[Talk-us] Maxweight in the USA

Andy Townsend ajt1047 at gmail.com
Mon Nov 2 21:59:09 UTC 2015

Just a heads up...

There's a bit of a discussion going on at the moment as to whether it 
makes sense to store SI units (or actually a derivative - metric tons) 
in maxweight tags.  I noticed a few changes (initially to other values 
in the UK), and commented on 
https://www.openstreetmap.org/changeset/35009662 , and the person making 
a changes (who's the author of one of the popular routers using OSM 
data) wrote a diary entry here: 
https://www.openstreetmap.org/user/karussell/diary/36220 .

The argument in favour of the change is that storing an SI derivative 
makes the data easier to consume; my counter-arguments are that (a) it 
makes it harder for mappers to verify values and (b) anything consuming 
data shouldn't assume the data is valid anyway (for "Bobby Tables" 
reasons if for no other).

Whilst doing this I noticed that a bunch of other "x tons" weight limits 
had had values changed a while back (see for example 
http://www.openstreetmap.org/way/32719427/history ).  That's now been 
changed to "maxweight=4.5359237" which is at least not heavier than the 
actual posted restriction.  However there are still some other integer 
values without units which implies metric tons (see for example 
http://overpass-turbo.eu/s/cqw ).  It may be that Pittsburgh has woken 
up one morning and decided to adopt SI units ahead of the rest of the 
country, but I doubt it.  Logically I'd expect a router encountering 
"maxweight=10" in the USA might want to interpret it as "10 US tons" 
rather than 10,000 kg, but based on the above I suspect that at least 
one router isn't going to do that.

The relevant wiki page http://wiki.openstreetmap.org/wiki/Key:maxweight 
does say "as of September 2014 only metric units of weight (metric 
tonnes or kilograms) are supported for this tag".  I'm unaware of any 
discussion prior to the 17 September 2014 change (not that that means 
that it didn't happen, just that I'm unaware of it).

I'm not from the US, and I'm not sure what the right answer is (if as a 
community you're happy entering maxweight=4.5359237 it'd certainly make 
everyone's lives easier), so I'm posting this here and then retiring 
back across the Atlantic :)


Andy (SomeoneElse)

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