Hi
Personally think that High Water Mark and Low Water Mark are very
relevant to people and to OSM.
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....
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.
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.
The OS recognises this and on their maps marks the coastline/MHW with a
dense line, but not on non-tidal waters.
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.
On 13/07/2019 16:04, Colin Smale wrote:
On 2019-07-13 13:35, Borbus wrote:
On Fri, Jul 12, 2019 at 9:11 PM Devonshire <[email protected]
<mailto:[email protected]>> wrote:
> Just because the coastline follows MLW as it goes around the coast
> doesn't mean it needs to follow every tidal waterway inland. That
> doesn't follow at all.
Why not? What is the meaning of "coastline"?
The Dart is one example of where it seems obvious where to "draw the
line" by taking a cursory glance at aerial imagery, but does this line
have any bearing on reality?
My feeling is that the natural=coastline tag is a misnomer and it should
really just be called "mean_high_water_level" or
"mean_high_water_spring" (I'm still unsure about whether OS show MHWL or
MHWS, I thought it was MHWL, which is between mean high water spring and
mean high water neap).
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.
Is there a meaning to "coastline" that makes it distinct from any other
high water level that can't be expressed with other tags? (Other tags
could be water salinity, presence of beaches, dunes, cliffs etc. that
are real physical features).
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.
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)
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.
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?
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