[Talk-GB] UK coastline data

Tony Shield tony.shield999 at gmail.com
Sat Jul 13 19:33:28 UTC 2019


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 <maps at fortyfivekev.co.uk 
>> <mailto: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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