Locating an inaccessible point with the help of only a Plane table - Intersection method

The intersection method is used to locate an inaccessible point with the help of only a Plane table. This method involves the following steps:

1. Set up the Plane table:
- Select a suitable location to set up the Plane table such that it provides a clear view of the area to be surveyed.
- Level the table using a leveling instrument to ensure accuracy in measurements.

2. Orient the Plane table:
- Orient the Plane table by sighting on two known points with the help of a surveying compass.
- Rotate the table until the line of sight between the two known points coincides with the line on the map representing those points.

3. Select two visible reference points:
- Identify two reference points on the map that are visible from the Plane table setup.
- These reference points should be easily identifiable and preferably located at a reasonable distance apart.

4. Observe and record angles:
- Observe and record the angles between the reference points and the inaccessible point from the Plane table.
- Use a suitable instrument such as a theodolite or a compass to measure these angles accurately.

5. Plot the lines of sight:
- Plot the lines of sight from the Plane table to the reference points on the map.
- Use a straight edge or ruler to draw these lines accurately.

6. Locate the inaccessible point:
- The inaccessible point can be located by extending the lines of sight from the Plane table to the point where they intersect on the map.
- The intersection of the lines represents the location of the inaccessible point.

Advantages of the Intersection method:
- The intersection method is relatively simple and requires minimal equipment (only a Plane table and a compass or theodolite).
- It is useful for locating points that are not directly accessible or visible from the surveying station.
- The method provides good accuracy if the measurements and angles are recorded carefully.

Limitations of the Intersection method:
- The accuracy of the method depends on the accuracy of angle measurements and plotting on the map.
- Errors in measurements or plotting can lead to inaccuracies in locating the inaccessible point.
- The method requires clear visibility between the Plane table and the reference points, which may not always be possible in certain terrain or weather conditions.

Therefore, the correct answer is option 'D' - Intersection, as this method involves locating the inaccessible point by intersecting the lines of sight from the Plane table to two visible reference points on the map.