Radiation method of Plane table surveying | Advantages and disadvantages of Radiation method

The Radiation method of plane table surveying involves locating a point by measuring angles from the instrument to various points on the terrain and then plotting them.

1. Procedure: The surveyor sets up the plane table at a point, draws lines of sight to various known points, and measures the angles.

2. Advantages:

   • Simple and quick method for surveying small areas.

   • Accurate for relatively open or uncomplicated terrain.

   • Does not require a lot of equipment, only a plane table and a protractor.

3. Disadvantages:

   • Not suitable for large or complex areas due to angle distortion.

   • Accuracy reduces as the distance from the station increases.

   • Relies on accurate measurements, which can be challenging on uneven terrain.

It’s best used for small-scale surveys or preliminary mapping where speed is a priority.

The radiation method is a classic technique used in plane table surveying, where the surveyor occupies a station at the center of the area to be surveyed. From this central point, rays or lines are extended to the surrounding points of interest, and measurements are taken directly on the plane table.

What is the Radiation method?

In the radiation method, the instrument is generally set up at a single point, from where all the details which are to be plotted are clearly visible. The rays are drawn from the instrument station to the points which are to be plotted. The distance between the instrument’s station and the point is measured with a tape or chain. After that, the distance is plotted in the drawing sheet to some suitable scale along with their respective rays.

This method is generally suitable for a survey when the distances are small and can be measured with a tape length. But if we measure the distance with a telescopic alidade which works on the principle of the tachometer, the radiation method can be applied to a greater distance.

What Makes Radiation Method Unique?

The radiation method offers several advantages over other surveying techniques:

1. Simplicity: It is straightforward and easy to understand, making it suitable for both novice and experienced surveyors.
2. Flexibility: The method can be adapted to various terrains and surveying scenarios, making it highly versatile.
3. Direct Measurements: With measurements taken directly on the plane table, there is minimal room for error, resulting in accurate data.
4. Cost-Effectiveness: It requires minimal equipment, reducing the overall cost of the surveying process.

The radiation method is one of the four methods of plane table surveying. Those methods of plane table surveying are –

(a) Radiation method

(b) Intersection method

(c) Traversing method

(d) Resection method.

Among these four methods, the first two methods are used for finding the location details of various objects. The other two methods are employed for establishing the plane table stations.

Instruments required for Radiation method of surveying

The instruments required for this type of surveying are as follows:

1. The drawing board of size 75 cm x 60 cm consists of a circular plate for attaching the tripod stand.
2. The plane or telescopic alidade
3. Spirit level.
4. The through compass or circular box compass
5. U-fork with plumb bob
6. Ranging rods
7. Tape
8. Arrows
9. Drawing sheets
10. Pencils

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How Does the Radiation Method Work?

The setting up of a plane table over a station in a plane table survey includes a number of steps. In general, there are five steps that are followed. 

Step-1 Fixing the tripod stand with the Plane table

At first, the tripod stand is attached to the bottom of the board by a wing nut. The wing nut is generally attached to a circular plate attached to the bottom of the board.

Step-2 Level the plane table

After fixing the tripod stand the table is leveled by adjusting the tripod legs. The level of the table is checked with the help of a spirit level. The spirit level is placed at a different place on the board to check its inclinations. The bubble should be at the center at every position of the plane table.  

Step-3 Center the plane table exactly over the station

After the leveling operation, a drawing sheet is fixed on the board. A pin is fixed on the station. Plot this station on the drawing sheet with the help of a U-fork and plumb bob. After that, the board is clamped and proper precautions are taken not to disturb it.

Step-4 Marking the N-S line

After centering the table and fixing a drawing sheet on the board, the north-south line is drawn on the sheet. This N-S line is found with the help of a through compass or circular box compass. This N-S line is very useful for the orientation of the plane table.

Step-5 Orientation 

The orientation operation is not performed in the radiation method of plane table surveying. Because in this method all the rays are drawn from a single central station. So, there is no chance of shifting the plane table over several stations.

The procedure of the Radiation method

The following are the necessary steps to conduct a  survey in the radiation method: 

1. Suppose T is the station on the ground from where all the station points A, B, C, D, E  and F are clearly visible.
2. The plane table is set at T. Now transfer the point “T” onto the drawing sheet with the help of a U-fork and plumb bob.  Thus Point “t” on the drawing sheet represents the station T on the ground. 
3. The next step is to fix the table by clamping it to the tripod stand.
4. Now set a point at point “t”. while touching the pin at t with the alidade, sight to A. Now draw the rays on the drawing sheet along the fiducial edge of the alidade.
5.  Similarly sight all the points B, C, D, E, F with the help of the alidade and draw the corresponding rays.
6.  Now the distances between the stations and the instrument (TA, TB, TC, TD, TE, TF) are measured in the field.
7. Finally, those distances are plotted in the drawing sheet along with their corresponding rays for establishing a,b, c, d, e, f on the drawing sheet.

Precautions are taken during Radiation method

For successfully conducting a radiation method few precautions are to be taken into consideration. Those precautions are as follows:

1.  The plane table must not be disturbed. So surveyor should be careful while working on the board.
2. The drawing sheet should be fixed properly. In case it gets disturbed the whole work may need to be repeated.
3. The leveling with a spirit level is done properly at different positions on the board, most importantly on the corners.
4. The N-S mark must be drawn on the drawing sheet. It helps in easy orientation and execution of the drawing on the field.

Advantages of Radiation method

1. The radiation method is very fast and suitable for locating objects from a single station.
2. There is no need for multiple instruments set up, hence fewer chances of error than other plane table methods.
3. As this method is conducted from a single station, no orientation work is needed.
4. The radiation method is the most rapid and useful method for filling in details.
5. As there is no need for orientation, fewer magnetic errors are there.

Disadvantages of the Radiation method

1. The radiation method does not provide very accurate results.
2. The radiation method is not suitable for surveying work in a wet climate.
3. This method is not suitable where the stations are not visible.
4. If in any case, the surveyor missed a measurement, the replotting of the survey work turns impossible. Then the whole radiation method is to be conducted again.
5. As there is no field book, replotting the map to a different scale is not possible.

Where is the radiation method used?

1. Topographic Surveys: Mapping land contours, features, and elevation changes.

2. Engineering Projects: Planning infrastructure development, road construction, and building layout.

3. Resource Exploration: Surveying for mineral deposits, water resources, and environmental assessments.

4. Land Management: Boundary delineation, land parcel mapping, and cadastral surveys.

The radiation method offers flexibility and precision, making it indispensable in diverse surveying applications. Whether in urban planning or natural resource management, this method provides valuable insights for decision-making.

FAQs about Radiation Method

1. What equipment is needed for the radiation method?

• The primary equipment includes a plane table, alidade, ranging rod, and measuring tape.

2. How is the central station determined in the radiation method?

• The central station is typically chosen based on factors such as accessibility and visibility to surrounding points.

3. What are the steps involved in conducting a survey using the radiation method?

• Step 1: Set up the plane table at the central station. Step 2: Orient the table to true north using a magnetic compass. Step 3: Sight a target point with the alidade and place a ranging rod at the desired location. Step 4: Record measurements and annotations directly on the plane table.

4. How is angular measurement handled in the radiation method?

• Angles are measured using the alidade, which typically features a vernier scale for precise readings.

5. What are the advantages of the radiation method over the traversing method?

• The radiation method is quicker to execute and requires fewer measurements, making it ideal for smaller survey areas.

6. Can the radiation method be used in rough terrain?

• Yes, the method can be adapted to various terrains by strategically placing ranging rods and adjusting for elevation differences.

7. How is scale accounted for in the radiation method?

• Scale is applied during the plotting phase, where measurements taken on the field are transferred to a scaled map or drawing.

8. Are there any limitations to the radiation method?

• While suitable for smaller areas, the method may not be practical for large-scale surveys due to limitations in visibility and accuracy over longer distances.

9. How can errors be minimized in the radiation method?

• Errors can be reduced through careful alignment of the plane table, precise measurement techniques, and cross-checking of data.

10. Are there any safety considerations when using the radiation method?

• Surveyors should exercise caution when working in potentially hazardous environments, such as construction sites or rugged terrain, and use appropriate safety gear.

Conclusion

The radiation method of plane table surveying offers a straightforward yet effective approach to gathering accurate spatial data. With its simplicity, flexibility, and direct measurements, it is a valuable tool for various surveying applications. By understanding the principles and techniques involved, surveyors can leverage the radiation method to achieve reliable results in their projects. So, why not consider incorporating the radiation method into your next surveying endeavor for optimal efficiency and precision?