Short Answer:
Errors in plane table surveying are small mistakes that happen during fieldwork, which affect the accuracy of the map or plan. These errors may occur due to improper leveling, incorrect centering, poor orientation, instrument defects, or human mistakes like wrong sighting or plotting.
The most common errors include misalignment of the table, incorrect ray drawing, and movement of the drawing sheet. These can be reduced by using instruments carefully, checking orientation properly, and making sure the table is stable and level throughout the survey process.
Detailed Explanation:
Errors in plane table surveying
Plane table surveying is a method where both observation and plotting are done at the site. While this method is simple and fast, it is also sensitive to errors. Since plotting is done directly in the field, any small mistake can lead to a wrong location being marked on the map. Therefore, it is important to understand the possible sources of error in order to avoid them and ensure the accuracy of the survey work.
1. Centering errors
This occurs when the table is not placed exactly over the survey station on the ground. Improper centering causes all plotted lines to be slightly shifted, which affects the accuracy of the final map. It becomes more noticeable when distances are long.
Prevention: Use a plumbing fork and plumb bob correctly and double-check before fixing the tripod.
2. Leveling errors
The plane table must be perfectly horizontal before use. If not properly leveled, sighting and drawing will be tilted, which leads to angle and distance errors.
Prevention: Use a spirit level and check leveling from more than one side of the board.
3. Orientation errors
Orientation means aligning the drawing sheet correctly in relation to the ground. If the table is not rotated properly to match the previous direction, all plotted points will be wrongly placed.
Prevention: Use a compass or sight previously plotted points using an alidade for correct orientation.
4. Sighting errors
These happen due to improper use of the alidade. If the alidade is not aligned properly or if the line of sight is disturbed, the drawn rays will be incorrect.
Prevention: Use a good-quality alidade and sight carefully without shaking the board.
5. Plotting errors
Plotting errors occur when measurements are not scaled correctly or lines are not drawn properly. Drawing with dull pencils, shaky hands, or on loose sheets can also cause mistakes.
Prevention: Use sharp pencils, proper scales, and draw lines gently with the help of rulers or straight edges.
6. Instrumental errors
If the instruments like the plane table, alidade, or spirit level are damaged, old, or not calibrated, they will give incorrect results.
Prevention: Always inspect instruments before use and replace damaged ones.
7. Sheet movement
If the drawing sheet moves during the survey, the plotted lines will not remain accurate. This is one of the most common and serious issues in fieldwork.
Prevention: Fix the drawing sheet tightly using clips or thumbtacks and protect it from wind.
8. Environmental factors
Strong wind, bright sunlight, or uneven ground can disturb the setup. A shaking table or unclear vision can lead to further mistakes.
Prevention: Use shading tools or work during calm weather. Avoid surveying during rain or heavy wind.
Conclusion
Errors in plane table surveying can happen due to poor centering, leveling, orientation, or plotting. Careful handling of instruments, accurate sighting, and proper drawing techniques help in minimizing these mistakes. Understanding these errors is important for ensuring accuracy and reliability in survey maps and fieldwork.