High and low water marks vary every day, the height of the tides vary a lot..  
Tidal ranges in the south west of England can be about 13 metres in height.   
Most measurements are made using Mean Sea Level, which doesn't change (rising 
sea levels aside).  When you look at the Yahoo images, how do you know what 
state the tide is?
 
cheers, Chris
Steve Hill wrote:
> I've also raised a question on the natural=coastline talk page about what 
> the coastline actually denotes - I can't see anything saying whether it is 
> the high water or low water line (I'd assume it's the low water line - 
> i.e. everything to the right of it is always sea, no matter what the state 
> of the tide, whilst stuff to the left of it is assumed to be land 
> (tagged as water=tidal if it is flooded at high tide).
> 
> 




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