Vertical lines are often used in architectural survey. Tie a heavy object with one end of a string. When it is stationary relative to the ground, the straight line where the rope is located is the plumb line, also called the heavy plumb line. Gravity direction line in the earth's gravity field. It is perpendicular to the horizon and is the reference line for field observation. The direction of the hanging weight when hanging freely is the direction of this line. The plane containing it is called a vertical plane.
Extended data
To judge whether an object is perpendicular to the ground, we can use the vertical line method, that is, a line plus a heavy object. This heavy object is called plumb hammer. The plumb hammer is acted by gravity, that is, by a component of gravity, so the line is perpendicular to the ground at an angle of 90 degrees. The weight of plumb hammer has nothing to do with the verticality of vertical line.
In general, regardless of the relativistic effect, the plumb line at the foot of the mountain and the mountain beside the plumb hammer cannot affect the verticality.