[OSM-talk] Bridges / viaducts for railways

[Andy Robinson (blackadder)]

Tom Chance wrote:
>Sent: 28 January 2008 5:57 PM
>To: Talk Openstreetmap
>Subject: [OSM-talk] Bridges / viaducts for railways
>
>Hello,
>
>I'm a bit confused about these tags:
>
>railway=viaduct
>bridge=yes
>cutting=yes
>embankment=yes
>

railway=viaduct doesn't sound right.

Sticking my civil engineering hat on for a minute,

viaduct=true|yes|1 would be correct for a multispan bridge carrying anything
other than water (which would be aqueduct=true|yes|1)

A bridge is a single span structure carrying anything other than water.
Water doesn't have a single span equivalent, just use aqueduct= still.

A cutting or an embankment is what the transport corridor has been built on
when the natural ground rather than a structure is being considered. Almost
all transport corridors have traditionally been built using the "Mass Haul
Curve" principal which attempts to balance the amount you cut out with the
amount to fill to make embankments. This process keeps the road or rail
alignment as close to a constant or manageable grade as possible while
minimizing the distances you have to transport spoil.

For sections of a route not sitting on a structure there are actually three
conditions:
cutting
embankment
at_grade

The latter is where no excavation has been made (other than the removal of
topsoil and any other unsuitable surface deposits).

Cheers

Andy

>I assume that 'viaduct' covers these sort of elevated rail lines so that
>bridges are just for, well, bridges over other ways:
>http://www.mybrightonandhove.org.uk/images/uploaded/scaled/railway_viaduct_
>aerial.jpg
>http://www.betterpublicbuildings.gov.uk/assets/images/finalists_2006/jamest
>own/jamestown_large.jpg
>http://www.24hourmuseum.org.uk/content/images/2005_1745.JPG
>
>Why is it not a property like bridge, cutting etc. and will it render
>correctly? Should it be changed to viaduct=yes?
>
>The train lines around south London are pretty much always one of the
>above and I'd like to get the tagging right.
>
>Kind regards,
>Tom
>
>
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