Errors in Plane Table Surveying

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Plane table surveying is a type of surveying in which plotting and field observations are done at the same time. In this article, errors encountered while conducting plane table surveying are briefly explained.

Errors in Plane Table Surveying

The errors in plane table surveying are of three types :

1. Instrumental errors
2. Errors in plotting
3. Errors due to manipulation and sighting

1. Instrumental Errors

Instrumental errors are the primary source of errors in plane table surveying which can occur in the following ways :

• Errors will occur if the top surface of the plane table is not flat or contains undulations.
• The fittings of the tripod and plane table should be tightly fastened. Loose fittings can make the plane table unstable and cause errors while drawing.
• The magnetic compass used in plane table surveying should represent accurate direction otherwise an error may occur due to the wrong orientation of plane table.
• When the beveled edge or fiducial edge of the alidade is not straight or curved, an error occurs in the drawing.
• Both sight vane and object vane of alidade should be perpendicular to the base of an alidade- if not, there will be an error in sighting.
• Faulty Spirit level or level tube may not keep the plane table in horizontal position, therefore an error may occur.

2. Errors in Plotting

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.