Types of Ranging Rod: Direct and Indirect | Linear Measurements| Surveying

Ranging rod is of two types i.e., direct ranging and indirect or reciprocal ranging. 

Type # 1. Direct Ranging:

Direct ranging can be done by any of the following two methods:

(i) By judgment of eye, and

(ii) By a line ranger.

(i) Ranging by judgement of Eye:

The surveyor stands about 2m behind the ranging rod at the starting point of the line, while the assistant holds the ranging rod at arm’s length near the point where it is desired to fix the intermediate station. The surveyor then looking to the end ranging rod directs the assistant, by using hand signals, to move to the right or left until the three ranging rods appear to be exactly in a straight line.

The surveyor then checks the position of the rod by sighting over the lower ends of the three rods in order to avoid the errors due to non-verticality of the rods and then finally he signals the assistant to drive the rod into the ground.

The following hand signals are usually used by a surveyor to direct the assistant to move to the desired position:

(a) Slow sweeps with right hand; Move slowly a small distance to the right.

(b) Rapid sweeps with right hand; Move rapidly a long distance to the right.

(c) Slow sweeps with left hand; Move slowly a small distance to the left.

(d) Rapid sweeps with left hand; Move rapidly a long distance to the left.

(e) Right arm extended; Go on moving to the right.

(f) Left arm extended; Go on moving to the left.

(g) Right arm up and moved to the right; Make the rod vertical by moving its top to the right.

(h) Left arm up and moved to the left; Make the rod vertical by moving its top the left.

(i) Both hands above head and then brought down; The position of the rod is correct.

(j) Both arms extended forwarded horizontally and then brought down; Fix the rod.

A handkerchief may be used in signaling by the surveyor when the assistant has to read signals from a long distance.

(ii) Ranging by a Line Ranger:

A line ranger (Fig. 2.16) is a small reflecting instruments used for ranging long lines when it is difficult to go to either end of the line. It consists of two right-angled isosceles triangular prisms placed one above the other.

The surveyor takes approximate position of the intermediate station say P with his face at right angles to the line and with the instrument held near the level of the eye. The rays of light from A enter the upper prism and after being reflected from the hypotenuse enter the eye at right angles to the line AB.

Similarly rays from B enter the lower prism and after being reflected from the hypotenuse are also seen by the same eye. Thus the images of the rods at A and B are seen directly in the upper and lower prisms respectively.

The observer will continue to move forward and backward at right angles to the line AB until both the images coincide exactly. If the two images coincide as shown in (fig. 2.16 c), the required point P is then vertically below the centre of the instrument.

Type # 2. Indirect or Reciprocal Ranging:

Let X and Y be the two intermediate points to be fixed in line with the end points A and B (Fig. 2.17 a). Select two approximate positions of X and Y on height ground say x₁ and y₁ (Fig. 2.17 b) such that a person at x, can see y₁ and the end ranging rod B and similarly a person at y₁ can see x₁ and the end ranging rod A. Then the person at x₁ directs y₁ to move to y₂ to be in line with B and then the person at y₂ directs x₁ to move to x₂ to be in line with A. Again x₂ directs y₂ to move to y₃ to be in the line with B.

They continue to do so till they take up such positions that no further movement of either X or Y is required. Then X and Y are the desired positions of the intermediate points in line with A and B.

Reciprocal Ranging across a Valley or Depression:

X and Y are the two stations at the ends of a valley. Let the intermediate points A, B, C etc. are to be fixed in line with X and Y across the valley. To do this, the surveyor at X will direct his assistant at A to be in line with Y so that the top of ranging rod at A is brought in line with the bottom of the ranging rod at Y (Fig. 2.18). The ranging rod is then fixed at A. The survivor at X again directs his assistant to move downwards to B so that the top of the ranging rod at B is in line of sight pointing to the bottom of rod at A.

This process is continued till a point is reached near the lowest portion which is invisible from X. The surveyor then goes to the other end-station Y. He will then direct the assistant to move to the position F such that top of the ranging rod at F is in line of sight pointing F such that top of the ranging rod at F is in line of sight pointing to the bottom of the rod at X. The process is repeated as before. X, F, G …B, A and Y are then in the same straight line.