Understanding Plane Table Surveying and Resection

Plane table surveying is a graphic method of surveying where the field work and plotting are done simultaneously. It's particularly useful for small-scale mapping.

Several methods are employed in plane table surveying for different purposes. These methods include radiation, intersection, traversing, and resection.

The Purpose of the Resection Method

The question asks specifically about the purpose of conducting the resection method in plane table surveying. The primary goal of the resection method is to determine the unknown location of the plane table station (where the instrument is placed) by sighting known points whose positions have already been plotted on the drawing sheet.

Imagine you set up your plane table at a point 'P' whose location on the map is unknown. You have a map with two or more known points (say, A, B, C) already plotted on it. The resection method allows you to find the position of point 'P' on your map by drawing rays from the plotted positions of A, B, and C towards the corresponding physical points in the field. The intersection of these rays on the map gives you the location of 'P'.

Comparing Resection with Other Plane Table Methods

Let's briefly look at the purposes of other common plane table methods to highlight the unique role of resection:

• Radiation: Used for plotting details (objects, features) around a known instrument station. Rays are drawn from the instrument station towards the field points, and distances are measured and plotted along these rays.
• Intersection: Used for plotting inaccessible points. The point is sighted from two or more known instrument stations, and the intersection of the rays drawn from these stations gives the point's location on the map.
• Traversing: Used for surveying a line or series of lines (a traverse) by occupying successive stations, orienting the table at each, and plotting the lines.
• Resection: Used to determine the location of the instrument station itself by sighting known, plotted points. This is crucial when setting up the table at a point not previously surveyed or marked.

From this comparison, it is clear that determining the location of the instrument is the distinct purpose of the resection method.

How Resection Works (Basic Idea)

The basic principle involves:

1. Setting up and leveling the plane table at the unknown station 'P'.
2. Orienting the table. This can be done using a trough compass or by the back-sighting method if a known point on the traverse line is occupied. For resection using three points, orienting might involve the "three-point problem" solution.
3. Sighting at least two (preferably three or more) known points (A, B, C) in the field whose corresponding positions (a, b, c) are already plotted on the map.
4. Drawing rays on the map from the plotted points (a, b, c) towards the alidade held against the sight on the field points (A, B, C). These rays are essentially lines connecting the plotted known points to the unknown instrument station on the map.
5. The intersection of these rays on the map gives the plotted position 'p' of the instrument station 'P'. If three points are used, ideally, the three rays intersect at a single point (the point of resection).

Therefore, the core purpose of the resection method is to establish the exact position of the plane table on the map by using known, pre-plotted points. This makes it possible to continue surveying from this newly located station, either by radiation or other methods.

Based on the options provided and the purpose of the resection method in plane table surveying, the correct choice relates directly to finding the instrument's position.

Revision Table: Resection Method Key Points

Additional Information on Plane Table Surveying Techniques

Plane table surveying is advantageous because it allows the surveyor to compare the plotted features on the map with the actual ground features while in the field, reducing the possibility of errors and omissions. It is a relatively simple method but can be affected by weather conditions (like wind and rain) and the quality of the drawing sheet and instruments.

The accuracy of the resection method depends heavily on the accuracy of the plotted known points and the skill of the surveyor in sighting and drawing rays. When using three known points, if the rays do not intersect exactly at a single point, they form a small triangle called the "triangle of error". The instrument station's position is estimated to be inside this triangle, and methods exist to reduce or eliminate this error.

Resection is often used in combination with other methods. For example, one might traverse a main line and then use resection to set up at intermediate points or stations not on the main traverse line to fill in details using radiation or intersection.