[Tagging] length=

[Mike Thompson]

Thanks to everyone for sharing their thoughts.

> I personally recommend to use the length key while mapping street cabinets as nodes.

Agree, length makes sense on nodes


> The way in OSM is only a (sometimes not precise) drawing of an existing feature and can be different from the reality.

How precise is the value of the "length" tag? From what is the value derived?


> To be more precise : OSM is based upon a 2D geospatial database.
In my mind, a road climbing a mountain won't have the same length in
reality than in the DB : the Z dimension may have influence too.

For most highways this will have little difference.  10% (10 vertical
meters per 100 horizontal meters) is considered a steep highway grade.
In this example, the actual road length will be 100.5 meters
(sqrt(10^2 + 100^2)). For ski slopes and aerial trams, yes, it will
have a larger influence.


> it is quite often used on bridges and tunnels, where the actual (official) length can be observed.

Perhaps "official_length" or "signed_length" might be more indicative
of what actually is being recorded.