Method of Measuring Angles along the Horizontal Plane as the Vertical Axis Tilt

The tilt of the vertical axis will influence the results when you measure angles along the horizontal plane.One way to decrease the impact is to level before each position;the other method is to read out a number on a graduated bubble tube,which is equipped on the vertical plane of the theodolite,and then use the number to correct the result.However,both of them are cursory modification.What's more,there is still no measurement to this problem as many theodolites without graduated bubble tube equipped on the vertical plane.Then the assumed object method is presented to solve this problem.