On Sat, 6 Jul 2019 at 12:42, Mateusz Konieczny <matkoni...@tutanota.com> wrote:
It is not feasible to do for a typical mapper to record "light level in > lux". > Sadly, however, it is the only objective way of specifying the light level. And even then, it's easy to do it wrong if you don't account for the angle of incidence. Aim the sensor at the light and you'll get a higher reading than if you lay it flat on the ground. but it is not helping with problem what would be a good threshold between > lit=yes and lit=no > on footways > In the UK, BS 5266 requires a minimum illumination of 1 lux along the centre line of escape routes. Up until 2011, BS 5266 required a minimum of 0.2 lux along the centre line of escape routes. IIRC, the 0.2 lux figure was approximately equivalent to the light of the full moon, which was deemed adequate for some work in shipyards during WW II. Certainly it's more than necessary to take an unhurried stroll along a footpath after your eyes have acclimatized to the darkness. It may be the minimum that was once considered adequate to allow evacuation from a dark building after a power cut (no time for eyes to acclimatize) but it's not the minimum needed to follow a reasonable footpath if you're not in a hurry. So it is preferable that everyone has their own definition of what is > lit=no/yes and > recommend that "in case of doubt is it lit=yes or lit=no feel free to > choose either"? > It's hard for most mappers to accurately measure. It's hard to agree on a suitable figure, because the amount of light necessary depends very much on the nature of the path (an asphalted footpath can be safely followed with far less light than is required for an unmade path over rocky terrain). OTOH, if there are lights along the path, it is clear that it is lit. If it's the sidewalk of a lit road, it's lit. In any other cases, it's probably safer to say it's unlit. -- Paul
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