[Talk-GB] UK coastline data

Colin Smale colin.smale at xs4all.nl
Sat Jul 13 15:04:55 UTC 2019

On 2019-07-13 13:35, Borbus wrote:

> On Fri, Jul 12, 2019 at 9:11 PM Devonshire <maps at fortyfivekev.co.uk> 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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