[Tagging] units and notations for depth

[Tony Shield]

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.

TonyS

On 19/02/2019 08:01, Colin Smale wrote:
>
> On 2019-02-19 04:03, Graeme Fitzpatrick wrote:
>
>> Not opposition, but for tidal areas, isn't this going to have the 
>> same problems of whether you mark the "coastline" at the high- or 
>> low-tide line?
> Coastline is MHW, that is settled isn't it?
> Water heights are more problematic though, because there are different 
> datums across the world. AFAIK there is no global default datum apart 
> from WGS84 geoid height which no mariners ever use directly. All water 
> levels/heights therefore need to be related to a datum.
> Depth however is local and is just the difference in height between 
> the water level and the bottom. Can you even have a negative depth? 
> Floating above the water?
>
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