Line Ranger
It is an optical instrument used for locating a point on a line and therefore useful for ranging. It consists of 2 isosceles prisms placed one over the other and in an instrument fixed with handle. As to reflect the rays the diagonals of the prisms are silvered.
The surveyor holds the instrument in hand to locate point C on line AB and stands near the approximate position of C. the ranging rods at A and B appear separated If he is not exactly on line AB, as shown in given figure. Till the images of ranging rods at A and B appear in a single line Surveyor moves to and fro at right angles to the line AB as shown in given figure. Only when the optical square is exactly on line AB It happens. Thus the needed point C is located on the line AB.
It needs only one person to range, this is its advantage. Occasionally the instrument should be tested by marking 3 points in a line and standing on middle point observing the coincidence of the ranging rods. If the images of the 2 ranging rods do not appear in the same line, one of the prisms is attuned by operating the screw provided for it.
Line ranger