The standard direction includes: (1) true meridian direction: the direction that points to the north and south poles of the earth through a point on the ground is called the true meridian direction of that point, which is determined by astronomical survey or spiral theodolite. (2) Magnetic meridian direction: the direction of a point on the ground when the needle is stationary is called the needle meridian direction of that point. (3) Axis meridian direction: 1, also called coordinate longitudinal axis direction, is the direction of geodetic coordinate system. (Because China is located in the northern hemisphere, the north direction is usually the standard direction. ) 2. Using Gaussian plane rectangular coordinate system, in each 6-degree or 3-degree belt, take the central meridian of the belt as the coordinate longitudinal axis direction. Therefore, for the linear direction in the seat belt, the coordinate longitudinal axis direction of the seat belt is regarded as the standard direction.