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

[Casey_boy]

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).