[Talk-us] Highway Tagging Consensus to Improve OSM (and address some of 41 latitude's concerns)

Phil! Gold phil_g at pobox.com
Fri Oct 15 17:08:42 BST 2010

A blog post at 41latitude[0] has sparked a discussion on
talk at openstreetmap.org.  One of the comments there was that about half of
the points made concern inconsistent tagging in the US.  (Most of the rest
concern map rendering, which is more global in scope.)  I'd like to
discuss some of those things here, get a rough consensus, and, if
possible, update the wiki to consistently reflect the consensus.
Specifically, I plan to lay out several points in this email, and once
they've been discussed for a bit, poll the list to see how much consensus
there is on each issue.  Since this email is rather long, I ask that
replies focus on only one of the major points per reply; if you'd like to
comment on several points, please write several replies.

  [0]: http://www.41latitude.com/post/1310985699/openstreetmap-critique

I'll take things roughly by the headings on the 41 latitude piece.

== "Uneven road coverage" ==

He complains that road coverage looks uneven because different regions
have differing percentages of their roads with classifications above
residential.  This is mostly the result of different TIGER surveys
classifying things differently.  The best we can do, I think, is to try to
apply classifications consistently as we clean up the TIGER data.  I'd
also like to designate a discussion about consistency of road
classification in the US as being off topic for this thread, since that's
a very large, somewhat contentious issue that would probably drown out the
other issues I'd like to discuss.

== "Some of OSM’s Roads Look Like State Borders" ==

He points to some areas in Iowa and Ohio where there are roads that
alternate between motorway and trunk classification.  Without going too
deeply into specific criteria for road classification, are the different
sections of those really that distinct from each other?  Should there be a
rule of thumb that if a road seems to alternate between classifications
that it's better to tag the whole thing at the lower classification?  What
would be a good cutoff for that?  (If a road is downgraded as it goes
through a city but there's a high-grade bypass, it makes sense to tag the
two areas separately.  If it goes back and forth every few miles, maybe
it's not worth recording the higher classification.)

== "Hyphens" ==

There's a lot of inconsistency in tagging in road's ref= tags.  The main
wiki pages (Interstate Highways, United States road tagging) specifically
call for using spaces between the network designation and the network
number.  A lot of people still use dashes for Interstates, because thet's
how they're commonly written (and because "I-5" is more obvious than
"I 5", which might be read as "15").

We now have route relations with very good, specific rules that separate
road network and road numbers in easily parseable ways.  Does it make
sense to keep the "always spaces" rule (and possibly use the route
relations to update all the routes to match that rule)?  Should we go with
the way that people usually write the routes (dashes in Interstates,
spaces in US Highways and state roads)?

This seems like a very nitpicky point, but consistency will enhance the
overall impression of OSM's data.

== "Inconsistent State Prefixes" ==

This is another very inconsistent area.  The main US wiki page (United
States road tagging) says to use the state's two-letter postal code
(optionally with a US: prefix).  In practice, usage varies wildly,
generally based on how individual states prefer to represent their routes:
in states like Maryland that use their postal code, usage is pretty
uniform; in states like Ohio that use a "SR" prefix, usage is mixed
between local customs and the postal code standard.

A further complication is the presence of county roads.  The wiki doesn't
mention any standard for those.  From what I've seen, they mostly end up
as "CR" or whatever the local nomenclature is.

Should we use the postal code everywhere for nationwide consistency or
should we use the prefixes that locals use?  If we use postal codes, what
should we do about county or town roads?

== "No Prefix" ==

Some roads only have their number in the ref= tag.  I think a standard of
"don't do that; always put network information in the ref= tag, too"
should be uncontroversial, especially since the route relations should
have the network and number information separated for any data consumers
that want that.

== "Semi-Colons" ==

He shows a road that has a ref= of "I 88;56".  I think it should be
completely uncontroversial to say that each part of a semicolon-delimited
ref should have the appropriate network information in it.

== "Colons" ==

He shows a road tagged "US:OH 18".  This is really about what to use for
state prefixes, which is discussed above.

== "Parentheses" ==

Hw shows a road tagged "(36)".  This is also about prefixes (or not) for
state roads and should be discussed above.

== "Acronym Markers" ==

I haven't seen this around me, but apparently there are roads that use the
initials of the road's name as a ref=.  Is this in keeping with the other
uses of ref=, i.e. that the road is a member of a particular network and
this is the road's designation within that network?  If not, if it's just
the initials of a major road, my opinion is that it probably shouldn't
have a ref= tag.

== "Some Interstates Show Exits—Others Don't" ==

This is really just a problem with map coverage, not tagging convention,
but I'd like to ask about consensus on name= and ref= tags for
motorway_junctions.  ref= is pretty obviously the exit number, but
although some wiki pages (Interstate Highways, in particular) say or imply
that everything on the exit sign should go into the name= tag (including
the junction road but also further destinations like towns and distant
roads).  I think it makes more sense to just have the junctioned road (or
really significant destination road, like when the junctioned road is
almost always just a means to get to another major road) in the name= tag
and use the destination sign relation for the other information.

== "Coming Up" ==

There are more posts coming, but he mentions "City Boundaries" which made
me think about the TIGER CDPs.  Is it worthwhile to keep administrative
boundaries based on TIGER CDPs?  They're usually used for areas with
significant population density but no incorporated towns, so at best they
represent a very fuzzy idea of where a particular town's borders are and
at worst extend a town's boundaries well beyond what any residents would
consider appropriate.

I'd really like to know what other people think about these topics.  If
there's good consensus on them, I'd like to update the wiki to match the
consensus (and possibly clean up incorrectly-tagged things so that the
existing data will serve as a good example for new mappers).

...computer contrarian of the first order... / http://aperiodic.net/phil/
PGP: 026A27F2  print: D200 5BDB FC4B B24A 9248  9F7A 4322 2D22 026A 27F2
--- --
    <posix> this guy _is_ crazy
<stargazer> posix: from the looks of Enlightenment he's on LSD
    <posix> LSD is nothing compared to what this guy's on..
                       -- Seen on #unix
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