Ranging out Survey Lines

The operation of fixing intermediate points on a straight line between the end points of a survey line is known as ranging. Ranging should be done before measuring a survey line by a chain. If the survey line is short and the terminal points are visible clearly, it is easy to lay the chain in straight line. If the survey line is long and end points are not clearly there is a need for fixing intermediate ranging rods to maintain alignment. Ranging may be done by eye or by using an instrument ranger or theodolite. For important work, a theodolite is used. There are two methods commonly used for ranging Direct ranging, Indirect ranging. Direct ranging is possible only when the end points are visible. Indirect ranging is adopted when end points are not visible.

1. Direct ranging

When two ends of the survey line are inter visible then direct ranging methods are adopted. These are of two types (i) Ranging by eye (ii) Ranging by line ranger.

Ranging by eye

When both the ranging rod at the end of the survey line are visible, this method is adopted. In this type of ranging, the man ' holding the ranging rod is asked to move in between the ranging rods. The ranger standing about 2 to 3 m. away from the first ranging rod directs the man holding the ranging rod to come in line with the first and last ranging rod. One all the three are in the same line, the intermediate ranging rod is fixed. Similarly ranging is done for other points used for fixing intermediate points of the chain lines. To fix the intermediate point on the chain line, the man holding the instrument moves approximately in line with the end points and holds the instrument at the level of the eye. The observer sees both the ranging rods in the instrument. He moves left and right till the images of the ranging rods at starting and end point of the survey line are in one line . In this method only one observer is enough for ranging.

2. Indirect Ranging or Reciprocal Ranging

As shown in the figure A and B two end points of the survey line. Two points Pi and Qi are selected near the survey line such that from Pi, Qi and B end and from Qi Pi and Qi Pi and A are visible. After this two persons holding ranging rods will go to points Pi and Qi. The man standing at Qi looking at B directs man at Q1 to Q2 like this P1, Q2 and B come in one straight line. Similarly man standing at Q2 looking at A instruct man at Pi to come to P2, that A, P2 and Q2 are in one line. The same process is continued till APQ and B are in one line.

Chaining on Sloping Ground

Since a plan is the horizontal projection of the ground it represents, all measurements must be made ,horizontally, or, if made along a slope, these should he reduced to equivalent horizontal distances.

 

Thus, if Fig. 3.9 represents a section of sloping ground between. A and B, the distance required for plotting and/or computations is AC, not AB. Since the sloping distance is always greater than its horizontal equivalent, it becomes necessary to give allowance for slope. This can be done in two ways—either ' by stepping (Direct Method) or by measuring along the slope and calculating AC from the known length AB and its angle of slope ∞° (Indirect Method).

 Direct Method

By Stepping—

This method consists in measuring the line in short horizontal lengths. Suppose we have to measure the distance AB, starting from Aim the uphill direction. Tile follower elevates his end of the chain until it is stretched horizontally, with the other end held by the leader, resting on the ground. The rear handle should be vertically above the station peg A. Simultaneously the follower directs the leader into proper alignment with B, and the leader marks the other point of the chain on the ground with the arrow. This is repeated until the entire length is gone over in four step Thus all the four component distances a, b; c, d are measured which, when totalled, give the horizontal distance AC.

It will readily be appreciated that for accurate results:—

(1) The chain should be stretched in small steps, of 20 to so links at a time, or else the error due to sag will be significant. A steel tape, which is much lighter, is the best for this kind of work.

(2) Steeper the slope, shorter should be the steps. Length of the steps need not be uniform.

(3) Height of the handle above the ground, i.e., the length of the plumb bob, should not exceed 1.5 meter (or 5 feet).

(4) To judge the horizontality of the chain, the surveyor should stand clear on one side and direct his chainman.

To ensure that the outside edge of the rear handle is vertically above the marked point, either a plummet may be used with the plumb bob on the mark or the follower should support the handle of his chain by grasping it against a spare rod held on the mark, truly vertical. In chaining downhill, the follower stands on the rear handle of the chain and directs the leader into line. The leader lifts up the chain until it is horizontal, and marks the corresponding point on the ground under the end of the chain, by using a plummet, a spare rod or a drop-arrow weighted at its end. The leader drags forward the chain for the next length and repeats till the entire length is gone over. Chaining may be uphill or downhill, but downhill is preferable. This method of stepping is practicable whether the slopes are uniform or otherwise, it is certainly preferable when the ground is uneven, undulating and gradient are short

 

Then D= l cos ∞ however, the slope is irregular, and made up of varying inclinations, the ground is divided into sections of, more or less, uniform slope and the slope. length of each section measured and its corresponding angle of slope observed with a clinometer Suppose l1 /2, l4, etc., (Fig. 3.11) are measured lengths and ϴ1, ϴ 2, ϴ 3, etc., their corresponding angles of slope. Then the horizontal distance AB=lcos ϴ 1+12 cos ϴ 2+ 13 cos ϴ 3=∑ l cos ϴ

Fig: Indirect Method of Chaining

Second Method

Hypotenusal Allowance Method –if only the distance between two points is required, horizontal distances may be computed after measurements in the field, as in the first method. However, if a number of intermediate points have to be located. Correction will have to be applied to every chain length measured on the ground. In this case each chain arrow is set forward, beyond each chain measured on the ground, by an extent equal to calculated correction. This can be determined as follows:---

Fig: Hypotenusal Allowance (=CD).

Let ACE (Fig. 3.12) be a section along the sloping ground, angle of slope being 0. Let AB, the horizontal equivalent of AC be=1chain (100 links). With A as centre and as radius draw an arc cutting AC in D. Then the allowance to be given for a chin length AD is DC, so as to allow for greater length along the hypotenuse ; this distance DC is known as hypotenusal allowance. DC=AC—AD—AC—AB---AB sec 8—A131------AB (see ϴ -1). or, the correction in links-100 (sec ϴ —1), by which extent each chain arrow must be set forward to mark one chain horizontal distance. For example, if ϴ =10°, hypotenusal allowance or the correction DC-100 (sec 10°-1) =100 (1.0154-1)=1.54 links.