A massive geographical survey of the Great Wall has been launched by China's State Administration of Cultural Heritage and State Bureau of Surveying and Mapping.

The survey will be completed by 2007 and the basic statistics of the Great Wall, including its length and layout, will be released in 2008, officials said in Beijing [Images] on Thursday.

Local governments have been gathering statistics on the Great Wall since the 1980's. "Due to limited knowledge and technology at the time, most parts of the Great Wall remain unknown to us," director of the SACH, Shan Qixiang said.

"It's necessary for the government to organise a scientific survey so we have a comprehensive and accurate understanding of the Great Wall," Shan said.

The wall is generally considered to start at the Jiayuguan Pass in Gansu Province and stretch 6,000 km to the Shanhaiguan Pass on the shores of Bohai Bay in the east, yet no one knows for sure the ancient wonder's exact length.

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

The survey has already started in the Hebei and Henan provinces.

Scientists and historians say they will focus their work on the portion of the Great Wall near Beijing, which was built during the Ming Dynasty (1368-1644).