Beijing, Feb 12 (Xinhua) China will launch a four-year geographical survey of the Great Wall, which is on the World Heritage of Unesco, in April to determine its exact length, layout and current conditions. The Great Wall, an ancient defence facility in China, has been widely known to stretch more than 5,000 km.

Field research of the section built in the Ming dynasty (1368-1644) will finish in the early half of next year. According to China's State Administration of Cultural Heritage (SACH) and State Bureau of Surveying and Mapping (SBSM), results of it will be announced in a few months.

Field research results of the Great Wall built in earlier periods will be announced in the following two years.

The survey will be conducted in 13 provinces, autonomous regions and municipality, including Beijing, Hebei, Shanxi, Gansu and Inner Mongolia.

In 220 B.C., sections of earlier fortifications in north China were joined together to form a united defence system against invasions from the northern nomadic tribes. Construction continued up to the Ming dynasty, when the Great Wall became the world's largest military structure.

Local governments have been gathering statistics on the Great Wall since the 1980s. But due to limited knowledge and technology, much of the Great Wall is still a mystery.

Archaeologists and historians had urged the government to organise a scientific survey so people in the world can have a comprehensive and accurate understanding of the Great Wall.

The departments will jointly establish a database based on the results of their survey to facilitate future research and protection of the Great Wall, according to SACH.