RANGING STRAIGHT LINES

Ranging Straight Lines.

Ranging a line means establishing a set of intermediate points on a straight line between two points already fixed on the ground. The simplest case occurs when the two points are intervisible and a start is made from one end of the line. Let A and B, (Fig. 13), be the two points between which it is necessary to establish a number of intermediate points. Having set up vertical ranging poles at A and B, move a short distance behind the point B, the point from which it is proposed to work, to a point C, so that on looking towards A the ranging poles at A and B appear to be in a straight line with the eye.

Standing at C, get an assistant to hold a pole near some intermediate point d, and, using suitable signals or shouts, get him to move his pole right or left of the line until A, d and B, all appear to be in a straight line. If required, points g and h beyond A can be lined in in a similar manner.

The assistant should be made to stand to one side of the line during these operations, so that his body does not obstruct the sight to the distant point. He must also hold his ranging pole vertical by supporting it loosely between the forefinger and thumb so that it tends to hang vertical under its own weight.

Sometimes the whole of the intermediate line cannot be seen from the ends. In fig. 14 there is a gulley between A and B, and neither point can be seen from points inside the gulley. Establish points d and e on the line between A and B on the edges of the gulley and mark them by ranging poles. Then move to d and from behind d line in a pole at f on the straight line between d and e and possibly another pole at g between f and e. If necessary, move to f and put in intermediate poles at h and i. In this way, the line can easily be laid out over the gulley.

Another case arises when the ends of the line are not intervisible. This problem can easily be solved when a theodolite is available by making a survey of the relative positions of the two ends and calculating a bearing between them which can be laid out on the ground. In heavy bush country, where heavy clearing is involved, this is the easiest method, even if lines have to be specially cut for the legs of a preliminary traverse. For many purposes, however, such an elaborate procedure is not necessary, and a line can be established by ranging pole alone or by ranging pole and chain.

In fig. 15 a hill intervenes between A and B, so that these points are not intervisible. If a point C on the hill can be chosen such that it views A and B, it can be ranged in by a line ranger or by the method now to be described. Intermediate points between A and C and between C and B can then be ranged in in the ordinary way.

Choose the point C which, as closely as can be judged by eye, is on, or very close to, the line AB and line in a point D between A and C. On going to D, (Fig. 16) stand behind the pole there and look in the direction of B. In all probability it will be seen that D, C and B are not in a straight line. From D line in a point E between D and B. Proceed to E and from a point behind it see if E, D and A are on line. If not, line in the point F on the line EA. Proceeding in this way, keep moving the poles closer and closer to AB until, after a few trials, they are seen to lie on it.

If, as in fig. 17, it is not possible to choose a point between A and B from which both, points can be seen, estimate the direction of A from B as closely as possible, and range and chain a straight-line BD in that direction, leaving intermediate numbered pegs along BD at the end of each chain length. From A lay out a line AE perpendicular to BD at E and measure the length of AE and the chainage of E. Then, points such as f can be found by drawing a perpendicular to BE at F and laying out Ff such that

 Ff = EA x BF/BE