[Tagging] units and notations for depth

[Colin Smale]

On 2019-02-19 11:11, Tony Shield wrote:

> Depth of water in tidal areas can vary enormously. People using depth for navigation and general use would expect to use depth at MLWS (Mean Low Water Springs) and add increments based on tide tables, I suggest that OSM does the same. Use the tidal tag and perhaps a tidal-range tag, in metres, tidal_range is the difference between MHWS (Mean High Water Springs) and MLWS. 
> 
> note - Spring tides are the highest and lowest forecast tides and are based on the relative position of the earth, moon and sun.

Indeed, they are astronomical predictions, but not forecasts. They are
not in themselves good enough for planning a (route to a) berth in a
harbour for example. There are many other factors influencing the actual
water level, like global/local weather conditions and seiche waves.
Depth is of course also affected by dredging operations... 

Should OSM aim to replicate the Admiralty Charts and tide tables?
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