[OSM-talk] Rocky beaches

[David Groom]

----- Original Message ----- 
From: "Cartinus" <cartinus at xs4all.nl>
To: <talk at openstreetmap.org>
Sent: Monday, April 14, 2008 10:24 PM
Subject: Re: [OSM-talk] Rocky beaches


> Lots of coastline in OSM comes from PGS data. According to
> http://wiki.openstreetmap.org/index.php/PGS_whitepaper
>
> "This new shoreline is an approximation of the High Water Line; it is NOT 
> a
> Mean High Water Line since the source data have not been tide 
> coordinated."
>
> This of course says nothing about other coastline data.
>
> Dutch topographic maps don't consider tidal flats a part of the land. It 
> is
> coloured the same colour blue as the sea, but it has lots of black dots in
> it.
>
> -- 
> m.v.g.,
> Cartinus
>

The approach I have been taking is to tag natural=coastline as an 
approximation of high water.  Any area of sand which is normally above the 
high water mark I have tagged as natural=beach.  To tag the coastline as the 
low water line would I believe in many instances give an outline which would 
not be recognisable to most people as "the coast".  As an example see the 
area to the North-East of Ryde, Isle of Wight, UK [1]. On the low-res Yahoo 
image you can see a spit of light yellow / green extending over 1 Km from 
the marked coast.  This is Ryde Sands, a sandbank which is uncovered at low 
water.  Had I used the outline of this to tag natural=coastline it would 
have resulted in the outline of the Isle of Wight including this sandbank, 
and  would not have been anything like what anyone would expect a "map" of 
the Isle of Wight to look like at low zoom levels.

I'm currently not tagging anything below the high water mark, because I 
hadn't felt there was an appropriate approved tag.  I guess what I would 
argue is natural=coastline should be tagged as the high water line (so the 
outline of the coast looks 'correct' at low zoom) and that features below 
high water be marked as a separate layer, rendered on top of the blue for 
the water at higher zoom levels.

David

 [1]  www.openstreetmap.org/edit.html?lat=50.7346&lon=-1.1201&zoom=13