Short Answer:

Resection in plane table surveying is a method used to determine the location of the plane table station on the ground when its position is not known. This is done by sighting and drawing rays from the table to at least two or three known points whose positions are already marked on the drawing sheet.

The point where these rays intersect on the sheet gives the exact position of the instrument station. Resection is helpful when the surveyor starts work from an unknown place but can see previously plotted features.

Detailed Explanation:

Resection in plane table surveying

Resection is one of the four primary methods used in plane table surveying. It is mainly applied when the location of the plane table station is unknown, but the surveyor can see two or more known and already plotted points on the drawing sheet. By sighting these known points from the unknown station and drawing rays back to them, the exact position of the instrument station is found where the rays intersect on the sheet.

This method is extremely useful in real field conditions where the surveyor needs to resume work from a new or shifted location without starting the whole survey again.

Instruments used:

Plane table and tripod
Alidade (for sighting)
Drawing sheet
Spirit level (for leveling the table)
Plumbing fork and plumb bob (for centering)

Steps in the resection method:

Set up the plane table at the unknown ground point and level it properly.
Orient the table by aligning it using known points (usually two or three points) already plotted on the sheet.
Sight the known points using the alidade and draw lines (rays) back to their plotted positions on the sheet.
The intersection of these rays gives the location of the plane table station on the map.
Once the station is plotted, the surveyor can start plotting new details from that point.

Common techniques of resection:

Two-point resection: Uses only two known points. It’s faster but less accurate because slight errors in orientation can lead to big mistakes.
Three-point resection: Uses three known points and is more accurate. This method is widely used and preferred in the field.
Trial and error method: The table is oriented, and rays are drawn to see if they meet at one point. If not, the table is reoriented and the process is repeated until the correct position is found.
Lehmann’s method: A special graphical technique used to correct orientation and improve accuracy when rays do not intersect at a single point.

Importance of resection:

It saves time when returning to a survey after a break.
Helps when the table is moved accidentally or intentionally.
Useful in rough terrains where chaining is difficult but known landmarks are visible.
Allows flexible continuation of surveys from new locations.

Limitations:

Requires clear visibility to at least two known points.
Slight error in orientation affects accuracy.
Less reliable if only two points are used.

Conclusion

Resection in plane table surveying is a practical method used to find the position of the instrument when it is not previously known. By sighting and drawing lines to already plotted points, the station’s location is identified. It is commonly used in fieldwork for resuming or continuing surveys efficiently and accurately.