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

Fri Jan 29 22:31:00 GMT 2010

```At 2010-01-29 14:00, Dale Puch wrote:
>Your talk of "absolute levels of elevation" got me thinking.
>The tag has nothing to do with a measurement, only order of the layers.

What I'm saying, though, is that, if you tag with an eye on making the
layers reflect reality, it allows you to tag new features in the future
without too much collision with existing features.

>Some examples might be an elevated (built up earth) interstate in the US
>next to a city road.  Both are layer 0 but not at the same elevation.

I'd say it depends on how high it is in relation to the street. The
question is, what do the streets that cross the freeway do? If the street
underpasses go under the freeway without either the freeway or the roads
changing much in elevation, it makes sense to make the freeway layer 1 and
roads layer 0. If, OTOH, the roads have to drop substantially in elevation,
then I would make the freeway and the road segments away from the freeway
layer 0, and make the underpasses layer -1.

As I said before, I don't think this has much to do with tagging something
as a bridge or a tunnel - that is a matter of adherence to official
categorization and/or rendering.

I consider the "ground" (layer 0) level for a given area to be the position
of the majority of the streets and street-level features.

>  A road on a cliff 1000 feet next to a river at the bottom are both layer 0.

I disagree. As far as the exact values to use, I'd want to look at the
surrounding area, but I'd say the cliff has to be at least one layer higher
than the river regardless. What happens when they build a bridge over the
river to the cliff on the opposite side? You then have to raise the layer
of the bridge by one, even if the ground truth doesn't reflect this (i.e.
bridge at-grade with the cliff). If there was a cable car already at layer
1, you'd have to raise it too to avoid collision with the bridge, etc.

>An elevated train track above city streets would be layer 1 (and probably
>not tagged at bridge)

Agreed, but why is this different from the freeway case, which will also
have streets under it at some points?

>Personally I would tag the top roads in Chicago as layer 0 and the ones
>underneath as layer -1 and tunnel.

There are, of course, places like this where a consistent "ground level"
doesn't really exist, as traffic, natural features, and creative use of
limited space have created more of a dual-layer structure, but these are
more the exception than the rule I think.

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