[Tagging] layer=-1, rivers, bridges and tunnels

Richard Z. ricoz.osm at gmail.com
Wed Apr 2 14:03:38 UTC 2014

On Tue, Apr 01, 2014 at 11:21:51PM -0700, Bryce Nesbitt wrote:

> April 1st aside: the number of important implicit assumptions is relatively
> small.  Rivers under, power lines over, closed ways under except if they're
> tagged building, etc.  Currently this type of layering is implicit in
> various bits rendering software, but
> it could be formalized at the tag definition level to help meet certain
> mapper expectations.

there are some more assumptions, rivers are visible and not under forests, residential
areas maybe others.
The way mapnik handles this is black magic.

Also, when formalizing this we should make a distinction between rendering
order and vertical object relations.

> In the case of the river/highway layer warning: if the warning had never
> existed, chances are the various workaround schemes would never have come
> up.  Rivers would run under roadways, and tagging would be needed only in
> the rare case of a ford or an arroyo with no culvert.

That warning is indeed the cause of lots of hassle. It is not wrong but
until it is balanced by more complete validation of layers, rivers etc
it seems to cause more damage than good.


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