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

Dale Puch dale.puch at gmail.com
Fri Jan 29 22:00:04 GMT 2010

Your talk of "absolute levels of elevation" got me thinking.
The tag has nothing to do with a measurement, only order of the layers.  So
the most important factor is what is considered layer 0

For me anything above ground would (almost) always be a bridge and anything
below (almost) always a tunnel.
Things like buildings and openspace or plants would be a good indicator of
layer 0  Another would be if you were standing there, are you above ground,
below or on the ground.

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.  A
road on a cliff 1000 feet next to a river at the bottom are both layer 0.
An elevated train track above city streets would be layer 1 (and probably
not tagged at bridge)  Personally I would tag the top roads in Chicago as
layer 0 and the ones underneath as layer -1 and tunnel.

Dale Puch

On Fri, Jan 29, 2010 at 12:02 AM, Anthony <osm at inbox.org> wrote:

> On Thu, Jan 28, 2010 at 10:13 PM, Alan Mintz <Alan_Mintz+OSM at earthlink.net<Alan_Mintz%2BOSM at earthlink.net>
> > wrote:
>> Columbus Blvd is more puzzling. It may be layer=-1 through this whole
>> area, or the whole waterfront may be lower than the street level is to the
>> west of I-95. This is where I think a local survey of the actual elevations,
>> and tagging of layers by absolute levels of elevation makes sense.
> I-95 in that area (before, within, and after the "tunnel") is built in a
> trench.  I don't think that means it should all be at layer=-1 though.
> Tagging of layers by absolute levels of elevation would be great, except
> for the fact that "absolute levels of elevation" can be extremely hard to
> come by.  Some sort of "relative levels of elevation" would probably be
> easier.  Break the problem up into two parts and let people work on the two
> parts independently.  1) Choose a "surface level" for all points on the
> earth and tag node heights relative to that surface.  2) Figure out, for all
> points of earth, where that "surface level" lies with respect to one or more
> datums.
> Part 2 (which I picture as http://en.wikipedia.org/wiki/File:Geoida.svg)
> is cool, but probably isn't going to interest a lot of OSMers who would,
> nonetheless, be interested in mapping part 1.
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