[Tagging] How to tag flood prone points and areas?
Warin
61sundowner at gmail.com
Sun Sep 1 22:57:31 UTC 2019
On 1/9/19 10:31 pm, François Lacombe wrote:
> Hi all,
>
> flood_prone=yes doesn't sound to be good semantics.
Think those are the words used on signs, so for English areas they make
sense..
> Should we rewrite it as floodable=* with 3 or four big level of
> probability (or causes, or whatever) instead?
I don't think floodable is right .. to me it says that a feature has a
tolerance for flooding, once the flood is past the feature is ok. Where
as flood_prone says the feature may be under water (flooded) from time
to time. After the flood flood_prone says nothing about the recovery of
the feature from the flooding.
>
> Many people raised concerns about yes/no tags and the key name seem to
> contain two distinct information (floodable + probability) while the
> value meaning could be improved.
>
> Furthermore, such work can be useful for many hazard description.
> This proposal is interesting :
> https://wiki.openstreetmap.org/wiki/Proposed_features/hazard
>
> Floods can also occur on river banks surroundings when hydropower is
> in operation upstream
> https://lists.openstreetmap.org/pipermail/tagging/2018-July/037973.html
> Here is what is often displayed : https://imgur.com/a/TLhZcgE
>
> All the best
>
> François
>
> Le dim. 1 sept. 2019 à 14:07, Joseph Eisenberg
> <joseph.eisenberg at gmail.com <mailto:joseph.eisenberg at gmail.com>> a écrit :
>
> For `flood_probability` to be useful and verifiable in some way, there
> should be a link to the source in the changeset, or perhaps also
> source: flood_probability= on the object.
>
> Such statistical features like "1% risk of flood per year" can't
> really be verified by individual mappers, since they are based on
> calculations of the floodplain geometry and historical observations of
> floods over many decades. So it's probably more useful to have these
> mapped in official sources which are kept up-to-date, rather than
> importing the data from the external source into OSM, and then having
> to maintain it in our database.
>
> I agree that if there is a sign that says "this area prone to
> flooding", then "flood_prone=yes" is verifiable and helpful to add,
> since that's representing a feature that can be checked when the area
> is next survey.
>
> On 9/1/19, Paul Allen <pla16021 at gmail.com
> <mailto:pla16021 at gmail.com>> wrote:
> > On Sun, 1 Sep 2019 at 05:24, Warin <61sundowner at gmail.com
> <mailto: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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