Plane Table Surveying – Overview
Plane table surveying is a type of surveying that combines plotting and field observations. In plane table surveying, the surveyor uses a plane table, an instrument mounted on a tripod, to observe distant objects and mark their locations on a plane table sheet. The surveyor then walks to the observed object and records additional data such as measurements and angles. This method is used to accurately measure and map large areas.
Common Errors in Plane Table Surveying
Despite its accuracy, plane table surveying is not free from errors. These errors can arise due to incorrect observations, incorrect calculations, incorrect plotting, incorrect levelling and incorrect measurements. Common errors include incorrect readings of angles, incorrect plotting of points, wrong levelling of the plane table, incorrect measurements and incorrect calculations. In some cases, these errors can be prevented by double-checking the observations and calculations.
Errors in Plane Table Surveying
The errors in plane table surveying are of three types :
- Instrumental errors
- Errors in plotting
- Errors due to manipulation and sighting
1. Instrumental Errors
Error Sources in Plane Table Surveying
Errors can arise in plane table surveying due to several factors. The top surface of the plane table has to be flat, with no undulations, for accurate results. Moreover, the tripod and plane table must be tightly fastened to avoid instability. The magnetic compass used should also be accurate in providing the direction. Additionally, both the sight vane and object vane of the alidade should be perpendicular to the base of the alidade. Lastly, the Spirit level or level tube should be in working condition to keep the plane table in a horizontal position.
2. Errors in Plotting
Poor Quality Drawing Sheet May Affect Plotting Scale
Poor quality drawing sheets can cause errors in plotting due to their susceptibility to expansion or contraction with temperature changes. This can cause the scale of the drawing to become distorted, and it is important to use a well-seasoned drawing sheet to ensure accuracy in plotting.
Using the Right Pencil for Plotting
Using the right pencil for plotting is essential to avoid errors in plotting. A thin, sharp pencil should be used to ensure accuracy, especially when dealing with very small scale drawings. Using a pencil with a thick tip can distort the scale of the drawing, resulting in errors.
3. Errors of Manipulation and Sighting
Errors of Manipulation and Sighting
Manipulation and sighting errors can be classified under personal errors or man-made errors. These errors can occur due to a number of cases, such as improper clamping, unstable drawing sheet, inaccurate levelling, centering errors, improper orientation and improper sighting.
Improper Clamping
One of the main causes of errors in manipulation and sighting is improper clamping of the plane table to a tripod. The plane table should be firmly clamped in place so that it does not move while drawing or changing location. If not clamped correctly, errors can occur.
Unstable Drawing Sheet
The drawing sheet of the plane table should be firmly fixed to the plane table using thumb screws or clips. If the sheet is not firmly fixed, errors can occur while plotting.
Inaccurate Levelling
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. If the levelling is not done properly, the sight vane can be inclined with the vertical axis, resulting in errors in plane table surveying.
Centering Errors
Centering errors can occur when the plane table is not accurately placed over a ground point. To avoid these errors, centering should be done with precision using a plumbing fork.
Improper Orientation
When there are multiple instrument stations, the orientation of the plane table is compulsory. If the orientation is not done properly, errors can occur in the angular position of points.
Incorrect Sighting
Improper sighting through vanes can lead to errors in the direction of the object. To avoid this, the sighting should be done in such a way that the object is bisected exactly in the middle.
Improperly Fixed Tripod
An improperly fixed tripod can also cause plotting errors. To ensure accuracy, the tripod should be firmly fixed to the ground.
