Plane table surveying is a widely used method in which plotting and field observations are conducted simultaneously. While this technique is efficient and effective for many surveying tasks, it is prone to various errors that can affect the accuracy of the results. Understanding these errors and their causes is essential for minimizing inaccuracies and improving survey outcomes.
This article explores the types of errors encountered in plane table surveying, categorized into three main types: instrumental errors, errors in plotting, and errors due to manipulation and sighting.
Types of Errors in Plane Table Surveying
Errors in plane table surveying can broadly be divided into the following categories:
- Instrumental Errors
These errors arise from defects or imperfections in the instruments used during the survey. - Errors in Plotting
These occur during the plotting phase due to poor quality materials or improper techniques. - Errors of Manipulation and Sighting
Personal or man-made errors caused by improper handling of instruments and sighting techniques.
1. Instrumental Errors
Instrumental errors are the most common source of inaccuracies in plane table surveying. These can occur due to:
- Uneven Plane Table Surface
If the top surface of the plane table is not flat or has undulations, it can distort the drawings and lead to significant errors. - Loose Fittings
The fittings of the tripod and plane table must be tightly fastened. Loose fittings can make the plane table unstable, leading to inaccuracies during plotting. - Magnetic Compass Accuracy
A faulty magnetic compass can provide incorrect directional readings, resulting in the wrong orientation of the plane table. - Defective Alidade Edges
The beveled or fiducial edge of the alidade must be straight. If it is curved or uneven, drawing errors will occur. - Misaligned Alidade Vanes
Both the sight vane and object vane of the alidade must be perpendicular to its base. Misalignment causes errors in sighting objects accurately. - Faulty Spirit Level or Level Tube
An improperly functioning spirit level or level tube may fail to keep the plane table horizontal, introducing errors in readings and plotting.
2. Errors in Plotting
Plotting errors occur when inaccuracies arise during the process of drawing on the plane table. The most common causes include:
- Poor Quality Drawing Sheet
Drawing sheets that expand or contract due to temperature changes can distort the scale of the drawing. Using a well-seasoned drawing sheet is essential to avoid these issues. - Thick Pencils
A thick pencil can alter the scale of the drawing, particularly in small-scale surveys. It is recommended to use a pencil with a thin and sharp tip for precise plotting.
3. Errors of Manipulation and Sighting
Manipulation and sighting errors, also known as personal or man-made errors, are often a result of improper handling or technique. These include:
- Improper Clamping
The plane table should be clamped securely to the tripod. Any movement during plotting or location changes can lead to errors. - Unstable Drawing Sheet
The drawing sheet should be firmly fixed to the plane table using thumb screws or clips. A loose or unstable sheet can cause plotting inaccuracies. - Improper Levelling
The sight vane must always be vertical while sighting objects. This requires the plane table to be levelled accurately. Poor levelling results in an inclined sight vane, leading to errors in direction. - Centering Errors
The plane table must be centered precisely over the ground point. Errors in centering can be avoided by using a plumbing fork for accurate alignment. - Incorrect Orientation
When working with multiple instrument stations, proper orientation of the plane table is essential. Improper orientation results in angular errors for plotted points. - Sighting Errors
During sighting, the object must be bisected exactly in the middle. Any deviation results in directional inaccuracies. - Unstable Tripod
The tripod must be firmly fixed to the ground. An unstable tripod introduces errors during plotting and sighting.
Conclusion
Plane table surveying, though efficient, is prone to various errors that can compromise the accuracy of the survey. By understanding and addressing the sources of these errors—instrumental defects, poor plotting materials, and improper handling—surveyors can minimize inaccuracies and improve results.