[OSM-talk] Misclassified roads
richard at systemeD.net
Mon Jul 14 13:33:14 BST 2008
Steve Hill wrote:
> If you don't go by the definitions in Map Features, what definitions do
> you go by? As far as you are concerned, what is the difference between an
> unclassified and a tertiary? If we don't have some agreed definition, the
> tags become meaningless since the meaning will vary widely depending on
> who surveyed the road.
> For example, if someone is writing a route planner for HGVs, it is wise to
> have it try and avoid unclassified roads as they are defined by the wiki.
> But it is not sensible to avoid a high quality dual carriageway (which
> seems to match some other people's definitions of an unclassified road).
I think the first thing to establish is that the word "unclassified",
in itself, doesn't particularly mean anything; nor does "tertiary".
(Just as "primary" in OSM-speak actually means "non-primary" UK
roads!) They're words, nothing more. You could just as easily call
them highway=level1 [motorway], level2 [trunk], etc.
I posted earlier in the thread that I'd define a tertiary road as
something like "significant through route for non-local use, other
than an A/B/M road". It's a descending scale of localness:
highway=motorway is the road that you use to get from one end of the
country to another, highway=residential is the one that delivers you
right to your house but nowhere else, and the rest are stages in
between. That's how I understand our highway tagging system.
As it happens, the UK (and several other countries) have a road
classification system that does exactly the same. It makes sense to
align OSM's definitions with theirs where possible, so we do. But in
the UK this doesn't work for anything below "secondary".
So to reiterate: I'd use highway=tertiary for a useful, good-quality
route that might form part of a wider journey; I'd use
highway=unclassified for one predominantly intended for local use.
That dual carriageway off the A4067 is an interesting example - should
it be tertiary (clearly very good quality) or unclassified (it doesn't
really go anywhere apart from the industrial estate)? Both would seem
to have merit.
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