The walls and border fortifications that make up the Great Wall include those by the [M 002 / Qin from 221-206BC, the Han from 202BC to AD220], the Northern Qi from AD550 to AD577, the Sui from AD589 to AD618 and, most recently, the [P 010 / Ming] from 1369 to 1644. The ruins have never been surveyed and no one is even sure exactly where all of them are. Technological advances in recent years means this situation may well be remedied. However, it is expected to be many years before studies on the Great Wall can be compared to investigations into other walls, such as Hadrian's wall in northern Britain.
