Plane Table Surveying And Its Types | Terminologies, Errors And Advantages Of Plane Table Surveying

Introduction Of Plane Table Surveying:

• Plane table surveying is a method of conducting surveys in which a plane table is used as the primary instrument.
• The plane table itself is a flat, level table that is mounted on a tripod.
• The surveyor stands at the table and makes observations and measurements directly on its surface, allowing for on-the-spot mapping and data collection.

Plane Table and Its Types:

• The size of a plane table can vary depending on its purpose and the scale of the survey. Generally, plane tables are designed to be portable and compact for ease of transportation.
• The size of the board is usually 750 mm * 600 mm with a thickness of about 20 mm.
• The table is typically made of a smooth material, such as plywood or metal, to facilitate accurate measurements and marking.

There are different types of plane tables available, each with its own features and suitability for specific surveying tasks. Here are a few examples:

1. Simple Plane Table:

• This is the most basic type of plane table.
• It consists of a flat table mounted on a tripod.
• The surveyor uses a sighting device, such as an alidade or a telescopic sight, to measure angles and distances to determine the positions of points on the ground.
• The simple plane table requires manual calculations and is suitable for relatively simple surveys where a high level of accuracy is not critical.

2. Johnson Plane Table:

• The Johnson plane table is an improved version of the simple plane table.
• It incorporates additional features for increased accuracy and convenience.
• The Johnson table is equipped with a built-in magnetic compass, which helps the surveyor establish the magnetic meridian.
• This allows for the orientation of the table with respect to magnetic north.
• The Johnson table also includes a leveling device, such as a spirit level or a pendulum, to ensure that the table is perfectly horizontal.
• The leveling device helps maintain the accuracy of measurements taken on the table’s surface.

3. Coast Plane Table:

• The coast plane table is specifically designed for coastal surveying and mapping.
• It includes additional features that are essential for hydrographic surveys and the creation of nautical charts.
• The coast table is equipped with a tidal scale, which allows surveyors to account for changes in sea level due to tides.
• It also incorporates a tide clock, which provides information on the timing of high and low tides.
• Furthermore, the coast table includes a declinometer, which accurately measures the magnetic declination or the angle between true north and magnetic north. This information is crucial for precise navigation and mapping in coastal areas.

Alidade:

• An alidade is a sighting device used in surveying and mapping to measure angles and determine directions.
• It consists of a straight edge or ruler with a sight or telescope mounted on it.
• The surveyor uses the alidade to align with points of interest on the ground and measure the horizontal and vertical angles between them.

There are two main types of alidades: plain alidade and telescopic alidade.

1. Plain Alidade:

• A plain alidade, also known as a non-telescopic alidade or open-sight alidade, is a simple sighting device without magnification.
• It typically consists of a ruler or straightedge with a sighting notch or slit at one end.
• The surveyor looks through the slit or notch and aligns it with the target point or object.
• By reading the graduation on the alidade’s ruler or scale, the surveyor can determine the angle between the line of sight and a reference direction, such as the magnetic north or a baseline.

Plain alidades are straightforward to use and do not require complex adjustments. They are suitable for basic surveys and rough measurements where high precision is not necessary. They are commonly used in conjunction with simple plane tables or as attachments to other surveying instruments.

2. Telescopic Alidade:

• A telescopic alidade, also known as a transit or theodolite, is an alidade with a built-in telescope for magnified observations.
• It provides higher accuracy and precision compared to plain alidades.
• The telescope allows the surveyor to view targets with more clarity and make more precise angular measurements.

Telescopic alidades typically have a vertical circle or arc for measuring vertical angles and a horizontal circle or vernier for measuring horizontal angles. They also have a leveling mechanism to ensure the alidade is precisely horizontal. This leveling mechanism, often aided by a bubble level, allows for accurate measurement of both horizontal and vertical angles.

Plumbing Fork:

• A plumbing fork, also known as a plumb bob, is a tool used in surveying to establish vertical alignment.
• It consists of a weighted object suspended from a string or wire.
• Its uses include centering surveying instruments, determining the verticality of structures, checking for settlement, aligning cables or poles, and aiding in leveling and layout operations.
• A plumbing fork is a versatile tool that helps ensure accurate measurements and vertical alignment in surveying tasks.

Level Tube:

• A level tube, also known as a spirit level or bubble level, is a tool used in surveying to determine if a surface or line is level or horizontal.
• It consists of a transparent tube filled with a liquid (usually alcohol or oil) and an air bubble.
• The tube is mounted on a surveying instrument, such as a level or theodolite.
• When the bubble is centered within two marked lines or indicators on the tube, it indicates that the line or surface being measured is level.

Level tubes are essential for ensuring the accuracy of measurements and maintaining a level reference during surveying work. They are commonly used in conjunction with leveling instruments to establish horizontal planes and align measurements.

Trough Compass:

• A trough compass, also known as a compass trough or compass rule, is a tool used for drawing arcs, circles, and curves during surveying and drafting.
• It consists of a rectangular or curved ruler with a pivot point at one end and a sharp point or pencil attachment at the other end.
• The compass is placed on a drawing surface, and the pivot point allows the ruler to rotate freely.
• By adjusting the distance between the pivot point and the pencil attachment, the surveyor can create arcs and circles of various sizes.

Trough compasses are commonly used in planimetric surveying and mapping to draw accurate circular features such as curves, arcs, and circles. They are particularly useful when precision and consistency are required in drawing curved lines.

Drawing Sheet:

• A drawing sheet refers to a flat surface or paper on which surveyors and drafters create maps, plans, and other graphical representations.
• It provides a blank canvas for documenting survey data, recording measurements, and illustrating the results of surveying work.
• Drawing sheets used in surveying typically have standardized sizes, such as A0, A1, A2, or architectural sheet sizes like 24″ x 36″ or 36″ x 48″.
• These sizes provide ample space for detailed drawings and annotations.
• The drawing sheets may also include grid lines or coordinate systems to aid in accurate positioning and scaling of the survey data.
• Surveyors use drawing sheets to present their findings, communicate survey results, and create accurate maps and plans that can be used for various purposes, including land development, engineering projects, and infrastructure design.

Terminologies In Plane Tabling:

In plane table surveying, there are several terminologies that are commonly used:

1. Centering: Centering refers to the process of setting up the plane table at a specific location on the survey site. It involves placing the tripod firmly on the ground and adjusting the legs to ensure stability. The plane table is then placed on the tripod and leveled using a leveling device or a spirit level. Centering is important to ensure that the plane table is positioned accurately at the desired survey point.
2. Orientation: Orientation in plane table surveying refers to aligning the plane table with respect to a known reference direction, such as the true north or a specific baseline. This is typically achieved by sighting a distant point with an alidade or a telescope and rotating the plane table until the sighting line aligns with the desired reference direction. Proper orientation is crucial for accurate mapping and measurement.
3. Back Sight: A back sight, also known as a backsight or BS, is a surveying measurement taken from a known reference point or a previously established survey point to the plane table. It helps establish the starting point or reference for subsequent measurements. Back sights are used to determine the orientation and position of the plane table relative to the reference point.
4. Fore Sight: A fore sight, also known as a foresight or FS, is a surveying measurement taken from the plane table to an unknown point or feature of interest. It involves sighting the target using an alidade or a telescope and measuring the angles and distances to determine its position relative to the plane table. Fore sights are used to map and locate various points and features on the survey site.
5. Radiation: Radiation refers to a method of surveying in which a series of rays or lines are drawn from the plane table to various points of interest on the ground. These rays represent the directions and distances to the surveyed points. The intersection of multiple radiation lines can be used to determine the positions of points and create a map or plan.
6. Intersection: Intersection is a technique used in plane table surveying to locate a point by taking observations and measurements from two or more known reference points. The lines of sight or radiation from these reference points are drawn on the plane table, and the intersection of these lines gives the position of the desired point.
7. Resection: Resection is the opposite of intersection. It is a technique used to determine the position of the plane table itself by taking observations and measurements from two or more known points on the ground. By drawing the lines of sight or radiation from these known points on the plane table, the point where these lines intersect indicates the position of the plane table.
8. Plane Table Traversing: Plane table traversing involves a series of measurements and observations taken with a plane table to determine the relative positions of points along a traverse or a survey line. By taking fore sights and back sights and using the methods of radiation, intersection, and resection, the surveyor can map out the traversed line and calculate the coordinates or positions of the surveyed points.

Setting Up of The Plane Table:

Setting up a plane table involves a series of steps to ensure that the table is fixed securely, centered accurately, and leveled properly. Here’s an explanation of the process:

1. Fixing the Plane Table:

a. Choose a suitable location: Select a stable and flat area for setting up the plane table. Clear the ground from any obstructions or debris that may hinder the stability of the setup.

b. Attach the tripod: Place the tripod at the chosen location and unfold its legs. Attach the plane table head to the tripod securely. Make sure the tripod is stable and firmly planted on the ground.

c. Secure the plane table: Fix the plane table to the head of the tripod using the clamps or screws provided. Ensure that the plane table is firmly attached and doesn’t wobble or move.

2. Centering the Plane Table:

a. Rough centering: Roughly position the plane table at the approximate center of the tripod head. You can achieve this by eye estimation.

b. Rough leveling: Adjust the leveling screws or legs of the tripod to roughly level the plane table. This can be done by observing if the bubble on the leveling device is roughly centered.

c. Visual centering: Look through the sighting device (such as an alidade or telescope) and align it with a distant object or a target point. While keeping the sighting device aligned, gently rotate the plane table until the object or target point is visually centered in the crosshairs or sighting notch of the instrument.

d. Fine centering: Move the plane table in a small circular motion around the tripod head while observing the target point through the sighting device. Continue the circular motion until the target point remains visually centered throughout the rotation. This ensures that the plane table is accurately centered.

3. Leveling the Plane Table:

a. Bubble leveling: Use the leveling device (often a circular bubble level) on the plane table to check its levelness. Adjust the leveling screws or legs of the tripod to bring the bubble into the center position, indicating that the plane table is perfectly horizontal.

b. Cross bubble leveling (if available): Some plane tables have a cross bubble level, which allows for checking both horizontal and vertical leveling simultaneously. Adjust the leveling screws or legs to center both bubbles, ensuring accurate leveling in both directions.

c. Recheck centering: After leveling the plane table, recheck the centering by aligning the sighting device with the target point or a reference direction. The target point should remain visually centered, confirming that both centering and leveling have been accurately achieved.

Orienting the Plane Table:

• Orienting the plane table is a crucial step in plane table surveying as it establishes the reference direction and alignment of the table.

There are several methods to orient the plane table:

• orientation with a trough compass
• orientation by back sighting
• orientation by the two-point and three-point problems

1. Orientation with a Trough Compass:

a. Place the plane table on the tripod and ensure it is securely fixed.

b. Set up a trough compass on the plane table. The compass should be placed at a suitable location on the table, such as near one corner.

c. Level the trough compass by adjusting the leveling screws or legs until the bubble is centered within the leveling vials.

d. Rotate the plane table until the magnetic needle of the compass is aligned with the magnetic north-south direction.

e. Verify the alignment by sighting a known reference point or a distant object through the trough compass and aligning it with the corresponding point on the plane table.

f. Once the alignment is achieved, the plane table is considered to be properly oriented.

2. Orientation by Back Sighting:

a. Set up the plane table at the desired location and ensure it is securely fixed.

b. Select a known reference point or a previously established survey point in the vicinity.

c. Use an alidade or a telescope to sight the reference point and take a back sight measurement. This involves aligning the line of sight with the reference point and reading the angle indicated on the alidade or telescope.

d. Rotate the plane table until the line of sight coincides with the reference point on the plane table.

e. Once the line of sight and the reference point are aligned, the plane table is considered to be oriented.

3. Orientation by Two-Point and Three-Point Problems:

a. Set up the plane table at the desired location and ensure it is securely fixed.

b. Identify two or three known reference points in the survey area.

c. Choose two of the reference points for the two-point problem or three reference points for the three-point problem.

d. Sight the selected reference points using an alidade or a telescope, and plot the lines of sight on the plane table.

e. Determine the intersection point of the lines of sight. This point represents the position of the plane table.

f. Align the plane table with the calculated intersection point, ensuring that the lines of sight coincide with the plotted reference points.

g. Once the alignment is achieved, the plane table is considered to be oriented.

Errors In Plane Tabling:

In plane tabling, like any surveying method, there are several sources of errors that can affect the accuracy of the measurements and resulting mapping. Here are some common errors encountered in plane tabling:

1. Instrumental Errors: These errors arise from imperfections in the plane table and the surveying instruments used, such as the alidade or telescope. Instrumental errors can include misalignment of the sighting device, incorrect leveling of the plane table, or inaccuracies in the measurements provided by the instruments. Regular calibration and maintenance of the instruments can help minimize instrumental errors.
2. Human Errors: Human errors can occur due to mistakes made by the surveyor during the surveying process. These errors can include misreading measurements, incorrect plotting of points on the plane table, improper alignment of the sighting device, or inaccurate estimation of angles or distances. Diligent care, attention to detail, and proper training can help reduce human errors.
3. Natural Factors: Various natural factors can introduce errors in plane tabling. These can include atmospheric conditions, such as refraction or mirage, which can affect the accuracy of angle measurements. Weather conditions like wind or vibrations can also cause instability in the plane table, leading to errors. Surveyors should be aware of these factors and try to minimize their impact through appropriate adjustments and precautions.
4. Stationing Errors: Stationing errors occur when the plane table is not properly centered or leveled on the survey site. If the plane table is not accurately centered, oriented, or leveled, it can lead to significant errors in the resulting mapping. Careful attention should be paid to centering and leveling the plane table, as well as ensuring proper orientation using known reference points.
5. Observational Errors: Errors can arise from inaccuracies in observing and measuring the angles and distances to points on the ground. These errors can be caused by parallax, which occurs when the observer’s eye is not precisely aligned with the sighting device, resulting in an apparent shift in the observed point. Taking multiple observations and averaging the results can help reduce observational errors.
6. Terrain and Ground Conditions: The nature of the terrain and ground conditions can also introduce errors in plane tabling. Uneven surfaces, rough terrain, or obstacles in the line of sight can make it challenging to accurately sight and measure points. Surveyors should carefully choose their survey locations and consider the impact of the ground conditions on the accuracy of the measurements.

To minimize errors in plane tabling, surveyors should follow proper procedures, use calibrated and well-maintained instruments, pay attention to details, and account for any known sources of error. Regular checks, cross-checks, and verification of measurements can help identify and correct errors during the surveying process.

Advantages And Disadvantages Of Advantages And Disadvantages:

Plane table surveying offers several advantages and disadvantages. Here are some of the key advantages:

Advantages of Plane Table Surveying:

1. Simplicity and Ease of Use: Plane table surveying is relatively simple and straightforward compared to other surveying methods. It does not require complex instruments or advanced technical knowledge, making it accessible to surveyors with varying levels of experience.
2. Real-Time Mapping: Plane table surveying allows for real-time mapping, as the surveyor can directly observe and plot points on the plane table during fieldwork. This immediate visual feedback can be beneficial in understanding the survey area and making on-site decisions.
3. Flexibility and Adaptability: Plane table surveying can be applied in a variety of terrains and surveying situations. It is adaptable to both small and large-scale surveys and can be used in areas with limited access or difficult terrain where other methods may be impractical.
4. Relatively Low Cost: Plane table surveying equipment is generally more affordable compared to high-precision instruments used in other surveying techniques. This makes it a cost-effective option, especially for small-scale projects or when budget constraints are a consideration.
5. Improved Spatial Perception: Plane table surveying provides a direct visual representation of the surveyed area on the plane table itself. This visual feedback helps surveyors gain a better understanding of the spatial relationships between points and features, enhancing their perception of the surveyed area.

Disadvantages of Plane Table Surveying:

1. Limited Accuracy: Plane table surveying is generally considered less accurate than modern surveying methods such as GPS or total stations. The accuracy of the measurements depends on factors such as the quality of the instruments, the skill of the surveyor, and the terrain conditions. It may not be suitable for projects that require high levels of precision.
2. Time-Consuming: Plane table surveying can be time-consuming compared to other surveying techniques, especially when a large area needs to be surveyed. The process of setting up the plane table, sighting and measuring points, and plotting them on the table can be more time-intensive compared to automated methods.
3. Limited Efficiency in Data Collection: Plane table surveying requires manual observation, measurement, and plotting of points. This can be time-consuming and may limit the amount of data that can be collected within a given time frame. For projects that require extensive data collection, other methods may be more efficient.
4. Dependency on Clear Line of Sight: Plane table surveying relies on a clear line of sight between the instrument and the target points. Obstacles such as vegetation, buildings, or uneven terrain can hinder the visibility and accuracy of observations, making it challenging to survey certain areas.
5. Subject to Environmental Factors: Plane table surveying is susceptible to environmental factors such as wind, vibrations, atmospheric conditions, and natural disturbances. These factors can introduce errors and affect the accuracy of measurements, requiring additional precautions and adjustments.