[talk-au] natural=land v natural=coastline

[James Livingston]

On 06/10/2009, at 2:12 PM, John Smith wrote:
> Lake Eyre etc is so big they used natural=coastline... Although this
> comes back to the question the other day, where does the coastline
> start/end, legally speaking it cuts across bays, it doesn't go round
> them or up rivers...

I looked into this a while back and it's somewhat contentious. The  
best I could figure out (which is quite possibly wrong) is that a  
coastline ends and a river starts when there are no longer any tidal  
significant effects. It's easiest to see if you have a slope, such as  
a sandy beach. Consider the following, where at some point the high  
and low tide levels get close enough to be negligable

1   sand
2 ---- high tide ---\
3 sand / water       ---------- 6
4 ---- low tide --- /
5   water

(1) is a polygon with natural=beach;surface=sand
(2) is the way with natural=coastline
(3) is a polygon with water=tidal;surface=sand
(6) is waterway=riverbank

At the point where it changes from coastline to riverbank, you  
obviously need to have the coastline run across the river, so as to  
form closed shape. Whether you're supposed to have the riverbank do  
the same to form a closed shape, I don't know.