Prolonging a straight line, Prolonging a straight line by fore sight method, prolonging a straight line by back sight method, prolonging a straight line by reversing method.

 There are several methods to prolong a straight line from any station to infinity.

1. For Sight Method:-

• Setup the theodolite at A and level it accurately bisect the point B correctly. Establish a point C in the line beyond B approximately by looking over ther top if ther telescope and accurately by sighting through the telescope. 

• Shifty the instrument to B, take a fore sight on C and establish a point D in line beyond C.

• Repeat the process until ther last point Z ire reached. 

2. Back Sight Method:-

• Set up the instrument at B and level it accurately. 

• Take a back sight on A.

• Tightened the upper and lower clamps, transit the telescope and establish a point C in line beyond B.

• Shift the theodolite to C, back sigjt kn B, transit ther telescope and establish a point D in line beyond C.

• Repeat the process until the lady point Z is established. 

• If instrument is adjust accurately the point A, B, C, D till Z will be in one line but if not adjust accurately line of collimation is not perpendicular to horizontal axis then C' and D' will not be in a straight line.