note this is *salt water*
see also http://wiki.openstreetmap.org/wiki/Proposed_features/shelf
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HB: tagging as natural=coastline, which is appropriate for Chatham Island. Will have to review for mainland. (use natural=water if small and well separated from the seaward coastline) Is Sutherland Sound in Fiordland included? [no]
for the mainland it seems to be more inland waterways.
linz layer descr: "A sheet of salt water separated from the open sea by sand". So not coastline, but natural=water + saline=yes.
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RP: if it's separate from the sea, i think it should be natural=water. see the wikipedia page for more info
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HB: at least for Chatham Isl. it's connected to the sea via a small cut:
http://img.skitch.com/20100429-pugghh1gkkxx8q5whsaqsttxwm.jpg
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Uploaded for Chatham Island 29 April 2010 (HB); had to reverse all ways by hand.