Direct Ranging

Direct ranging is the ranging conducted when the intermediate points are intervisible. Direct ranging can be performed by eye or with the help of an eye instrument.

Ranging by Eye

As shown in figure-1 below, let A and B are the two intervisible points at the ends of the survey line. The surveyor stands with a ranging rod at the point A by keeping the ranging rod at the point B. The ranging rod is held at about half metre length.

Fig.1.Direct Ranging

The assistant then takes the ranging rod and establishes at a point in between AB, almost in line with AB. This is fixed at a distance not greater than one chain length from point A.

The surveyor can give signals to the assistant to move traverse till the rod is in line with A and B. In this way, other intermediate points are determined.

Ranging by Line Ranger

The figure-2 below shows a line ranger that has either two plane mirror arrangement or two isosceles prisms that are placed one over the other. The diagonals of the prism are arranged and silvered such that they reflect incident rays.

Fig.2. Ranging by Line Ranger

In order to handle the instrument in hand a handle with hook is provided. The hook is to enable a plumb- bob to help transfer the point to the ground.

In order to range the point 'P', initially two rods are fixed at points A and B. By eye judgment, the surveyor holds the ranging rod at P almost in line with AB.

The lower prism abc receives the rays coming from A which is then reflected by the diagonal ac towards the observer. The upper prism dbc receives the rays from B which is then reflected by the diagonal bd towards the observer. Hence the observer can see the images of the ranging rods A and B, which might not be in the same vertical line as shown in figure-2(c).

The surveyor moves the instrument till the two images come in the same vertical line as shown in figure-2(d). With the help of a plumb bob, the point P is then transferred to the ground. This instrument can be used to locate the intermediate points without going to the other end of the survey line. This method only requires one person to hold the line ranger.