Using infrared laser rangefinders and global positioning system modern technology, a survey released that the Ming Dynasty section of the Great Wall of China is 8,851.8 kilometers (5,500 mi), some 2,551 kilometers (1,585 mi) longer than previous estimates.
Historical accounts put the wall's length around 5,000 kilometers (3,107 mi), however it was not actually physically measured.  The actual constructed wall measurements come to 6,259 kilometers (3,889 mi). The remaining 2,595.2 kilometers (1,613 mi) of the defensive wall system are trenches, hills and rivers according to the survey project which traversed mountains and deserts.
Sandstorms buried some sections of the wall, and these hills of dirt and sand may erode completely in 20 years. The new technologies helped to discover some of these wall areas which were buried. 
The Great Wall of China was declared a World Heritage Site in 1987. In 2005, a conservation project began to preserve the Great Wall. The last two years involved mapping and surveying by the State Administration of Cultural Heritage and the State Bureau of Surveying and Mapping, and will see completion in another 18 months. So far only the portion of the wall built during the Ming Dynasty (1368 to 1644 AD) has been measured, and the remaining sections constructed in the Qin Dynasty (221 - 206 BC) and Han Dynasty (206 BC - 9 AD) have yet to be investigated. Besides discovering hitherto unknown sections of the Great Wall, researchers analysed its construction and the challenges presented to preserve it. Many areas of the wall have undergone restoration processes since 1949.
The wall was constructed of packed earth in places, however the majority was brick and stone construction. Sandstorms, extreme weather, highway construction, commercial expansion, and tourism are the main threats to the Great Wall of China. 

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Modern technology maps new sections of the Great Wall of China