Plane table surveying is the graphical method of surveying that is carried out to prepare maps, and plans and also to collect details (like topographic details) through field observations. This surveying is carried out utilising a plane table hence it is named as plane table surveying. The plane table generally refers to the instrument that is used in surveying and provides a solid and level surface on which field drawings, maps and charts are made. 

It is a fast and cheap method of surveying. Another benefit of using this method of surveying is that the omission of observations is eliminated, as the surveyor directly plots the details in the field. This surveying is a relatively fast method of surveying. The method employed is also simple and easy to perform.

As Plane Table Surveying is used extensively in the construction industries, it is essential to have a proper idea about the same. As far as the syllabus of various examinations like SSC JE CE, RRB JE Civil and GATE CE as well, the concept of Plane Table Surveying has become very important. This comprehensive article below will deal with Plane Table Surveying, including its principles, instruments, adjustments and many more. 

What is Plane Table Surveying?

Plane table surveying is a graphical method of land surveying where observations and plotting are done directly in the field. This technique allows surveyors to draw the map on-site while taking measurements, which reduces the chance of errors during data transfer. It is commonly used for small-scale surveys like property boundaries, road layouts, and field mapping.

The basic idea behind plane table surveying is to draw lines on the sheet that match the direction of features on the ground. When the table is properly oriented, each line on the paper runs in the same direction as the actual object it represents. This allows the surveyor to create a scaled drawing directly in the field by aligning the alidade with real-world features and sketching their positions accurately.

Instruments Used in Plane Table Surveying

To carry out plane table surveying effectively, several simple instruments are required. These tools help the surveyor to sight objects, measure directions, level the board, and plot points on the sheet. Since this is a graphical method, accuracy depends a lot on how well the instruments are handled and maintained. Here are the essential instruments used in plane table surveying:

• Plane Table: A flat wooden drawing board mounted on a tripod, used to hold the drawing sheet and serve as the base for plotting.
• Alidade: A sighting device used for drawing straight lines toward the objects being surveyed. It has a ruler and two sight vanes.
• Trough Compass: A small compass mounted in a trough to determine magnetic north and help with orientation of the table.
• Spirit Level: A leveling device used to ensure that the board is horizontal, which is important for accuracy.
• Plumbing Fork and Plumb Bob: These help in centering the table directly over the ground station point.
• Drawing Accessories: These include pencils, drawing sheets, clips, pins, erasers, and a scale for drawing measurements neatly on the paper.

Methods of Plane Table Surveying

Plane table surveying offers flexibility in how data is collected and mapped, depending on the terrain and purpose. Different methods can be used to plot objects and boundaries based on how many stations are involved and whether direct sighting is possible. Each of the four major methods serves a specific need, from simple point plotting to complex positioning when the station itself is unknown.

Methods of Plane Table Surveying

Objectives of plane table surveying 

• Creating a detailed and accurate map of the surveyed area.
• Establishing control points and reference markers for future surveys or construction projects.
• Determining the distances, angles, and directions between different points on the ground.
• Gathering data for designing infrastructure projects, such as highways, pipelines, and railways.
• Assisting in land management and land development projects.
• Determining the distances, angles, and directions between different points on the ground.

Setting Up the Plane Table

To get accurate results during a plane table survey, it is important to set up the table correctly at every survey station. Proper setup ensures that the plotted lines on the sheet match the directions and distances on the ground. Any error in this stage can lead to incorrect mapping. The setup involves aligning, leveling, and securing the equipment before actual observations begin. Below are the key steps involved in setting up the plane table:

• Centering: This involves placing the plane table exactly above the ground point marked for observation. A plumbing fork with a plumb bob is used for this task.

• Levelling: The table is adjusted to become perfectly horizontal using a spirit level. This ensures accuracy when drawing lines on the sheet.

• Orientation: The table is turned so that the directions on the paper match the directions on the ground. This is done either by using a compass (trough compass) or by back-sighting from a known point.

• Fixing the Sheet: The drawing sheet is pinned or clipped firmly to the board so that it does not move during the survey.

Plane Table Surveying: The Basic Idea Behind It

Plane table surveying is a method that involves placing a drawing board, known as a plane table, on the ground and using it as a reference surface. The surveyor visually observes and plots the positions of points on the ground directly onto the drawing sheet. By measuring angles and distances, the relative positions of these points can be determined and mapped accurately.

Plane Table Surveying Instruments

Following are the accessories of plane table surveying: 

Drawing Board

The drawing board is made up a of well-seasoned wood, so as to counteract the effect of warping and damages due to weathering. The upper surface of the drawing board is kept smooth. The table at the centre of the underside is attached to the tripod with the help of a screw and wing nut. The table can be clamped in any position using the wing nut. Plane tables are available in the following different sizes as follows; 

Fig.1. Drawing Board

Alidade

An alidade is a straight-edge ruler having some sighting device. It is used for sighting objects and drawing lines in Plane Table Surveying. Plane Alidade is a straight-edge ruler about 450 mm long, made of metal or wood. One of the edges is bevelled and graduated. The alidade is provided with a sight vane at each end. The sight vanes have hinges at the lower end so that they can be folded down on the ruler when not in use. One of the sight vanes is provided with a narrow slit and is used as an eye vane. 

The object vane is open, and it carries a hair or thin wire at its centre. The two sight vanes are open, and it carries a hair or thin wire at their centre. The two sight vanes provide a definite line of sight. The bevelled edge of the ruler is also known as the fiducial edge. The line of sight of the alidade is in the same plane as that of the fiducial edge or in a plane parallel to it. Nowadays, Telescopic Alidades are also much in use, in place of Plane Alidades. When the points too high or low are to be sighted, the accuracy and the range are considerably increased by providing a telescopic alidade.

Fig.2. Alidade

Spirit level

The other name used for spirit level is level tube. The essential condition in plane table surveying is that the board should be level. This level tube is either tubular or circular.

It is placed on the board in two positions mutually at right angles, and the bubble is centred in each position to make the board horizontal.

Fig.3. Spirit Level

Trough Compass

It is one of the vital instruments employed in plane table surveying. Generally, it is 15 cm long and is provided to plot the magnetic meridian (N-S direction) to facilitate the orientation of the plane table in the magnetic meridian. At the extremities of the trough compass, there are graduated scales with zero at the centre and marking up to 5° on either side of the zero line. Longer sides of the trough compass are parallel and plane such that they can be used as a ruler for drawing the line or for placing the compass such that it coincides with a line already drawn on the drawing sheet. 

Fig.4. Trough Compass

U-Frame or Plumbing Fork

A plumbing form is a U-shaped piece of the metal frame. It is used for the centering of the plane table over the station.

One end of the frame is pointed and is kept over the drawing sheet touching the plotted position of the instrument station, while the other end of the frame carries a plumb bob.

The position of the plane table is adjusted until the plumb bob hangs over the station occupied by the instrument.

The use of a plumbing fork is justified only if the plotting is done at a large scale and the rays are short. Whereas, small-scale mapping, which is usually done with a plane table, using a plumbing fork is a sheer waste.

Fig.5. Plumbing Fork

Waterproof Cover

During rainy days, a waterproof cover is a must so that the surveying operation can go on without much disturbance. 

Paper or Drawing Sheet

The paper or drawing sheet used in Plane Table Surveying should be of the best quality to withstand rubbing and scrubbing. 

Tripod

An open frame type light tripod is usually used in the simplest form of plane tables. Levelling of the board is achieved with the tripod legs and checking the horizontality of the board with the help of two spirit levels fixed at right angles to each other in a block of wood.

Fig.6. Tripod

Temporary Adjustments of Plane Table

Surface of the board should be perpendicular to the vertical axis of the instrument which can be achieved by placing a spirit level over the plane table.

The Fiducial Edge of the Alidade should be a straight line which can be checked by drawing a line along the ruling edge, reverse the alidade and place it against the ends of the line. Again a line is drawn, which should coincide with the previous line. If the two lines do not coincide, the edges are corrected.

The Two Vanes should be perpendicular to the base of the alidade and can be done by setting the alidade on the corner edge of a building or on a suspended plumb bob. If they do not coincide, adjust the hinges till the vane coincides with the plumb line. 

Levelling

In plane table surveying, it is the operation of bringing the plane table in a horizontal plane. Levelling of the board is done with the help of a spirit level and by employing one of the following methods:

• Application of ball and socket arrangement.
• Tilting of the board ordinarily.
• Adjusting the legs of the tripod.

Orientation

• It is the operation of keeping the plane table parallel to the position it occupied at the first station. In such a case, all the lines plotted will be parallel to the corresponding lines on the ground.
• If the position of the board is different at successive stations, the relative positions of the plotted details will not remain the same as the relative positions of the details on the ground.
• As a result, the plotted work of the previous stations cannot be connected to that of the successive stations.
• During orientation the table is rotated and the plotted position of the instrument station is also disturbed and shifts relative to the ground stations.
• Therefore, operations of orientation and centering are therefore interrelated. Orientation of the plane table can be closed using a trough compass, back sighting or by resection. 

Centering

• It is the operation of bringing the plotted station point exactly over the ground station. A Plumbing Fork is used for centering the plane table.
• First of all, the pointed end of the plumbing fork is kept on the paper while the other end is attached with a plumb bob.
• Then, the board is continuously moved until the bob hangs exactly over the peg of the station.
• It should be noted that exact and proper centering is done only in case of large-scale mapping. And for small-scale mapping, an error of about 30 cm is permissible. 

Arrangement of Plane Table Surveying Equipment

Plane Table Surveying equipment is arranged in 4 steps as follows:-

Fixing of Plane Table

Attach the tripod stand to the plane table. Using paper clips or thumb screws, arrange the drawing sheet on the plane table. From first to last, the sheet should be in one position.

Levelling of Plane Table

It is the operation of bringing the plane table in a horizontal plane. Levelling of the board is done with the help of a spirit level. 

Centering of Plane Table

The table should be centered by using a plumbing fork, by which we can arrange the plotted point exactly over the ground point. 

Orientation of Plane Table

Whenever we are using more than one instrument station, orientation is essential. It can be done by using a compass or back sighting. In this case, the plane table is rotated such that plotted lines in the drawing sheet are parallel to corresponding lines on the ground. 

Methods of Plane Table Surveying

There are four methods of plane table surveying that are discussed below. 

Radiation method of plane table surveying 

• In this method, the instrument is set up at a station and rays are drawn to various stations which are to be plotted.
• The distances are cut to a suitable scale after actual measurements. This method is suitable only when the area to be surveyed is small and all the required stations to be plotted are clearly visible and accessible from the instrument station.
• The scope of the method is increased when the distances are measured with the help of a tacheometer.

Intersection method of plane table surveying

• In this method, two stations are selected such that all the other stations to be plotted are visible from these. A line joining these two stations is called a base line. The length of this base line is measured very accurately. 
• Rays are drawn from these stations to the station to be plotted. The intersection of the rays from these two stations gives the position of the station to be plotted on the drawing sheet. Sometimes, this method is also called graphical triangulation.
• This method is most commonly used for plotting details. It is preferred when the distance between the stations is too large, the stations are inaccessible, or the ground is undulating. For example broken boundaries, which can be very conveniently plotted by this method.

Traversing method of plane table surveying 

• This method is similar to compass or theodolite traversing. The table is set at each of the stations in succession.
• A foresight is taken to the next station and the distance is cut according to a suitably chosen scale.
• This method is most suited when a narrow strip of the terrain is to be surveyed, e.g. survey of roads, railways, etc.
• This method can be used for traversing both open as well as close traverses.

Resection method of plane table surveying

• This method of orientation is employed when the plane table occupies a position not yet plotted on the drawing sheet.
• Resection can be defined as the process of locating the instrument station occupied by the plane table by drawing rays from the stations whose positions are already plotted on the drawing sheet.
• The point representing the resection of two rays will be the station to be located, provided the orientation at the station to be plotted is correct, which is seldom achieved. This problem can be solved by any of the methods, such as resection after orientation by back ray, by two points, or by three points.
• This method is employed when the surveyor feels that some important details can be plotted easily by choosing any station other than the triangulation stations. The position of such a station is fixed on the drawing sheet by resection.

The Three-Point Problem

Three points and their positions in the field are known in this condition. The plane table is positioned in such a way that all three points are visible. So, by sighting those three points, we can find the location of the equipment. This can be accomplished in a variety of ways, including the ones listed below:

• Tracing Method
• Lehmann Method
• Analytical Techniques
• Graphical method

Tracing Method

The plane table in the tracing method is located at a point from which three points are visible. The table is oriented in relation to the three points' plotted lines. Place the tracing paper on the drawing sheet and sight the three points again, this time plotting the radiating lines. The tracing paper is then moved above the drawing sheet until the three radiating lines intersect the previously plotted points on the map. Finally, the position of the plane table is marked.

Lehmann Method

This method is also known as the trial and error method. It is very commonly used in field measurements because it gives fairly accurate results. 

Analytical Methods

In this method, various points are sighted from the station and values of angles and lengths are recorded. From these values, the location of unknown points can be determined easily.

Graphical Method

In this method too, the angles and lengths are recorded by sighting from the plane table station and then the recorder data is plotted on the graph. Finally, the location of unknown points can be determined using these data. 

The Two-Point Problem in Plane Table Surveying

The two-point problem consists in locating the station occupied by the plan table on the plan. The observation of two well-defined points visible from the instrument station and whose positions have already been plotted on the plan is chosen. Then, a new station at the required position is established by perfectly bisecting these points.

Fig.7. Two-Point Problem

Following are the procedures to be adopted for the Two-point Problem:

Suppose P and Q are two well-defined points whose positions are plotted on map and q. It is required to locate a new station at A by perfectly bisecting P and Q.

1. Another station B is selected on the ground, which is visible from A.
2. Set the plane table at B, with touch of p and q  bisect the P and Q with the help of alidade.
3. Alidade is centred on b bisecting the point a_{1}.
4. Set the table on A. Bisect the point P along p and ray is drawn, same method Q and q drawn.
5. The alidade is placed along the line PQ and the table is turned to bisect R. At this position the table is said to be perfectly oriented.
6. The alidade is centred on p and q, the points P and Q and bisected rays are drawn. These rays intersect at a point a. This would be the exact position of A.

Errors in Plane Table Surveying

• Instrumental Errors: These errors are caused by imperfections or misalignment of the plane table, alidade (sighting device), or other surveying instruments used in the process.
• Observational Errors: Errors can occur due to inaccurate readings or observations made by the surveyor while measuring angles, distances, or directions.
• Natural Errors: Environmental factors such as atmospheric refraction, wind, temperature changes, and uneven terrain can introduce errors in the survey measurements.
• Human Errors: Mistakes or inaccuracies in reading, recording, or plotting the observations can lead to errors in the final survey results.
• Scale and Projection Errors: Errors may occur during the process of scaling down the measurements to fit the drawing sheet or during the process of projecting the surveyed points onto the map.

Advantages and Disadvantages of Plane Table Surveying

Plane table surveying is a popular technique for quick and simple surveys, especially where direct visual access to the area is possible. However, it is not suitable for every situation. Understanding its strengths and limitations helps in deciding when to use this method.

The table below highlights the key advantages and disadvantages:

Precautions To Take During Plane Table Surveying 

The following are some of the precautions that must be taken during plane table surveying:

• The plane table must be properly oriented at each station, or else the lines drawn from the new station will not correspond to the previous lines.
• The rays must be carefully drawn using only a straight ruler.
• The drawing sheet must be held in place with paper clips. This eliminates the possibility of the drawing sheet contracting or expanding.
• To ensure accuracy, check lines must be plotted.
• The alidade should be carefully centered.

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