Zheng He (Chinese: Zheng He ; 1371-1433 or 1435) was a Chinese mariner, explorer, diplomat, fleet admiral, and court eunuch during China's early Ming dynasty. He was originally born as Ma He in a Muslim family, later adopted the conferred surname Zheng from Emperor Yongle. Zheng commanded expeditionary voyages to Southeast Asia, South Asia, Western Asia, and East Africa from 1405 to 1433. His larger ships stretched 120 meters or more in length.

Zheng He's sailing charts, the Mao Kun map, were published in a book entitled the Wubei Zhi (A Treatise on Armament Technology) written in 1621 and published in 1628 but traced back to Zheng He's and earlier voyages. It was originally a strip map 20.5 cm by 560 cm that could be rolled up, but was divided into 40 pages which vary in scale from 7 miles/inch in the Nanjing area to 215 miles/inch in parts of the African coast.  Investigation into folios 19V to 20R of the Mao Kun Map which covers the Indian Ocean including the South India, Sri Lanka, the Maldives, and East Africa suggests that it is a composite of four maps, one for Sri Lanka, one for South India one for the Maldives and one for around 400 km of the East African coast, no further south than 6 degrees south of the Equator. Each of these maps is positioned at a different orientation to fit with the ocean currents and winds required of a sailing chart, rather than a formal map. The analysis also suggests that Arabic-speaking pilots with a detailed knowledge of the African coast were involved in the cartography.  There is little attempt to provide an accurate 2-D representation; instead the sailing instructions are given using a 24-point compass system with a Chinese symbol for each point, together with a sailing time or distance, which takes account of the local currents and winds. Sometimes depth soundings are also provided. It also shows bays, estuaries, capes and islands, ports and mountains along the coast, important landmarks such as pagodas and temples, and shoal rocks. Of 300 named places outside China, more than 80% can be confidently located. There are also fifty observations of stellar altitude.

Using a quote from the above article, answer the following question: What is the sailing charts ?
the Mao Kun map, were published in a book entitled the Wubei Zhi (A Treatise on Armament Technology) written in 1621 and published in 1628