In Egypt, the incense hall of the chapel must face east, so how does this "east" map? In order to determine this direction, the ancient Egyptians first observed the rising and falling positions of the stars on the horizon, and determined a plane in it, thus obtaining the north-south line, and then drew the east-west line perpendicular to the north-south line by a special surveyor. The surveyors set up two piles along the north-south line and another pile at a far place, so that it forms a right triangle with the first two piles. Enclose the three piles with a rope. If the ratio of the lengths of three sides of a triangular pile is 3: 4: 5, then the angle opposite to the largest side is a right angle. With a right angle, it is not difficult for surveyors to determine the east-west line according to the north-south line. The ancient Egyptians made full use of the characteristics of right triangle here, which is also their long-term experience in surveying and mapping.