How long is China's Great Wall? One reason the question is so hard to answer is that the wall is discontinuous. Parts of it are built from stone, parts from rammed Earth. In places it is little more that a ditch, and in some places there is no wall at all, just fortifications along natural barriers such as rivers.
To add to the complexity, the earliest fortifications date to the first emperor of China, Qin Shi Huang. Some of these can still be seen today but most of the modern wall dates from the Ming dynasty (before 1700 AD).
Another reason it is so hard to measure the length of the Great Wall is that in some parts, the wall is built over the steepest parts of mountainous areas. Traditional field surveying could not easily measure its real length in 3D space.
Recently, the International Society of Photogrammetry and Remote Sensing sponsored a project to determine the true length of the wall. ISPRS president Chen Jun was the chief scientist on the project. He led a team of about 800 professionals from geomatics and cultural heritage disciplines.
The team used 1:35,000 air photos to generate a 3D digital model of the topography. This allowed the team to measure the slope length using digital photogrammetric tools.
On 18 April, they revealed that the Great Wall built in the Ming Dynasty era stretched 8851.8 km, further than the previous estimate of around 6000 km. It consists of 6259.6 km of stone wall, 359.7 km of trench wall, and 2232.5 kilometers of natural defensive barriers such as hills and rivers.
As the result of the two year investigation, Chen says all the important items of the Great Wall have been surveyed and strip maps at 1:10,000 scale have been produced. A special GIS is now under development that will combine the results of heritage investigation with a geospatial database.
This database will be used as an operational system in the protection, research and sustainable development of the World Heritage site.
Source: www.asmmag.com
