[Tagging] Coastal beach definition for mapping.

[Christoph Hormann]

On Monday 02 April 2018, Warin wrote:
> The present OSM wiki defintion for beach
>
> is "Coastal beaches should be mapped down to the mean high water
> spring line (natural
> <https://wiki.openstreetmap.org/wiki/Key:natural>=coastline
> <https://wiki.openstreetmap.org/wiki/Tag:natural%3Dcoastline>)"
>
> (from https://wiki.openstreetmap.org/wiki/Tag:natural=beach)
>
> I think this is incorrect .. they should be mapped past the high
> water mark to the low water mark.

This has been a disputed subject for a long time.  In any case the wiki 
clearly does not reflect the current use of the tag here though.  The 
following situations exist frequently in reality:

a) coast at high water line, beach only above high water line.  This 
leads to very narrow beaches since the area correctly to be tagged as a 
beach is only between the regular high water line and the extreme 
(storm flood) high water line.

b) coast at high water line, beach down to lower end of beach (either 
the low water line or the transit of the beach to a tidal flat - 
sometimes, in particular in the UK, the tidal flat is also incorrectly 
tagged natural=beach).

c) coast at an intermediate water level (the level shown in whatever 
image is used), beach ends at this water level (i.e. mappers directly 
draw what they see in the image).

All variants are common, (a) in my experience is not more common than 
(b).

For clarity regarding the difference between beaches and tidal flats:  A 
beach is formed by waves, it therefore always has a significant slope 
and is rarely wider than a few hundred meters.  A tidal flat is a flat 
area exposed at low tide that is shaped by the tidal currents.

-- 
Christoph Hormann
http://www.imagico.de/