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<p>Hi</p>
<p>Personally think that High Water Mark and Low Water Mark are very
relevant to people and to OSM.</p>
<p>Yeah - tides are a nuisance and can never be predicted with total
accuracy and with Global Warming HWM and LWM will change over
time. Then there are Highest and Lowest Astronomical Tides, and
then tides which increase or decrease according to weather
conditions (pressure and wind) (New Orleans tonight is a good
example). There are probably a few others which I have
forgotten....<br>
</p>
<p>Knowing the inter-tidal area at Hunstanton is important, as are
those in Morecambe Bay and the River Dee(North Wales/England)
where paths cross the area. <br>
</p>
<p>How many beaches are there on the Thames? and what is the
inter-tidal ground like - sand, shingle, mud . . . .And what and
where is the access? These questions are what OSM is about. <br>
</p>
<p>The OS recognises this and on their maps marks the coastline/MHW
with a dense line, but not on non-tidal waters. <br>
</p>
<p>OSM needs the equivalent of MLW - as far as I know its not
defined (and I do not feel competent to define) - and I think that
Borbus is on the good path.<br>
</p>
<div class="moz-cite-prefix">On 13/07/2019 16:04, Colin Smale wrote:<br>
</div>
<blockquote type="cite"
cite="mid:666e8a7d3d37b6bde9f14a192fa14a26@xs4all.nl">
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<p>On 2019-07-13 13:35, Borbus wrote:</p>
<blockquote type="cite" style="padding: 0 0.4em; border-left:
#1010ff 2px solid; margin: 0">
<div dir="ltr">On Fri, Jul 12, 2019 at 9:11 PM Devonshire <<a
href="mailto:maps@fortyfivekev.co.uk" moz-do-not-send="true">maps@fortyfivekev.co.uk</a>>
wrote:<br>
> Just because the coastline follows MLW as it goes around
the coast<br>
> doesn't mean it needs to follow every tidal waterway
inland. That<br>
> doesn't follow at all.<br>
<br>
Why not? What is the meaning of "coastline"?<br>
<br>
The Dart is one example of where it seems obvious where to
"draw the<br>
line" by taking a cursory glance at aerial imagery, but does
this line<br>
have any bearing on reality?<br>
<br>
My feeling is that the natural=coastline tag is a misnomer and
it should<br>
really just be called "mean_high_water_level" or<br>
"mean_high_water_spring" (I'm still unsure about whether OS
show MHWL or<br>
MHWS, I thought it was MHWL, which is between mean high water
spring and<br>
mean high water neap).</div>
<div dir="ltr"> </div>
</blockquote>
<div dir="ltr">The data included with Boundary-Line would appear
to be mean high water (springs) according to the User Guide and
Technical Specification, although in some places it is referred
to as the High Water Mark and High Water Line.</div>
<div dir="ltr"> </div>
<blockquote type="cite" style="padding: 0 0.4em; border-left:
#1010ff 2px solid; margin: 0">
<div dir="ltr">Is there a meaning to "coastline" that makes it
distinct from any other<br>
high water level that can't be expressed with other tags?
(Other tags<br>
could be water salinity, presence of beaches, dunes, cliffs
etc. that<br>
are real physical features).</div>
<div dir="ltr"> </div>
</blockquote>
<div dir="ltr">Salinity is too variable to be useful. My vote is
to stick to MHWS, or whatever the prevailing law states as the
edge of the land.</div>
<div dir="ltr"> </div>
<div dir="ltr">How about creating an OSM tidal prediction model?
Then we could take all the WGS84 elevations that are near the
coast in OSM, and make our own model, and make it open source.
How hard can it be? (PS I know exactly how hard it would be, but
it would be a typical OSM attitude to reject existing standards
and roll our own)</div>
<div dir="ltr"> </div>
<div dir="ltr">Just for completeness, even MHWS is not the limit
of where the water comes to. It's a mean value, averaged over a
long period; statistically, half the high tides at spring tide
will encroach further landward than MHWS. Every tide is
different. But you have to draw the line somewhere.</div>
<div dir="ltr"> </div>
<div dir="ltr">When is our coastline fit for purpose? It seems to
be a rendering hint, to colour one side of the line "blue" and
the other side various colours. Do we need a rendering hint to
separate the sea from an estuary? It might also be said to form
a useful polygon to allow the dry bits of the world to be
excised from the global database in a convenient way. What do we
want here?</div>
<div dir="ltr"> </div>
<div dir="ltr"> </div>
<div dir="ltr"> </div>
<br>
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