In the field of surveying, specifically in plane table surveying, various methods are employed to accurately plot features and objects on a map directly in the field. The question describes a particular method where the surveyor uses two distinct instrument stations to locate objects by drawing lines of sight (rays) and observing where these lines intersect.

Intersection Method in Plane Table Surveying

The method described in the question is known as the Intersection method. This technique is predominantly used for plotting points or objects that are inaccessible or located at considerable distances, making direct measurement challenging or impossible. These could include features like distant hilltops, river banks, or structures across obstacles.

Working Principle of Intersection Method

The core principle of the Intersection method involves observing the same object from at least two different survey stations. By drawing rays from each station towards the object, the point where these rays meet on the drawing sheet determines the object's precise location.

Let's break down the typical steps involved in the Intersection method:

• Choosing Stations: Two suitable stations, say 'A' and 'B', are selected on the ground. The positions of these stations must either be already plotted on the map sheet or can be accurately plotted before proceeding.
• First Setup (at Station A):
  • The plane table is set up at Station A, ensuring it is properly leveled and oriented. Orientation means aligning the map sheet parallel to the ground it represents.
  • From the plotted position of 'A' on the map, the alidade (a sighting device) is used to sight towards the target object (e.g., an inaccessible tree, 'T').
  • A ray, or line of sight, is then drawn on the map sheet along the edge of the alidade, extending from 'A' towards 'T'.
  • This process is repeated for all inaccessible objects to be plotted from Station A.
• Second Setup (at Station B):
  • The plane table is then carefully moved and set up at Station B.
  • At Station B, the table is once again leveled and precisely oriented. A common way to orient the table at Station B is by back-sighting to Station A (if 'A' is visible) or using a trough compass. Correct orientation is critical for accurate plotting.
  • From the plotted position of 'B' on the map, the alidade is again used to sight towards the same target object 'T'.
  • Another ray is drawn on the map sheet from 'B' towards 'T'.
• Locating the Object:
  • The point where the ray drawn from Station A intersects with the ray drawn from Station B for object 'T' is the accurate plotted position of object 'T' on the map.
  • This intersection point effectively triangulates the position of the object without requiring direct distance measurements to it.

Plane Table Surveying Methods Comparison

Understanding the distinctions between various plane table surveying methods helps clarify why the Intersection method is the correct answer based on the question's description.

The question's description, "rays are drawn from station No. 1 towards objects then the plane table is shifted to second station, again from station No. 2 the rays are drawn towards the same objects then the cutting point represents the respective objects on map," perfectly aligns with the procedure and purpose of the Intersection method in plane table surveying.