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<div class="moz-cite-prefix">On 2012-12-17 07:50, Clay Smalley wrote
:<br>
</div>
<blockquote
cite="mid:CAHR3E+qVcyEaSMD1_V7Ftk3KZmdSf9Jpvnk33iVar9y4NVWt6g@mail.gmail.com"
type="cite">I noticed the majority of the trackage of the San
Francisco Muni lines are tagged as layer=1, while the streets
along which they run have no layer tag (an implied layer=0).
<div>If the Muni lines are layer=1, it is my understanding that
the Muni lines should be physically above the street.</div>
<div>Since this is not the case and the lines run at street level,
should I remove the layer tag on these specific tracks (to imply
layer=0)?</div>
<div><br>
</div>
<div>(Of course, some of these lines run through tunnels where
they are tagged layer=-1, and on bridges where they are tagged
layer=1 correctly. The layer tag on these bits of track would
remain untouched.)<br clear="all">
<div><br>
</div>
-- <br>
Clay<br>
</div>
<br>
</blockquote>
<br>
A level is an altitude. A layer is a drawing opacity. Although OSM
does not tag for the renderer, it uses the tag <b>layer=*</b>. It
defines <b>layer</b> as the relative "position" (is that
"altitude"?). In fact, the only effect of assigning a layer is that
upper layer objects hide lower layer ones (it's not a "mind your
step" warning ;-))<br>
It's interesting to keep all the rails in the same layer to avoid
splits and layer =+1 may be needed for them to show at some places.
My reaction would be that the person having cared to explicitly set
the level might have had something on his mind.<br>
<br>
A bridge is a piece of concrete that is under -- relative altitude
-1 -- an uninterrupted foil of macadam. It shows just out of each
side of the road, like rails and the macadam hides it (that's, to
me, how the maps render it too). It can be tagged using a short
additional segment <strike>over</strike>underlaying the road. Yet,
the instructions and practice are to put it at layer +1 and to
unnecessarily split and even interrupt the road.<br>
Strange.<br>
You say that trams run at altitude -1 in a tunnel. As I see it, a
tunnel is layer=+1 even if the tram goes down (underground level) to
pass under it.<br>
Very complicated.<br>
<br>
I have traced lengths of streams <br>
<ul>
<li>stream as a constant layer=-2 way, uninterrupted end to end
(even if they "don't look so deep"),<br>
</li>
<li>roads are at level 0 <br>
</li>
<li>and bridges and culverts at level -1, in the manner mentioned
above.</li>
</ul>
If the stream comes to a pond, it continues to flow in a way drawn
across the pond, at the bottom of it, just what happens if the pond
depletes, all in a very uninterrupted stream way.<br>
Very neat, uniform, consistent and simple.<br>
<br>
Fortunately, streams are always one-way, have no speed limits,
etc... and it's easy to keep them in a single thread. It would be
possible for roads too with my SEGMENT idea but my e-mail wasn't
replied.<br>
Roads are even split unnaturally and unnecessarily by bridges.<br>
<br>
Cheers, <br>
<br>
<table>
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<td valign="top">André.</td>
</tr>
</tbody>
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