Plane Table Surveying - Instruments and accessories, Method of setting up, Advantages & Disadvantages, Procedure

What is Plane Table Surveying?

Plane table surveying is a method of surveying that involves observing and recording the horizontal and vertical positions of objects from a fixed point (the plane table) using an alidade (a straight edge with sights or a telescope). The surveyor places the plane table at a point of interest and levels it using a spirit level. The alidade is then used to sight various features on the ground, and the surveyor notes the angles and distances to each feature on a sheet of paper attached to the table. These measurements are then used to create a map or plan of the surveyed area. Plane table surveying is often used in areas where traditional surveying equipment may not be practical or accurate enough.

Instruments and accessories used in Plane Table Surveying: -

Plane table surveying

1. Plane Table: A table that is levelled and used as a base for surveying.

The plane Table is made of well-seasoned good quality teak or pine wood and is available in different sizes,

• Small size plane table – 50cm x 40cm x 1.5cm
• Medium size plane table – 50cm x 50cm x 1.5cm
• Large size plane table – 75cm x 60cm x 1.5cm

2. Alidade: An instrument used for measuring angles.

3. Spirit level: A tool used to identify whether something is levelled.

4. Tripod stand: Used to support the plane table at the required height.

5. Plummet: A small weight that is used to check if the instrument is levelled vertically.

6. Drawing board: Where a sheet of paper is attached and used to draw sketches of the survey site.

7. Chain: Used to measure distances in the survey area.

8. Ranging rod: A rod that is used to mark distances for measuring right angles.

9. Compass: Used to determine the bearing of a survey line.

10. Pegs: Used to mark the boundaries of the survey area.

11. Poles: Used to mark the location of various points in the survey area.

12. Drawing instruments: Includes set squares, protractors, and other tools used to make accurate sketches of the area.

Method of setting up the plane table:

There are three methods involved in the setting up of a plane table in plane table surveying. They are,

1. Leveling the plane table

2. Orienting the plane table

3. Centering the plane table

1. Levelling: The first process involves levelling the table using a spirit level. This ensures that the table is placed on a level surface so that accurate readings can be taken.

2. Orientation: The second process involves orienting the table in the correct direction. This is done by sighting a prominent point in the field and aligning the table with it using the sighting rule.

Orientation of the plane table shall be done in two ways,

• Orientation by back sighting
• Orientation by a magnetic needle

 i. Orientation by backsight:

Let’s understand this through an example. Let’s take two stations on the ground namely, A and B. Consider A as the first station and B as the second station.

Now, the plane table is leveled and centered at station A. The ground station A is brought over the drawing sheet using a U fork and it is marked as “a”.

Now mark a point on the sheet using a small pin. Now place the working edge of the alidade in touch with the pin and sight station B. When station B is sighted, clamp the plane table in position.

Now measure the distance between A and B on the ground and plot it on the sheet with a respective scale and mark the station B on paper as “b”.

Now bring the plane table to B and sight A and do the same process to check the line ab on the sheet.

This method gives a better orientation of the plane table and is always preferred.

ii. Orientation by the magnetic needle:

Let’s take two stations A and B on the ground.

The plane table is set up at the station A and leveled using the spirit level. The centering is done by U fork with plumb bob so that the point a on the sheet is exactly over station A.

Then the trough compass is placed on the right-hand top corner of the sheet. Adjust the table in such a way that the compass shows a “0-0” direction (North). Then clamp the table in position and draw a line indicating north on the sheet.

Now place the alidade by touching point ‘a’ on the sheet and try to sight the station B. When station B is bisected using a sight vane, measure the actual distance on the ground between A and B and plot it in the sheet using a suitable scale (like 1cm =10m) and mark the point b on the sheet.

Now the table is shifted to the station B and centered over B so that the point b on the sheet is just over station B. After checking leveling and centering, the trough compass is placed in the sheet in the exact same position along the north line precisely.

Adjust the table clockwise or anticlockwise until the needle coincides exactly with the “0-0” mark of the compass.

While turning the table, care enough care should be taken not to disturb leveling. If disturbed, leveling needs to be adjusted immediately.

When centering, leveling and the needle is exactly at 0-0 at station B, the orientation is said to be perfect.

This method is suitable when the local attraction is not suspected in the surveying area.

3. Centering: The third process involves centering the instrument over the point to be surveyed. This is done by sighting two or more known points on the field and adjusting the table until the lines of sight intersect at the point to be surveyed.

Why orientation of the plane table is important?

The orientation of the plane table is important for accurate measurement and mapping of the features of the terrain. If the plane table is not properly oriented, the measurements taken will be distorted and inaccurate. A misaligned plane table can lead to errors in the measurement of angles and distances, which can ultimately result in incorrect mapping. Therefore, it is important to ensure that the plane table is carefully aligned to the true north and levelled before making any measurements.

Advantages of Plane Table Survey: -

1. Efficiency: Plane table survey is considered to be very efficient and time-saving. It enables quick and accurate mapping of a region.

2. Cost-effective: Compared to other surveying methods, plane table survey is relatively low-cost because it doesn’t require complex equipment.

3. High Accuracy: Plane table survey is known for its high degree of accuracy due to the use of leveling and other precise instruments.

4. Simplified field work: Plane table survey has simplified field work. Operators do not require being highly skilled individuals and this makes it an accessible technique for conducting surveys even in remote locations.

Disadvantages of Plane Table Survey: -

1. Limited scope: Plane table survey can only survey a small area at a time. This makes it unsuitable for surveying large regions or areas.

2. Time consuming: Despite being more efficient than other surveying methods, plane table survey still requires a considerable amount of time to complete.

3. Requires skilled personnel: Although individuals without advanced training can conduct plane table surveys, a skilled operator is required to achieve the required level of accuracy.

4. Limited accuracy in terrain with steep slopes: In areas with uneven terrain, it may be difficult to achieve a high degree of accuracy with plane table surveys.

Plane Table Survey Procedure: -

1. Gather all necessary equipment and materials, including the plane table, tripod, alidade, paper, pencils, erasers, and measuring tools.

2. Set up the tripod and attach the plane table to it. Ensure that the plane table is level and stable.

3. Orient the table to magnetic north or true north using the compass and adjust it if necessary.

4. Select a reference station or point, such as a known benchmark or a visible landmark, and set up the alidade over it.

5. Look through the alidade's sight and align it with the reference point. Record this as the first observation.

6. Rotate the alidade and sight on the next point of interest on the survey area. Take note of the angle between the previous observation and the new observation. This will be the angular measurement or bearing.

7. Place a pencil on the paper and mark a dot to represent the first point of interest. This dot should correspond to the same location as the reference point on the table.

8. Place the alidade over the paper and align it with the marked dot. Hold the table steady and rotate it until the alidade lines up with the next observation.

9. Place a pencil on the paper and mark another dot to represent the second point of interest. This dot should correspond to the same location as the second observation on the table.

10. Continue this process for all the other points of interest on the survey area until the survey is complete.

11. Use measuring tools to measure distances between the points of interest and add these distances to the survey map.

12. After the survey is completed, review the map and make sure all the information is correct and accurate.