The Great Wall has fascinated William Lindesay since he first saw it on a map. In 1987, the Liverpool native made his first trek along 2,470 kilometers of the landmark, and he has called China home ever since.

He now spends his days hiking, researching, photographing and talking about the Great Wall, as the founder of a conservation group called International Friends of the Great Wall.

Mr. Lindesay, 55 years old, also leads weekend treks of unrestored portions from his guesthouse near Mutianyu, about 60 miles from Beijing, often rousing hikers at 3 a.m. so they're far enough up the mountain to see the wall as the first rays of dawn turn it golden.

He recently sat down with The Wall Street Journal in his map-lined Beijing study, to talk about skirting death in the Gobi, trying to measure the wall's length and his new book, 'The Great Wall Explained.' Below are edited excerpts from the interview.

Mr. Lindesay: I hope the first one has nothing to do with the length of the Great Wall. That's the biggest headache.

There are many Great Walls in China. A few years ago, the state announced that they had surveyed the Ming wall, which is the wall most tourists see a segment of on their trips to China. And the figure announced was 8,851 kilometers. That announcement was made in April 2009, and then at the same time the State Administration of Cultural Heritage announced that they were asking archeological teams in each provincial region containing wall to go out and measure it.

In June, the survey was complete, and the figure was 21,196 kilometers