Prolonging of a straight line by reversing method, Prolonging of a straight line by double reversing method, prolonging of straight line, reversing method.

 When the line is to be prolonged with high precision or when the instrument is in imperfect adjustment, the process of double reversing or double sighting is used. 

Suppose the line AB is to be prolonged to a point Z.

• Setup the theodolite at B and level it accurately. 

• With the face of instrument left, back sight on A and clamp both the upper and lower plates. 

• Transit the telescope and set a point C1 ahead in line. 

• Loosened the lower clamp, revolve the telescope in the horizontal plane and back sight on A. Bisect A exactly by using the lower clamp and its tangent screw. Now the face of instrument is right. 

• Transit the telescope and establish a point C2 in line beside the point C1.

• The exact position of the true point C must be mid- way between C1 and C2.

• Measure C1, C2 and establish a point C exactly midway, which lies on the true prolongation of AB.

• Shift the instrument to C, double sight on B establish ther point D1 and D2 and locate the true point D as before. 

• Continue the process until the last point Z is established.