Which little-known GPS function does this cache ask you to use?
The answer : position averaging
The Cheops V geocache (GCJG9J) is a virtual cache placed in May 2004 by Alan and Sarah on the Giza plateau.
The goal is to walk all the way around the Great Pyramid of Khufu, then send the owner your GPS track together with an estimate of the coordinates of the summit, 137 metres up, obtained with the position-averaging function — a GPS feature almost nobody uses, and that is precisely the point of the exercise. The monument remained the tallest structure in the world for more than four millennia.
The answer : position averaging