Official maps show the roads, the power stations, the lighthouse — but skip all the special spots where generations of Indian fishermen have moored boats or dried their silvery catch, prayed for the living and buried their dead.

So now the fishermen of southern India are learning a whole new skill: mapping their ancient land by app — plot by precious plot — to stop industry from swallowing life on the lagoon.

"I am just a fisherman. I knew nothing about mapping. But I had to learn this so that our livelihoods are not destroyed tomorrow by the government," said Bharath S. Raji, whose family has trawled Pulicat Lake in Tamil Nadu for generations.