Plane Table Surveying is a surveying method where field work and plotting are done simultaneously. However, like any surveying method, it is subject to various errors that can affect the accuracy of the final map or plan. Understanding these errors is crucial for obtaining reliable results.

Identifying Errors in Plane Table Surveying

Errors in Plane Table Surveying can generally be classified into several main categories based on their source. The common types of errors encountered are:

• Errors due to manipulation and sighting
• Instrumental Errors
• Errors of Plotting

Let's look at each category briefly:

Errors Due to Manipulation and Sighting

These errors arise from the surveyor's handling of the instrument and the process of taking sights. Examples include:

• Incorrect centering of the plane table over the station.
• Improper leveling of the plane table.
• Inaccurate orientation of the table (e.g., not pointing the alidade correctly at the backsight station during orientation).
• Errors in sighting through the alidade (e.g., parallax errors, not aligning the object and the fiducial edge accurately).
• Careless bisection of the object.

Instrumental Errors

These errors are caused by defects or imperfections in the surveying instruments themselves. For Plane Table Surveying, this primarily refers to the plane table and the alidade. Examples include:

• The drawing surface of the plane table not being perfectly flat.
• The edges of the alidade's sights not being parallel.
• The fiducial edge of the alidade not being straight.
• The mounting of the alidade's sights not being perpendicular to the fiducial edge.

Errors of Plotting

These errors occur during the process of drawing and marking details on the paper fixed on the plane table. This is where field measurements are translated into graphical representation. Examples include:

• Errors in drawing lines along the fiducial edge.
• Errors in measuring distances along the ray with the scale.
• Using a faulty or inaccurate scale.
• Shrinkage or expansion of the drawing paper due to changes in humidity.

Understanding Scale Error

While selecting or using a scale (which relates to plotting) can introduce errors, "Scale error" itself is not typically listed as a separate, primary category of error in the same way as Instrumental, Manipulation/Sighting, or Plotting errors are. Errors related to the scale often fall under the broader category of Plotting Errors (e.g., using an incorrect scale, errors in scaling distances on the paper). The scale is a factor in plotting, but "Scale error" isn't usually categorized as a distinct major source of error independent of the plotting process or instrument/manipulation issues.

Based on the standard classification of errors in Plane Table Surveying, Manipulation and Sighting errors, Instrumental errors, and Plotting errors are considered primary types. Scale error, while a potential source of inaccuracy, is usually subsumed within Plotting errors or considered a planning consideration rather than a fundamental category.

Therefore, Scale error is NOT a type of error in Plane Table Surveying, according to the common classification.