Abstract: Based on the least square adjustment by indirect observations,this paper gives the rigorous formulas for calculating the lateral break-through error caused by surveying errors of a surface traverse network for setting out a tunnel.Starting from this lateral break-through error and considering some examples,the influence caused by the different weights selected for the direction and distance observations,the optimization for the figure and observation programme of the traverse network are discussed.The advantages of adopting traverse network as a control for setting out a tunnel and some problems to which attention must be paid in the design are pointed out.