[Tagging] Bridges and layers

Richard Mann richard.mann.westoxford at googlemail.com
Mon Jul 26 17:12:32 BST 2010

On Mon, Jul 26, 2010 at 2:56 PM, Dave F. <davefox at madasafish.com> wrote:
>  On 26/07/2010 14:07, Richard Mann wrote:
>>  If you draw the
>> grey in the correct layer, then you get little semi-circular arcs of
>> grey at the end of bridges (if they are layer=1).
> I've never noticed this in Mapnik,or an other. Do you have examples of these
> please.


>> So renderers have to do something. Different renderers have come up
>> with different solutions, but all produce artefacts because there's a
>> piece of data missing
> What data is this?

That one way is at a higher layer than another that it connects to.
You can infer this from bridge=yes, but it doesn't always work, and
you can't infer it directly from tunnel=yes.

>> (which they could pre-process, sure, but there
>> are better uses of time). It would be more effective to give them the
>> data, and have renderers do it reasonably well consistently.
>> What I've suggested isn't the only solution, but it's the most
>> economical, I think.
> So your saying to save the renders time, the data collectors have to waste
> time adding new tags?
> It sounds like a rendering software problem to me & nothing to do with
> tagging.

It could be fixed by the renderer, though they haven't to date, and
some of them (me included) can't with the tools available.

> I still don't see how layer_change solves anything.

It fills a hole in the data model. At the moment, for almost all
rendering tasks, I only have to look at the tags on the way and it's
nodes. Without layer_change, I either have to pre-process information
from the attached ways, or simplify, or make a plausible guess. With
layer_change I just do what the tags tell me.

> AFAICS it just adds even
> more separated sections which cause other render problems such as one-way
> arrows overlapping & cutoff name & ref labels.

now that is the sort of stuff for renderers to solve; it's a much more
general problem than a few tunnel ramps

> incidentally, is there a reason you've joined the cycle path to the overhead
> power line?

Weren't me guv, but I've fixed it.

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