[OSM-talk] Metric / imperial bridge heights

[Richard Bullock]

> Mappers should be mapping what it is they find. If I find an 11'3"
>> clearance bridge with a 20mph limit beneath it then that is what I
>> want to map.
>
> Nobody is suggesting you shouldn't do that. I'll certainly express the
> view that when I drive under that bridge, my km/h speedometer and lack
> of feet and inches reckoning skills will mean that I'll want that
> translated into real money, but this is going to be possible wherever
> you choose to store this information. What I'm saying is that when we
> have tags that are documented as containing simple numbers interpreted
> as being in a particular unit, that you should either convert your
> data into that format or choose another tag where your preferred way
> of using it doesn't break with the already documented behaviour.

With speed limits - there is an exact conversion factor. 1 mph = 1.609344 
km/h exactly. It's not massively difficult to imput data in to OSM in km/h - 
just multiply the mph limit by 1.609344.

In the UK with bridge heights there isn't an exact conversion factor - 
mainly because a signed 11'3" bridge isn't 11'3" high. To get the signed 
height - you subtract 3 inches from the true height then round down to the 
next 3 inches. There will always be between 3 and 6 inches leeway.

When a UK bridge is signed in metric as well, you don't convert the imperial 
measure. You subtract 0.08m from the correct height measured in metres - and 
then round down to 1 decimal place. Thus the actual leeway will be between 
8cm and 18cm.

The regulations say that bridge heights must be reviewed every time the road 
is resurfaced or similar works occur.

This can lead to two bridges being signed the same in metric - but different 
in imperial - or vice versa. E.g. 
http://img204.imageshack.us/my.php?image=img7896am4.jpg