Should we be mapping both values for `maxheight` in the UK?

InsertUser · 2024-09-30T18:26:33.551Z

Apparently the rounding of the values is weird so you can’t always predict the metric value from the imperial or vice versa. Gallery here.

The wiki currently says you should pick either the metric or imperial and discard the other. I presume this means that any routing software that “previews” a sign in different units to the units in the tag could end up showing a different value to the value on the sign.

Is this worth tagging?

RedAuburn · 2024-09-30T19:36:32.596Z

IMO, we should just put the most accurate value in maxheight. It could be additionally tagged with something like maxheight:metric=4.9 maxheight:imperial=16'3" for cases where the sign should be rendered

InsertUser · 2024-09-30T19:38:25.750Z

RedAuburn:

maxspeed

eh? This is about heights under bridges etc.

Assuming that’s a typo: how do you know the most accurate when both are stated on the sign?

RedAuburn · 2024-09-30T19:39:31.194Z

Oops yeah. Well, you could measure it, but I’d go with the imperial as it’s units are smaller. The signs are meant to be authoritative afaik

Casey_boy · 2024-10-01T08:23:41.117Z

According to the Traffic Signs Manual, the imperial and metric values are calculated* independently of each other.

*I say calculated, not measured, as there is a specific process on how the measured values are presented on the sign:

Imperial height
The imperial figure shown on signs to indicate the available headroom should be at least
3 inches less than the measured height to allow a safety margin. If the resulting figure is not a
multiple of 3 inches, it should be rounded down to the nearest lower multiple of 3 inches.

Metric height
To obtain the metric figure shown on signs, the bridge height should be measured to two
decimal places, rounding down to the nearest 0.01 m. The following method is then used to
calculate the appropriate signed height:

if the second decimal digit is 8 or 9, delete it and sign the bridge with the remaining whole number and the first decimal digit;

if the second decimal digit is 7 or less, delete it and reduce the first decimal digit by 1. Sign the bridge with the remaining whole number and first decimal digit, as reduced.

For both imperial and metric:

The height shown on the sign must be to only one decimal place. The maximum
headroom that will normally appear on a sign is 4.9 m

So performing a conversion between units is pretty much always going to result in a different value than what has been signed.

TLDR: the two values are distinct and, technically, shouldn’t simply be converted (so mapping both might be useful). But in practice the result is likely to only be a few centimetres difference (so perhaps less useful).

mueschel · 2024-10-01T10:45:49.230Z

From the regulations the imperial restriction is 3" to 6" (7.5 cm to 15 cm) less than the actual height, the metric one is 8 cm to 17 cm less.
Which value is lower depends on the actual height and the interpretation of “at least” in the imperial regulation.

We don’t have metric or imperial cars, so you can always argue that it fits if your height is lower than at least one of the signed values. Hence the ‘maxheight’ tag, which describes the largest allowed size, should contain the larger of the two values. (If the actual interpretation of the law is that you need to be smaller than any of the signed values, then tag the lower one)

The precise values on the sign can be added using the usual ‘traffic_sign’ tag.

Andrew_Chadwick · 2024-10-01T12:01:50.344Z

Given what a maximum height is for, tag with the lowest of the two measurements.

Whatever weird rounding the authorities and builders (are supposed to) do to arrive at the 8-to-17 cm or 3-to-6 inch lower figures you see on the signs doesn’t matter. Convert, pick the lowest declared figure, and discard the other. I favour metric.

It’s nice to leave a maxheight:note= with the actual sign’s wording, if a structure has both. Not all will.

InsertUser · 2024-10-01T13:34:21.157Z

Casey_boy:

TLDR: the two values are distinct and, technically, shouldn’t simply be converted (so mapping both might be useful). But in practice the result is likely to only be a few centimetres difference (so perhaps less useful).

The first link in the first post has a plot of actual vs signed height for both metric and imperial. Which one rounds down the most changes back and forth.

mueschel · 2024-10-01T16:12:18.366Z

Andrew_Chadwick:

Given what a maximum height is for, tag with the lowest of the two measurements.

Why the lower one? For tags like ‘maxheight:physical’ I fully agree to tag the lower one (or the actual measurement).

‘maxheight’ doesn’t describe the height of an object, but the legal lower limit before you can be fined (either physically or financially). If you’re between the two numbers, you are still low enough to physically pass through, and you can dispute a fine by using the favorable system of units.

Andrew_Chadwick:

It’s nice to leave a maxheight:note= with the actual sign’s wording

I’d rather go for the proper tag for traffic signs. That would be something like this one from our database, which if fully machine readable and therefore might be shown by any software supporting traffic signs:
traffic_sign=GB:629.2A[4.5m,14’10"]

Andrew_Chadwick · 2024-10-01T20:48:09.100Z

mueschel:

Why the lower one? For tags like ‘maxheight:physical’ I fully agree to tag the lower one (or the actual measurement).

‘maxheight’ doesn’t describe the height of an object, but the legal lower limit before you can be fined (either physically or financially). If you’re between the two numbers, you are still low enough to physically pass through, and you can dispute a fine by using the favorable system of units.

Thankyou for restating something I am perfectly aware of.

OSM globally should not be interested in providing vagueness for legal disputes. This project is best-effort data trying to capture something that is verifiable on the ground, in this case, two numbers written on a sign for maxheight:legal.

By analogy, where you are expressing multiple legal restrictions as a tag, tag the smallest legally restrictive value into maxheight:legal. Do this just as you would tag the most physically restrictive - smallest - height you have personally surveyed into maxheight:physical.

Then when that’s done, put the smallest of maxheight:physical and maxheight:legal into maxheight, verbatim or converted.

Note that if it is desirable to establish a difference between a legal and a physical maxheight, this [maxheight] should be the lesser of the two.
– https://wiki.openstreetmap.org/wiki/Key:maxheight

mueschel:

I’d rather go for the proper tag for traffic signs. That would be something like this one from our database, which if fully machine readable and therefore might be shown by any software supporting traffic signs:
traffic_sign=GB:629.2A[4.5m,14’10"]

Fine if this can be done with specific editor support allowing people to gather the info very conveniently, with particular reference to StreetComplete and SCEE. Otherwise, a note suffices for all but pedants, and can be worked on later by people in the know about traffic sign things.

Absurd to express the same quantity twice in a functional tag that addresses one single concern, simply and machine-readably, unless you are (somehow) recording exactly what the sign says in a tag dedicated to that concern.