Plane Table Survey

Introduction: 

Plane table surveying is a method of field surveying where observations and plotting are done simultaneously. A drawing board mounted on a tripod is used, allowing for real-time plotting of survey points on paper. It’s ideal for small areas and provides accurate results without the need for extensive calculations.

Instrument used in plane table Survey:

1. Plane Table: 

A drawing board mounted on a tripod, used for plotting points directly in the field. It is usually rectangular or square and provides a flat surface for drawing.

2. Alidade: 

A simple alidade is a basic surveying tool used in plane table surveying. It consists of a straightedge with two sight vanes: a sight vane (eye vane) with slits for viewing and an object vane with a stretched thread to establish a line of sight. It's ideal for low-elevation surveys.

3. Plumbing Fork and Plumb Bob:

 Used for centering the table over a ground point. The U-shaped plumbing fork has a pointer on one arm and a plumb bob suspended from the other, ensuring the table is directly above the station peg.

4. Spirit Level:

 A tool containing a mercury bubble inside a curved tube, used to make sure the table is perfectly level during the survey.

5. Ranging Rods: 

Tall rods with colored bands, used to mark the positions of surveying stations and help sight and align lines over long distances.

 

 A narrow magnetic compass placed on the drawing board to help orient the plane table in relation to the magnetic north, ensuring proper alignment between different stations.

7. Drawing Accessories: 

Includes pencils, rulers, and clamps to assist in plotting points and securing the drawing paper on the Table 

8. Measuring tape:

 Measuring tape is used to measure horizontal distances between survey points. Typically made of materials like steel, fiberglass, or linen, the tape ensures accurate distance measurements between stations or objects in the field. 

Temporary adjustments of the plane table include three main steps:

1. Centering:

 This ensures the ground point is accurately represented on the paper. A plumbing fork and plumb bob are used for this. The board is shifted until the plumb bob hangs directly over the station peg. 

2. Leveling:

 This ensures the drawing board is parallel to the ground. It can be done by simply tilting the board, using a ball-and-socket arrangement, or adjusting the tripod legs.

3. Orientation: 

This process ensures that the board is parallel at different survey stations, so the lines on paper correspond to those on the ground. It can be done by:

  i)  Magnetic Needle: Using a magnetic needle to align the board, though this is less reliable.

  ii)  Back Sighting: A more reliable method where a line from the previous station is checked for alignment at the new station.

Methods of plane table survey : 

a) Radiation

Method It is the simplest method of plane table surveying. This method is only effective if the whole surveying is to be done from one single station i.e. The procedure is as follows:

1) A point P is to be selected in such a fashion that all the other points ( A B C D E) are seen easily from P.

2) Centering, leveling, and orientation must be done prior to surveying.

3) At first, by putting the alidade on point P a line of sight for station A is to be drawn.

4) After measuring the distance of PA on field, the measurement needs to be put on paper to a suitable scale.

5) Similarly, points b, c, d, and e are obtained on paper by drawing lines of sight for stations B, C and D and measuring the distances PB, PC, PD and PE on ground respectively. 

6) Points a, b, c, d, and e are joined on paper, as shown in the figure.

b) Intersection

Method In previous method it was possible to measure every distance on the field manually. In case of a mountainous terrain or rough surface where distances cannot be taken physically, it is best to use intersection method. The procedure is:

1) Two stations O1 and O2 are selected so that the points to be located on paper are easily seen from them.

2) The baseline (o1o2) is plotted on the paper. This is done in the way below: The table can be centered and leveled at station O1 and then after orienting at station O2, the distance O1 O2 can be accurately measured and put up to some scale on the paper. The line o1o2 can be drawn to some scale on the paper and then the board can be adjusted from station O1 by back sighting at station O2.

3) From station O1, rays for stations A, B are drawn etc.

4) Now moving the table to the new station and orienting it again the rays of stations A, B are drawn etc.,

5) The intersection of rays from stations O1 and O2 will give points a, b etc. on paper, as shown in the figur 

Merits of Plane Table Survey:

1. Direct Plotting: Fieldwork and plotting are done simultaneously, reducing the chances of mistakes.

2. Cost-effective: It is relatively inexpensive and does not require sophisticated instruments.

3. Simple Method: The method is straightforward and easy to use, making it suitable for small to medium-sized surveys.

4. Visualization: The surveyor can immediately visualize the area being surveyed, helping in better decision-making.

5. No Need for Field Notes: As observations are directly plotted on the map, field notes are often not required.

6. Ideal for Small Areas: It's most suitable for surveys of small areas where high precision is not a primary requirement.

Demerits of Plane Table Survey:

1. Weather Dependency: It is difficult to perform in adverse weather conditions such as rain or wind, as paper and equipment are exposed.

2. Not Suitable for Large Areas: It is less effective for large or very detailed surveys compared to more advanced methods.

3. Lower Precision: The accuracy is generally lower compared to other modern surveying methods like total station surveys or GPS surveys.

4. Bulky Equipment: The plane table and related equipment can be bulky and cumbersome to carry around.

5. Requires Stable Ground: The table must be perfectly horizontal and stable, which can be challenging on uneven or rough terrain.

6. Limited Use in Dense Areas: The method may not work well in dense vegetation or areas with many obstructions, as line of sight is important for accurate plotting.

The End