[Tagging] Data redundancy with "ref" tag on ways vs relations

David ``Smith'' vidthekid at gmail.com
Mon Jul 30 20:12:53 BST 2012

Route relations are good because they offer a structured format to identify
and describe a route, such as US Bike Route 25, or Fairfield County Highway
177.  Ref tags on ways now are a good place to use shorthand, like USBR 25
or CR 177.  When multiple routes overlap, the ref tag on the way is an
opportunity to summarize and/or prioritize.  For example, it would make
sense to me for all the ways of Interstate 465 around Indianapolis to have
their ref tag say just I-465, even though different parts of it may also be
parts of one or two Interstates and up to six US and/or state routes.  If
you're making a simple map of, say, a neighborhood or campus adjacent to
that highway, the summary I-465 will suffice. If you're generating driving
directions, I-465 alone will suffice and in this case matches exactly what
a local will tell you.  If you're making a roadgeeky map that uses correct
highway symbols wherever possible, you look at route relations and draw up
to 9 shields on part of the highway.  If you're making a map that hilights
one of those routes specifically, you use its relation which includes ways
whose refs just say "I-465" which is okay because you're probably not
labeling any roads besides hilighting the route of interest.

If you have a tool that says "US 136 includes ways whose ref tags say
'I-465' and this is an error" then you need to realize that route relations
and way ref tags serve different purposes and use cases, or at least that
sometimes it's impractical for them to match perfectly.
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