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

Richard Z. ricoz.osm at gmail.com
Fri Mar 14 15:45:00 UTC 2014

On Fri, Mar 14, 2014 at 11:51:47AM -0300, Fernando Trebien wrote:
> Hello everyone,
> This is a small issue that came up recently in Brazil. In my
> understanding, the layer tag has no specific meaning other than to
> specify a rendering order. The wiki, however, states that it is wrong
> to tag a whole river with layer=-1. The reason for that, as far as I
> could figure, is because current validators (such as JOSM's or
> KeepRight's) will not issue a warning on a waterway x highway crossing
> when their layers are different, leading some users into tagging the
> river with layer=-1 in order to get rid of warnings about missing
> bridges and tunnels. So, I think that the validation rule is
> inadequate: it should warn when a waterway crosses a highway that has
> no bridge or tunnel tag, regardless of the value in the layer tag.
> (Fords are the only exception, AFAIK.)

agree that the error check should be stronger, location, pipeline and covered 
being other exceptions.

> Do you agree that the river can be tagged with layer=-1 as long as
> this value is correct in relation to the layer of other
> nearby/crossing ways?

in theory yes. However "nearby" is a problem as rivers can be very long.

Many people simply tag rivers with layer=-1 without even thinking about
the fact that the rivers may now collide with tunnels some hundreds of miles

Furthermore there is a convention that if there is a crossing with a bridge, 
the bridge should have the layer tag and not the way bellow it. Similar tunnel.

For both tunnels and bridges layer is now considered mandatory thus it is
totally useless to put rivers at layer=-1 unless they are in a tunnel.


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