >> > http://www.openstreetmap.org/?lat=33.76931&lon=-84.53762&zoom=17&layers=0B00FTF.) 

I think a better example is http://www.openstreetmap.org/?lat=33.77333&lon=-84.56043&zoom=17&layers=0B00FTF . 

Osmarender has drawn the secondary road with its bridge then draws the motorway over both. 

> >> 
> >> I'd suggest to modify Osmarender rather than the data, then. 
> > 
> > No this is Tiger import data, the data arrived wrong and was half 
> > corrected. (much of tiger has intersecting nodes where there should be 
> > bridges. some bridge insertion went without layering), It missing all but 
> > implied layering of bridge-nature. What we can't tell without checking 
> > satellite view is whether the bridge is at grade level with the Railroad in 
> > a ditch, or if the bridge pitches up over the RR. 


> Anyway, I have seen places where people have cleaned up freeway 
> corridors, and neglected to tag "layer" on anything unless there are 
> bridges crossing over other bridges. Mapnik renders this fine 
> (actually that could be considered a deficiency of Mapnik in my 
> opinion) but it looks a little goofy in Osmarender. (On the other 

I agree; since the OSM wiki says bridge=yes needs a layer= tag, Mapnik is covering up mistakes in the data. That said, I'm also not a big fan of tags that _require_ other tags. Perhaps it should have been bridge=n where n is the layer. 

> hand, when one end of a "layer=0" bridge is directly at an 
> intersection with other "layer=0" streets, Osmarender renders this 
> beautifully and Mapnik makes it look odd.) Since some people consider 
> the entire "layer" tag to be "tagging for the renderer" these people 
> probably don't think it's important to add thorough layer information; 
> instead, they add just enough to make it look decent in "the 
> renderer". I am not of that opinion. 

To beat the "tagging for the renderer" horse for a minute: We want to avoid the following scenario: someone maps their local gym with shop=gym, name="Abs of Steel". The renderer does't do anything for shop=gym, so nothing shows up on the map. They then change the tags to something that does render, perhaps, leisure=sports_centre. 

However, in this case, they layer= tag is all about getting the map to be meaningful. 

I do see a legitimate question about whether a bridge over a river or canyon needs a layer tag. In Oregon, rivers are at level 0, just like roads. It sounds like some folks are mapping them at layer=-1. 

Maybe I should rephrase my question: is there any harm in adding a layer=1 tag to something that is already tagged bridge=yes? Would things be worse after doing this? I submit that the maps would look the same or better in most cases. 

Any objection to trying this on a smaller scale, say, just Portland, OR? 


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