Ranging using Line Ranger:

For more accurate ranging, instruments like line rangers are used. Line ranger consists of either two plane mirrors or two right angled isosceles prisms, as shown in Figure (a), silvered to reflect the incident rays.

Figure: Ranging Using Line Ranger

A handle is provided to hold the ranger in hand which also helps in transferring correct location of L (intermediate station) on ground. The line ranger is held at approximate position of L along line AB. Upper prism receives light rays from ranging rod at A and reflects it towards surveyor by diagonal ab. Similarly, the lower prism reflects the light rays from ranging rod at B by diagonal cd. The surveyor can, thus, see the images of both the ranging rods A and B. Simultaneously, if the instrument does not lie exactly along survey line AB, the two images do not coincide and a parallax is obtained as shown in Figure (b).

The observer removes this parallax by moving the line ranger sideways till the two images, respectively, of A and B coincide indicating the correct alignment of intermediate station L (Figure (c)). Apart from higher accuracy, another advantage of using the line ranger is that only one surveyor can perform the ranging operation as against two surveyors required for vision ranging.