The book is not easy to understand or decipher, as it uses archaic words and ancient expressions for directions. But once the "code" is cracked, its accuracy is unquestionable.

Fu Shibao, an officer at the border police station in Tanmen, said there used to be at least 1,000 such books in the seafaring town.

"Now there are only about a dozen books left," said Fu, who has been helping Hainan University to study and protect the remaining books. He said the true value of the books was realized after friction between China and the Philippines over Huangyan Island escalated sharply in 2012.

Zhou, who has studied genglubu for 26 years with his wife, said that a book provided by former sea captain Peng Zhengkai records 17 routes to the Xisha Islands and more than 200 routes to the Nansha Islands. This book also contains many other details, including the weather conditions and ocean currents for every month.

Fishermen in ancient times named 136 islands and reefs in the books for China, much earlier than they were named by other countries, Zhou said.

Many of the names are still used, and nine of them have become official names in English. The Paracel Islands, for example, are named from a Portuguese word meaning "stone reef", which originally comes from the Chinese.

There are 287 named locations in the South China Sea. Zhou said the ancient Chinese did not name the others because many of them are submerged or partly submerged banks.

Genglubu date to the Ming Dynasty (1368-1644), he said, adding, "We are the earliest owners of the South China Sea."

He said the routes, along with China's historical documents, archaeological discoveries and accounts from old seafarers and fishermen show that "waters around the Dongsha, Xisha, Zhongsha and Nansha islands had become fixed fishing grounds for Chinese fishermen in the Ming Dynasty".