Re: [OSM-talk] Metric / imperial bridge heights
In your letter dated Sat, 11 Oct 2008 12:57:18 +0100 you wrote: 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. Isn't that just conservative engineering? You make sure that any verhicle that is at most the posted height can pass safely, and when a verhicle does hit the ceiling, you know that it was not a just a tolerance issue. There is no need to make use of that information unless you actually have to (if there is no other way of reaching a destination) I guess that for routing you want to take the higher imperial heigh into account and put the metric value in a comment section. Do people actually enter 11'3 in a consistent way when tagging heights? ___ talk mailing list talk@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk
[OSM-talk] Metric / imperial bridge heights
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 ___ talk mailing list talk@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk
