Is there any historical basis to China's claim to sovereignty over waters within the the nine dash line, other than that the line appeared (first as an eleven dash line) on a map issued by China in 1947, based apparently on an earlier map from 1935? What is the historical basis for the claims underpinning the 1947 map?
The 2016 arbitration tribunal ruling declared China's claim to be invalid because it had never exercised sole sovereign control over the waters in question. So as far as can be ascertained by the external observer, the sole basis for the claim is that at some point in the early 20th century, China decided that it should be so. Is there anything historically substantive justifying the line?