[Tagging] units and notations for depth

[Colin Smale]

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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