Errors in Plane Table Surveying

Errors may occur during plotting as well and they could be due to :

• Use of poor quality drawing sheet may affect the scale of the drawing. It is due to the expansion or contraction of drawing paper against temperature changes. Well-seasoned drawing sheet will not expand or contract due to weather fluctuations.
• The pencil used for plotting should contain thin and sharp nose. Usage of a thick pencil may alter the scale of drawing especially in the case of very small scale drawings.

3. Errors of Manipulation and Sighting

Errors of manipulation and sighting come under personal errors or man-made errors. They may occur due to any of the following cases :

• The plane table should be clamped to a tripod in such a way that it should not move while drawing or changing location. Improper clamping causes errors.
• The drawing sheet of the plane table should be firmly fixed to the plane table using thumb screws or clips. If it is unstable, plotting errors will occur.
• The sight vane should always be in a vertical position while sighting objects. To make the sight vane vertical, the plane should be levelled accurately. Improper levelling may cause inclination of sight vane with vertical axis resulting errors in plane table surveying.
• The centering error occurs when the plane table is not exactly over the ground point. So, centering should be done carefully using plumbing fork.
• When there are multiple instrument stations, the orientation of the plane table is compulsory. Improper orientation leads to errors in the angular position of points.
• Improper sighting through vanes causes errors in direction of the object. Hence, the sighting should be done in such a way that the object should be bisected exactly in the middle.
• Improperly fixed tripod can also cause plotting errors. It should be firmly fixed to the ground.