A number of diverse sources are available including primary sources, secondary sources, archaeological sources, artifacts, inscribed tablets, cartographic sources, Google Earth, photographic sources, rephotographic sources, personal observation (exploration, travel) sources, myths and legends, oral histories and folktales.
I subscribe to the broadest definition of a primary source: that it is material that permits the closest, most enlightening insight into the subject. In three words: most direct evidence.
Although the Ming Wall is only part of the Great Wall story and subject, it is a main part. I will answer this question on sources therefore with regard to studying the Ming Wall.
The main primary source for study is on site: the extant Ming Wall. If we inspect a section of this Wall we can learn, or gain insights into, the following: the geography (location) of the structure, the age of (history) the structure (if an artifact such as a tablet can be found, either in situ or in proximity, the construction materials used and methods of building, and the operational and strategic functions of the structure. Looking at details such as design features can provide clues as to what weapons were used or what natural forces posed threats to damage the structure. Archaeological sources may also be forthcoming and revealing. For example, land mines deployed in approaches to the defences show us adept military strategy and technology; shards of water containers might reveal transportation of building material or basic needs of garrisoning troops. For these reasons the Ming Wall is an outstandingly rich and diverse primary source, an outdoor museum.
Off site, primary sources may exist in written forms. After compiling the ‘opening volume’ of China’s history within Shi Ji, or Records of the Grand Historian, Sima Qian was hailed as the father of Chinese historiography.
Each dynasty recorded its own history, nationally and locally, in many shapes and forms, and had its events recorded therein collected, synthesized and judged by scholars of the following dynasty, a tradition widely thought to have initiated and been maintained for the purpose of history serving as a mirror. The rationale was simple: what had happened in the past was a valuable lesson for the current monarch to achieve his goal of governing well and ruling for a long time. The main problem with records on paper is that they are easily damaged or destroyed. For example, the Ming’s central archive concerning the running of the country which was stored on the ‘Back Lake’, on an island of Nanjing’s Xuanwu Lake for safety, eventually went up in flames. The imperial archives Huangshi Cheng, or Imperial City of History, in Beijing are well preserved in a Fort Knox type building built c. 1535.
All sources have their strengths and weaknesses, and in the case of standard official histories concerning any Great Wall there are non-specific problems such as the tardy retrospective nature of the record, for they were usually written well after the event, and subjectivity. These problems among others strictly define such sources as being secondary in quality, particularly when compared with archaeological sources.
We can regard Sima Qian as a rare breed of chronicler: he claimed to have seen the Qin Wall himself, but we surmise that one reason for the Wall’s scant mention in various official (dynastic) histories throughout the ages, is that chroniclers or compilers were in the main sedentary creatures. While they recorded imperial decrees, court debates, responses to advisory correspondence (known as memorials), or took note of the authorization of financial resources to enable its construction, they did not get close to many Walls.
Returning to the Ming (1368-1644), its official history, Ming Shi, was compiled by Zhang Tingyu (1672-1755) and published c. 1739. The compiler had a unique resource to work from, for Ming Shilu, or Veritable History of the Ming, was preserved from floods, fires, thieves and vermin in the Imperial Archives mentioned above as a primary, literally truthful, source. This gargantuan resource details the life and times of 13 of the dynasty’s emperors. The collection includes edicts, memorials, court annals and biographies, among other documents.
Cartographic sources, produced within or outside China can reveal understanding on how the Wall evolved and how its builders used the landscape strategically. The modern map view, from satellites and presented via Google Earth, is a tremendous tool, providing a privileged insight into the macro-geography of the structure being studied. My recent research in Ömnögovi, Mongolia, was greatly assisted by this satellite perspective. Field knowledge supplemented by a view from Google Earth can help make sense of the structure’s twists and turns.
Narratives and maps from early explorers and travellers, especially Régis, Parish, Stein and Geil have provided extremely valuable primary and secondary source materials. Photographs, especially vintage photos taken early in the twentieth century, provide a view of the Wall when it was more complete, and longer, prior to a succession of damaging events that occurred throughout the mid twentieth century. Returning to the same locations to rephotograph sites has produced powerful primary source material for conservationists.
Local sources can be vivid: in some regions present day building methods remain unchanged from the era of the Great Wall builders, while farmers have inherited their own family’s historical role in Great Wall building via folktales from across the centuries.