If a stone falls and creates a hollow area, either due to land vibration or water loss and soil erosion, the wall should be mended as soon as possible. Otherwise, the damage may get worse, Luo says.

Some 1.2-km of the Kangtugou section of the wall was restored in 2004. The well-preserved section, located to the north of Guyang's county seat, is 5 meters deep on the outside, 2 meters deep on the inside, 2.8 meters wide at the top and 3.1 meters wide at the bottom, and runs for a length of 3 km.

"I came to work there by chance. I really admired the experts, thinking 'how come they knew so many things?' They told me a lot that I found really interesting," Luo says.

After a while, when he was more familiar with their work, he decided to give it a shot himself. The experts taught him how to photograph the relics — when to use a full view and when to use a close-up, for example — as well as to draw.

In 2006 and 2007, Luo followed the experts and trekked the length of the Qin Dynasty Great Wall in the county to survey the whole structure.

The team accurately ascertained the length of the Great Wall in Guyang and Luo also learned to map the site with GPS technology.

As a low-paid temporary worker, his colleagues wondered why he would bother to climb the mountains, exposing himself to the blazing sun for hours, and getting dirt all over his trousers, which sometimes wore out, simply to do the extra work.

"But after twice walking along the Great Wall step-by-step, I dare say I was one of those who knew it the best in our county," he says, adding that people would naturally turn to him when facing problems with the wall.

"My colleagues recognized my abilities, and I felt proud of myself. That feeling supported me over the long period serving as a temporary worker."

In 2007, he participated in further fieldwork at other cultural heritage sites in Guyang, and got to know the layout and condition of the sites by heart. As a result of that expedition, the registered immovable cultural heritage in Guyang grew from around 30 locations to 159.

In charge of the GPS facilities, Luo had to cover every corner of the archaeological sites, which ended up with him remembering every tree and telegraph pole.

"People are passionate when doing what they love," Luo says, adding that he would get excited, even when finding a pottery shard with patterns on it while on patrol.

It was not until 2012, after eight years working as a volunteer Great Wall conservator and temporary worker, that he finally got a bianzhi (official registration), which means his job is secure and comes with social insurance, healthcare and a pension.

To make up for the knowledge gap, he reads a lot about history, geography and meteorology, and combines his reading with what he sees on the spot and learns from the experts.

Nowadays, drones and advanced map software have made patrols easier, improving efficiency and accuracy and lightening the heavy backpacks they used to carry.

Local residents living near the Great Wall are of great help to Luo's work, as surprise findings might occur following a casual chat, either a new discovery or clues about potential thefts.

During a routine patrol in March 2018, several small, regular holes — around 30 centimeters deep and with a diameter of around 20 cm — near the beacon towers along the Great Wall at Yinhao town caught his attention.

When he went back to his village for tomb sweeping during the Qingming Festival holiday in April, he asked the other villagers whether there had been people digging around the relics. The villagers answered that two people had been seen doing so, but they thought they had been Luo's colleagues doing excavation.

Luo immediately went to check the site and spotted the same holes he had seen in Yinhao. He reported the situation to the police and asked Great Wall conservators to step up inspections.