Well I have been confused about Sancha for a while now. I think it's beginning to clear up. I first heard of Sancha looking at the map at the back of the book,
Explore The Great Wall. Then I found on other maps that there are two villages in Huairou named Sancha. They are at 40°26'30"N 116°39'30"E and 40°27'30"N 116°23'50"E. Reading further about the Great Wall I heard of Sanchakou and assumed it was another name for Sancha, e.g. the village was Sancha and the pass on the nearby Great Wall was Sanchakou. I guess this was right but it was the Sancha to the west at 40°27'30"N 116°23'50"E.
The more easterly Sancha village (40°26'30"N 116°39'30"E) is near the area that I plan to start my Huairou hike this year, the eastern extreme of the Great Wall in Huairou. Actually it's Hefangkou just north of the Yanqihu lake and just south of Sancha, at 40°25'54"N 116°40'18"E. From Qinglongxia I will be heading to Hefangkou and then west to Mutianyu, Jiankou, and from the Beijing knot taking the inner wall on to Sanchakou, Huanghuacheng, Zhuangdaokou, Xishuiyu, and Longquanyu. Later I might return to the Beijing knot to follow the outer wall from there.
I don't know if this will be accurate but I think it would make sense to use the coordinates you mentioned as Sanchakou and to refer to the more easterly Great Wall location (40°26'30"N 116°39'30"E) as Hefangkou. So I will change the Sanchakou location and add a
separate entry for Hefangkou.
Thanks again for your correction!