[OSM-talk] How do bridges work?

[Tom Hughes]

In a recent message Rick Collins <gnuarm.2006 at arius.com> wrote:

> At 09:00 AM 10/17/2007, you wrote:
>>In message <E1Ii8I4-0002tE-Sp at streetmap.vm.bytemark.co.uk>
>>         Rick Collins <gnuarm.2006 at arius.com> wrote:
>>
>> > Yesterday when I was viewing this section, I could actually see lines
>> > drawn to indicate the ends of the bridges.  Now I see nothing like
>> > that and even the bridges that look ok can't be distinguished from
>> > intersections.
>>
>>Only the tiles at home layer draws the bridge symbols - the mapnik layer
>>just draws the roads crossing (but stacked correctly where you can
>>tell the difference).
>
> But should the roads look like intersections?  There is a thin grey
> border at road edges and this line disappears where roads intersect.
> These "bridges" look the same way with the grey border missing at the
> points of intersection.

I don't know enough about it to comment - you're better off keeping
the discussion on list where people that know more about mapnik than
me can comment.

>> > I used a "bridge" = "yes" tag on all of the sections that should be
>> > passing on top.  They were not displaying correctly so yesterday I
>> > added a "layer" = "+1" tag to each of them.  This looks worse now
>> > than it did before I changed it.
>> >
>> > Am I still doing it wrong?  How do I get bridges to display correctly?
>>
>>Add bridge=yes (only use by tiles at home currently) and use layer
>>tags to indicate the correct relative ordering of the roads. So
>>normally the road on the bridge is layer=1. Complex interchanges
>>may need layer=2 etc to get everything stacked right.
>
> So what could be making the bridges display incorrectly?  Even the few
> that are close to "right" don't seem correct to me.
>
> One such bridge is at 39.4, -77.409.  The primary road, South Market
> St, is the bridge passing over the Interstate highway, I-70.

That looks like bridge span hasn't rendered at all - at a guess it
doesn't like "+1" as the layer. Try making it "1" instead.

Tom

-- 
Tom Hughes (tom at compton.nu)
http://www.compton.nu/