[Tagging] Definition of a Beach

[Christoph Hormann]

On Thursday 15 August 2019, Warin wrote:
> What is a beach?
>
> [...]

The question you actually wanted to ask i think is what does 
natural=beach mean in OSM.  This distinction between the meaning of a 
tag in OSM and the meaning of the terms used for key and value in 
English language is very important to keep in mind, in particular for 
native English speakers.

I had a thorough look at use of natural=beach in OSM back when i changed 
rendering in OSM-Carto and came to the conclusion that use of this is 
actually reasonably close to the core scientific definition of beaches, 
namely a wave formed accumulation of loose material at the shore of a 
waterbody.

See also

http://blog.imagico.de/reefs-and-beaches-in-the-openstreetmap-standard-style/

There are a number of notable exceptions from this

* natural=beach is also used for human made artificial beaches where 
sand does not occur naturally.  This is obvious since this is often 
hard to distinguish for the casual observer without in depth research.
* some use of natural=beach for rocky shores exists but it is minimal.
* sometimes use of natural=beach extents on costal dunes which are not 
water formed and therefore not part of the actual beach.
* in particular in the UK there is some atypical use of the tag - based 
on historic practice and use of OS data as a source apparently - of 
using natural=beach for what is indicated as 'Sand' in OS maps and 
wetland=tidalflat (or historically natural=mud) for what OS maps show 
as 'Mud'.  This is distinctly different from elsewhere in particular 
since it uses natural=beach for sand based tidal flats - like here

https://www.openstreetmap.org/way/67573161

How can you identify a beach on the ground:

* there are waves breaking, at least at some times, at the water line.
* ground has a significant slope so waves roll up the beach and water 
flows back in the typical fashion leaving a fairly smooth surface.
* the ground material grain size is somewhere between fine sand and 
medium sized stones - small enough to be moved by the waves when they 
are strong and large enough not to be suspended in and carried away by 
the water as the waves break.
* there are no tidal channels forming in the loose material since these 
are indicative of a tide dominated situation and not a wave dominated 
one - such cases would be suited to map with natural=wetland + 
wetland=tidalflat.

Where there is considerable variation is if and how the tidal part of a 
beach is mapped.  Mainly the following variants exist:

* mapping only the part of the beach above the high water line leading 
to a very narrow beach.
* the tidal part of the beach being mapped as or included in a tidal 
flat.
* the beach crossing the coastline and including the tidal part.
* the coastline being placed not at the high water mark but below, 
usually whereever the imagery used shows the water edge and ending the 
beach at this line.

This would be good to clear up and establish a well defined and 
intuitively usable mapping scheme.

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