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<div class="moz-cite-prefix">On 1/9/19 10:31 pm, François Lacombe
wrote:<br>
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<div>Hi all,</div>
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<div>flood_prone=yes doesn't sound to be good semantics.</div>
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<p>Think those are the words used on signs, so for English areas
they make sense.. <br>
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<div>Should we rewrite it as floodable=* with 3 or four big
level of probability (or causes, or whatever) instead?</div>
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<p>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. <br>
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cite="mid:CAG0ygLc6Rj4gMDxJn8e4ORn-ZY+Rq2fYdSdESP6HhRuMLszPuw@mail.gmail.com">
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<div><br>
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<div>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.<br>
</div>
<div><br>
</div>
<div>Furthermore, such work can be useful for many hazard
description.</div>
<div>This proposal is interesting : <a
href="https://wiki.openstreetmap.org/wiki/Proposed_features/hazard"
target="_blank" moz-do-not-send="true">https://wiki.openstreetmap.org/wiki/Proposed_features/hazard</a></div>
<div><br>
</div>
<div>Floods can also occur on river banks surroundings when
hydropower is in operation upstream</div>
<div><a
href="https://lists.openstreetmap.org/pipermail/tagging/2018-July/037973.html"
moz-do-not-send="true">https://lists.openstreetmap.org/pipermail/tagging/2018-July/037973.html</a></div>
<div>Here is what is often displayed : <a
href="https://imgur.com/a/TLhZcgE" moz-do-not-send="true">https://imgur.com/a/TLhZcgE</a></div>
<div><br>
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<div>All the best</div>
<div><br>
</div>
<div>François<br>
</div>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">Le dim. 1 sept. 2019
à 14:07, Joseph Eisenberg <<a
href="mailto:joseph.eisenberg@gmail.com" target="_blank"
moz-do-not-send="true">joseph.eisenberg@gmail.com</a>>
a écrit :<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left:1px solid
rgb(204,204,204);padding-left:1ex">For `flood_probability`
to be useful and verifiable in some way, there<br>
should be a link to the source in the changeset, or perhaps
also<br>
source: flood_probability= on the object.<br>
<br>
Such statistical features like "1% risk of flood per year"
can't<br>
really be verified by individual mappers, since they are
based on<br>
calculations of the floodplain geometry and historical
observations of<br>
floods over many decades. So it's probably more useful to
have these<br>
mapped in official sources which are kept up-to-date, rather
than<br>
importing the data from the external source into OSM, and
then having<br>
to maintain it in our database.<br>
<br>
I agree that if there is a sign that says "this area prone
to<br>
flooding", then "flood_prone=yes" is verifiable and helpful
to add,<br>
since that's representing a feature that can be checked when
the area<br>
is next survey.<br>
<br>
On 9/1/19, Paul Allen <<a
href="mailto:pla16021@gmail.com" target="_blank"
moz-do-not-send="true">pla16021@gmail.com</a>> wrote:<br>
> On Sun, 1 Sep 2019 at 05:24, Warin <<a
href="mailto:61sundowner@gmail.com" target="_blank"
moz-do-not-send="true">61sundowner@gmail.com</a>>
wrote:<br>
><br>
> You could add flood_prone=yes to the car park tag but
that will show the<br>
>> whole car park as affected, whereas it's only the
bit down this end that<br>
>> has a problem. Would drawing a separate area &
marking that as<br>
>> flood_prone=yes work?<br>
>><br>
><br>
> Better than nothing. If you feel adventurous, you
could try mapping it as<br>
> two, non-overlapping,<br>
> constituent areas of a multipolygon and see what
happens.<br>
><br>
>> I asked this question some time ago. I was told it
was not verifiable and<br>
>> therefore not for OSM.<br>
>><br>
><br>
> My opinion is that if there is signage/road markings
it's verifiable and<br>
> mappable. When we<br>
> map the speed limit of a road from signs the only
actual, verifiable<br>
> information we have is<br>
> the presence of the sign, but we assume the sign is
true and infer the<br>
> speed limit of the<br>
> road from it. Same thing here: sign says it's prone to
floods so we infer<br>
> the place is prone to<br>
> floods.<br>
><br>
> Where I differ from some is that I'd consider official
documents also<br>
> providing verifiability<br>
> provided their copyright permits it.<br>
><br>
> However there is the question of frequency, once in 10
year event, once in<br>
>> 100 etc. So I would add a sub tag or value about
frequency of the event..<br>
>> The key frequency is already in use. Period has
some use too, though the<br>
>> use looks to be years.. no wiki to say what it is?<br>
>><br>
><br>
> Period is the multiplicative inverse of frequency:
normalize the units,<br>
> multiply them together<br>
> and the result should be 1. Neither is appropriate in
this case. A<br>
> once-in-100-year event<br>
> does not occur at 100 year intervals, it has a
probability of 1% of<br>
> occurring (technically,<br>
> being equalled or exceeded) in any given year.<br>
> <a href="https://en.wikipedia.org/wiki/100-year_flood"
rel="noreferrer" target="_blank" moz-do-not-send="true">https://en.wikipedia.org/wiki/100-year_flood</a><br>
> So we should be tagging a probability. Technically,
exceedance probability<br>
> for floods.<br>
><br>
> Taginfo shows floodplain_probability used 77 times. Is
that sensible?<br>
> It's a floodplain or it isn't.<br>
> Also flood_probability 4 times (better) and
hazard:probability once. The<br>
> flood_probability value<br>
> in taginfo is "100y" rather than 1%. People who used<br>
> floodplain_probability divide into those<br>
> who expressed a large number like 100 (probably meaning
years) and those<br>
> who expressed<br>
> a small number like 1 or 0.5 (probably a percentage).
The only value for<br>
> hazard:probability<br>
> is "low" (which I consider to be effectively
meaningless).<br>
><br>
> I dislike floodplain_probability because it IS a
floodplain with a<br>
> probability of being<br>
> flooded, not a probability of an area being classified
as a floodplain.<br>
> Also because<br>
> it's been given both in terms of years and percentages
(except it's<br>
> impossible to be sure<br>
> because nobody has given units, so maybe the 100 means
it's 100% likely to<br>
> flood and<br>
> the 0.5 means it is likely to flood every six months).
It's a mess.<br>
><br>
> I'm fairly happy with flood_probability. There's
something nagging at the<br>
> back of my<br>
> mind saying I ought to be unhappy with
flood_probability, but it's not<br>
> telling me why.<br>
><br>
> I like hazard:probability, especially if we document
that it should be<br>
> tagged as a<br>
> percentage (and ignore or fix the sole value of
"low"). Only problem with<br>
> it is that<br>
> hazard=* is a proposal from 2007 that is supposedly
still active, so we'd<br>
> have<br>
> to do something about hazard=*. Then again there is
hazard_prone=* and<br>
> hazard_type=* which seem to have appeared in the wiki
without a proposal<br>
> and have a few thousand uses.<br>
><br>
> --<br>
> Paul<br>
><br>
<br>
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