[Tagging] How to tag flood prone points and areas?

[Paul Allen]

On Sun, 1 Sep 2019 at 05:24, Warin <61sundowner at gmail.com> wrote:

You could add flood_prone=yes to the car park tag but that will show the
> whole car park as affected, whereas it's only the bit down this end that
> has a problem. Would drawing a separate area & marking that as
> flood_prone=yes work?
>

Better than nothing.  If you feel adventurous, you could try mapping it as
two, non-overlapping,
constituent areas of a multipolygon and see what happens.

> I asked this question some time ago. I was told it was not verifiable and
> therefore not for OSM.
>

My opinion is that if there is signage/road markings it's verifiable and
mappable.  When we
map the speed limit of a road from signs the only actual, verifiable
information we have is
the presence of the sign, but we assume the sign is true and infer the
speed limit of the
road from it.  Same thing here: sign says it's prone to floods so we infer
the place is prone to
floods.

Where I differ from some is that I'd consider official documents also
providing verifiability
provided their copyright permits it.

However there is the question of frequency, once in 10 year event, once in
> 100 etc. So I would add a sub tag or value about frequency of the event..
> The key frequency is already in use. Period has some use too, though the
> use looks to be years.. no wiki to say what it is?
>

Period is the multiplicative inverse of frequency: normalize the units,
multiply them together
and the result should be 1.  Neither is appropriate in this case.  A
once-in-100-year event
does not occur at 100 year intervals, it has a probability of  1% of
occurring (technically,
being equalled or exceeded) in any given year.
https://en.wikipedia.org/wiki/100-year_flood
So we should be tagging a probability.  Technically, exceedance probability
for floods.

Taginfo shows floodplain_probability used 77 times.  Is that sensible?
It's a floodplain or it isn't.
Also flood_probability 4 times (better) and hazard:probability once.  The
flood_probability value
in taginfo is "100y" rather than 1%.  People who used
floodplain_probability divide into those
who expressed a large number like 100 (probably meaning years) and those
who expressed
a small number like 1 or 0.5 (probably a percentage).  The only value for
hazard:probability
is "low" (which I consider to be effectively meaningless).

I dislike floodplain_probability because it IS a floodplain with a
probability of being
flooded, not a probability of an area being classified as a floodplain.
Also because
it's been given both in terms of years and percentages (except it's
impossible to be sure
because nobody has given units, so maybe the 100 means it's 100% likely to
flood and
the 0.5 means it is likely to flood every six months).  It's a mess.

I'm fairly happy with flood_probability.  There's something nagging at the
back of my
mind saying I ought to be unhappy with flood_probability, but it's not
telling me why.

I like hazard:probability, especially if we document that it should be
tagged as a
percentage (and ignore or fix the sole value of "low").  Only problem with
it is that
hazard=* is a proposal from 2007 that is supposedly still active, so we'd
have
to do something about hazard=*.  Then again there is hazard_prone=* and
hazard_type=* which seem to have appeared in the wiki without a proposal
and have a few thousand uses.

-- 
Paul
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