Correct Answer - Option 2 : (ii) and (iii) only
Concept-
The measurement of distances consists of chaining and making offsets. During measurements, it is practically impossible to set out all the chain lines in a straightforward method because of a variety of obstructions to chaining and ranging in the field.
The difficulties can be overcome by running perpendicular and parallel lines or by running a few additional lines and measuring angles by some instrument. To find the best and rapid solution, the surveyor should have a good knowledge of geometric and trigonometric principles.
The obstacles may be divided into two classes. Those which do not obstruct the ranging (view) like ponds, rivers and fall in the category of obstacles to measurement.
The others are those which we cannot see across, i.e. both the chaining and ranging are obstructed, e.g. houses, stacks etc. and are known as obstacles to alignment.
Vision obstructed but chaining free-
Reciprocal Ranging-
When end station is not visible due to there being high ground between them intermediate ranging rod fixed on the line in an indirect way this method is known as indirect ranging or (reciprocal ranging).
Random line ranging-
In this method, a random line such as BQ (as shown in figure) is being laid such that R is visible from Q. A perpendicular QR is being erected at Q and measure its distance. Then the desired distance BR is being calculated using distance formula.
In laying the line BR, intermediate points are established first. These are laid by taking offset from the random line at distances calculated by using the method of similar triangle such as after locating the intermediate points on the line BR, the obstructions get cleared to make the end points visible.
Then, direct ranging is being carried out to obtain an extended continuous straight line.
