After fixing the plane table to the tripod, the main operations that are needed at each plane table station are
a. Levelling
b. Orientation
c. Centering

The correct sequence for this operation is

Explanation:

Levelling – First

• The plane table must first be levelled using a spirit level so that all angles and directions can be plotted correctly.

• Levelling ensures the drawing sheet lies in a horizontal plane, which is essential for accurate measurement.

• It is the first and foundational step before proceeding with any plotting.

Centering – Second

• After levelling, the table is centered over the station point using a plumb bob or plumbing fork.

• This ensures that the plotted station point on the drawing sheet lies exactly over the corresponding ground station.

• Proper centering avoids positional errors in the fieldwork and plotting.

Orientation – Third

• Once levelled and centered, the table is oriented using a backsight or compass, aligning it with the previous station or magnetic north.

• Orientation ensures that the direction of the lines drawn corresponds correctly to ground directions.

• Without correct orientation, angular measurements will be inconsistent.

Additional Information

• Plane table survey is a graphical method of surveying where field observations and plotting are done simultaneously on a drawing sheet.

• It operates on the principle of intersection and radiation, allowing direct plotting of angles and distances from a fixed station.

• It eliminates the need for field notes and is ideal for small-scale, moderately accurate surveys.

• The main instruments include a plane table, alidade, spirit level, plumb bob, tripod, and drawing accessories.

• The alidade is used for sighting and drawing rays, while the plumb bob helps in centering the table.

• A spirit level ensures proper levelling, and the tripod provides stability during observations.