[OSM-dev] One value per key? Why not one pair per object?

Simon Ward simon at bleah.co.uk
Tue Feb 3 07:32:45 GMT 2009

On Mon, Feb 02, 2009 at 03:44:37AM +0100, Frederik Ramm wrote:
> If two people each implement a renderer that draws cycleways in blue and 
> footways in red, then it is very likely that an object tagged 
> "highway=cycleway" and "highway=footway" will show up blue on one map 
> and red on the other.

Just as the mapper should tag in some defined way for it to render
correctly, multiple keys should be used only where it makes sense.  We
already say to tag highways in a certain way.  If the above case renders
incorrectly then it probably should be fixed.  The uniqueness of keys
(or lack of multi‐valued keys, whichever way you look at it) adds a
restriction in the tagging system where it is otherwise freeform.

A complex system that works is invariably found to have evolved from a
simple system that works.—John Gall
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