Errors in Plane Table Surveying
Plane table surveying, while a useful technique, is subject to various errors that can affect the accuracy of the final map. These errors can be broadly categorized into four main types:
- Instrumental Errors
- Plotting Errors
- Manipulation and Sighting Errors
- Errors due to Inaccurate Centering
1.Instrumental Errors
Instrumental errors arise from imperfections or defects in the surveying equipment. These errors can significantly impact the accuracy of the survey if not properly addressed:
a) Plane Table Surface:
- The top surface of the plane table may not be perfectly flat, potentially causing distortions in the drawn map.
- Warping of the table surface due to environmental factors (humidity, temperature) can introduce errors over time.
- Scratches or dents on the surface can affect the smoothness of drawing and measurements.
b) Alidade:
- The fiducial edge of the alidade may not be perfectly straight, leading to inaccurate sightings.
- The sighting vanes may be misaligned or bent, causing angular errors in observations.
- Wear and tear on the alidade’s moving parts can affect its precision over time.
c) Equipment Fittings:
- Loose fittings between the table and tripod can cause instability during surveying.
- Worn out leveling screws can make it difficult to achieve perfect horizontality.
- Faulty locking mechanisms may allow the table to shift during observations.
d) Magnetic Compass:
- A defective magnetic compass can lead to incorrect orientations.
- Magnetic interference from nearby metal objects or geological formations can affect compass readings.
- Declination errors if the compass is not properly adjusted for local magnetic variation.
e) Spirit Level:
- An inaccurate or damaged spirit level can result in the plane table not being truly horizontal.
- Bubbles in the spirit level may be sluggish or overly sensitive, leading to leveling errors.
f) Plumb Bob:
- An improperly weighted or damaged plumb bob can cause centering errors.
- Wind interference can affect the stability of the plumb bob, especially in open areas.
g) Drawing Sheet:
- Poor quality or unsuitable drawing paper can lead to distortions when exposed to humidity or temperature changes.
- Improper attachment of the drawing sheet to the plane table can cause shifting during surveying.
h) Measuring Accessories:
- Inaccurate or poorly maintained scales, protractors, or other measuring tools can introduce errors in plotting and measurement.
- Thermal expansion of metal rulers in extreme temperatures can affect precise measurements.
i) Trough Compass:
- Errors in the trough compass can lead to incorrect orientation of the entire survey.
- Magnetic anomalies or nearby metal objects can affect the accuracy of the trough compass.
Instrumental Errors Minimization Techniques
To minimize instrumental errors:
- Regularly inspect and maintain all equipment.
- Calibrate instruments against known standards periodically.
- Use high-quality, well-maintained equipment.
- Be aware of environmental factors that can affect instrument performance.
- Check for local magnetic anomalies or interference before relying on compass readings.
- Employ proper techniques for setting up and using each piece of equipment.
2.Plotting Errors
Plotting errors are a common source of inaccuracy in plane table surveying. These errors can significantly impact the final map’s quality and reliability:
a) Line Quality:
- Drawing fine, precise lines is crucial to minimize plotting errors.
- Inconsistent line thickness can lead to ambiguity in point and feature locations.
- Use of inappropriate drawing instruments (e.g., dull pencils) can reduce precision.
b) Error Accumulation:
- Errors tend to accumulate when the survey is extended from a small base to a larger area.
- This accumulation can result in significant distortions in the overall map layout.
- The effect is more pronounced in surveys covering large areas or with many intermediate stations.
c) Scale Errors:
- Incorrect application of scale can lead to systematic errors throughout the entire map.
- Failure to account for paper shrinkage or expansion can introduce scale errors over time.
- Inconsistent scale application across different parts of the map can cause distortions.
e) Intersection Errors:
- Inaccurate plotting of intersecting lines can lead to misplacement of features.
- As the angle of intersection between lines becomes more acute, the potential for error increases.
- Multiple intersections for a single point can help reduce this error, but care must be taken to resolve discrepancies.
Plotting Errors Minimization Techniques
To minimize plotting errors:
- Use high-quality, appropriate drawing instruments and materials.
- Employ precise drawing techniques, including the use of fine lines and careful measurement.
- Regularly check and verify plotted positions against known control points.
- Use mechanical plotting aids when possible to increase precision.
- Implement a system of checks and balances, such as closing traverses or using resection techniques.
- Be aware of the limitations of the plane table method, especially for large or complex surveys.
- Consider using digital plotting methods or computer-aided drafting tools for final map production to enhance accuracy and consistency.
3.Manipulation and Sighting Errors
These errors often result from surveyor carelessness or lack of attention to detail:
a) Non-horizontality of the Board:
- If the table top is not perfectly horizontal, especially perpendicular to its edges, both vanes of the alidade will be inclined, leading to incorrect point locations.
- This error can be magnified over longer distances and can significantly affect the accuracy of intersections.
- Regular checking and adjustment of the table’s level is crucial throughout the survey process.
b) Centering Errors:
- Inaccurate centering of the point on the sheet over the ground mark can introduce orientation errors, particularly significant in large-scale surveys.
- This error can propagate through the entire survey, affecting all subsequent measurements and plotted points.
- Precise centering becomes increasingly important as the scale of the survey increases.
c) Defective Sighting:
- Proper bisection of the ground station along the eye vane is crucial to avoid intersection errors.
- Errors can occur due to parallax if the surveyor’s eye is not properly aligned with the sighting vanes.
- Atmospheric conditions such as heat haze can affect the accuracy of sightings, especially over longer distances.
d) Board Movement:
- Any movement of the board after clamping and during sighting can lead to intersection errors.
- This can be caused by unstable ground, wind, or accidental bumping of the equipment.
- Even small movements can result in significant errors, especially when working at large scales or over long distances.
e) Incorrect Orientation:
- Failure to properly orient the plane table at each station can lead to systematic errors throughout the survey.
- This can occur due to improper use of the compass, misalignment with backsight points, or errors in the orientation process.
- Consistent checking of orientation using multiple methods (e.g., backsighting, compass, resection) can help minimize this error.
f) Parallax Errors:
- Parallax errors can occur when the surveyor’s eye is not properly aligned with the alidade sights.
- This misalignment can cause objects to appear in slightly different positions, leading to inaccurate plotting.
- Proper technique in aligning the eye with the sights and using any available fine-tuning mechanisms on the alidade is crucial.
Manipulation and Sighting Error Minimization Techniques
To minimize manipulation and sighting errors:
- Develop and follow a strict protocol for setting up and orienting the plane table at each station.
- Regularly check and adjust the level of the plane table throughout the survey.
- Use proper sighting techniques, including careful alignment of the eye with the alidade sights.
- Be aware of environmental factors that can affect sighting accuracy, such as heat haze or wind.
- Take multiple readings and use averaging techniques when possible to reduce the impact of individual errors.
- Implement a system of checks, such as closing traverses or resection, to verify the accuracy of the survey periodically.
- Provide thorough training to surveyors on proper techniques and potential sources of error.
4.Errors due to Inaccurate Centering
Inaccurate centering can introduce significant errors, especially in large-scale surveys and for short sights:
a) Angular Error:
- The difference between the actual angle on the ground and the plotted angle depends on the distances between stations and the centering error.
- This error can be particularly significant when working with short sights or in areas with high relief.
- The angular error can be calculated using trigonometric functions based on the centering offset and sight distance.
b) Positional Errors:
- Inaccurate centering leads to displacement of plotted points from their true positions.
- The magnitude of this displacement increases with distance from the station point.
- Positional errors can compound when multiple stations are involved in determining a single point.
c) Scale Dependency:
- The impact of centering errors varies with the scale of the survey. For small-scale surveys with longer sights, precise centering is less critical.
- As the scale increases (e.g., 1:500 vs 1:5000), the importance of accurate centering becomes more pronounced.
- The relationship between scale and centering accuracy is not linear; errors become exponentially more significant at larger scales.
d) Acceptable Error Limits:
- A formula is provided to calculate the acceptable centering error based on the map scale: d ≤ n/40 Where: d = acceptable centering error in metres n = number of metres represented by 1 cm on the map
- This formula helps surveyors determine the level of precision required in centering for a given map scale.
- It’s important to note that this is a general guideline, and more stringent standards may be necessary for high-precision surveys.
e) Practical Considerations:
- For very small-scale surveys, setting up the table within a reasonable radius (e.g., 5 meters for a 1:50,000 scale map) may be sufficient.
- In urban or built-up areas, physical constraints may limit the ability to achieve perfect centering, requiring surveyors to adapt their techniques.
- The use of centering devices or plumb bobs can significantly improve centering accuracy.
f) Error Propagation:
- Centering errors at one station can propagate through the entire survey, affecting all subsequent measurements.
- This propagation can lead to cumulative errors that may not be immediately apparent but can significantly impact the final map accuracy.
g) Mitigation Strategies:
- Use of forced centering devices or tribrachs can help maintain consistent centering between instrument setups.
- Implementing a traverse or resection method to periodically check and adjust for accumulated errors.
- Employing digital centering aids or laser plummets in modern surveying instruments can greatly enhance centering accuracy.
Centering Error Minimization Techniques
To minimize centering errors:
- Use appropriate centering devices and techniques based on the survey scale and required accuracy.
- Regularly check and adjust centering throughout the survey process.
- Be aware of the limitations imposed by the survey environment and adapt techniques accordingly.
- Implement error-checking procedures, such as closing traverses or using multiple control points.
- Consider using more advanced surveying techniques for high-precision requirements, especially at large scales.
