China's Great Wall far longer than thought: survey

However the project has also shown the World Heritage-listed site is in danger of disappearing in many places due to road construction and other forms of development, as well as extreme weather, the China Daily said.

The wall, built over centuries to keep foreigners out of China, stretches for 8,851.8 kilometres (5,488.1 miles), much further than common estimates of 5,000 kilometres, according to the findings of the survey.

The defensive structure includes 6,259.6 kilometres of actual wall, plus 359.7 kilometres of trenches and 2,232.5 kilometres of natural barriers such as hills and rivers.

The two-year mapping project, carried out by the State Administration of Cultural Heritage, involved using global positioning systems and infra-red technology.

Previous estimates were mainly based on historical records, rather than physically mapping each section. By tracking thoroughly across mountains and through deserts, unknown parts were uncovered.

State Administration of Cultural Heritage director Shan Jixiang was quoted as saying the survey had helped clear up an ancient mystery over its length but also highlighted many problems in trying to preserve it.

"The Great Wall is under great threat, climate change and the country's massive infrastructure building being the biggest two," the China Daily quoted Shan as saying.

The first parts of the Great Wall were built more than 2,000 years ago, then rebuilt and extended during the Ming Dynasty (1368 to 1644 AD) amid the threat of invading northern tribes.

In recent times, the wall has suffered extensively at the hands of modern development, with sections of it destroyed to make way for roads and other forms of construction.

The survey was part of a 10-year conservation project launched in 2005 that will now be able to keep detailed maps of the sections of the wall that need to be protected, and to maintain records of efforts to renovate or preserve it.

(c) 2009 AFP