Contour Mapping by Radial Method Using Levelling Machine & Plane Table

Contour Mapping by Radial Method Using Levelling Machine & Plane Table

Contour mapping is an essential technique in surveying, used to represent the 3D shape of land surfaces on a 2D map. Among various methods, the radial method combined with a levelling machine and plane table is effective, especially for hilly terrains or irregular topography. This method enables surveyors to produce accurate contour maps quickly and efficiently, especially when the control station is fixed, such as a hilltop or a central point.

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What is the Radial Method?

In the radial contouring method, all measurements are taken from a single central station, and lines (or rays) are radiated in multiple directions like the spokes of a wheel. Elevations are taken along these lines at regular intervals using a levelling machine such as a dumpy level or auto level. These observations are then plotted on a plane table to create contour lines.

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Instruments Used

1. Levelling Machine (Dumpy or Auto Level):
   Used to determine the relative elevation of points along each radial line.

2. Plane Table with Alidade:
   Used for plotting field observations directly onto the drawing sheet during the survey.

3. Ranging Rods & Staff:
   Assist in marking and sighting the locations along the radial lines.

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Procedure

1. Setup of Central Station:

A central point is selected—typically a high point like a hill or mound—where the plane table is set up and oriented.

2. Drawing Radial Lines:

From the central station, radial lines are drawn at equal angular intervals (e.g., every 15° or 30°), covering the whole area to be surveyed.

3. Levelling Observations:

Along each radial line, distances are marked (e.g., every 10 m), and the levelling staff is held at those points. The levelling machine reads the elevation differences between the central station and each marked point.

4. Plotting on Plane Table:

Using the distances and bearings, each point is plotted on the plane table with corresponding elevation noted.

5. Contour Interpolation:

After all points are plotted, contour lines are drawn by interpolating between points of known elevations using equal contour intervals (e.g., every 1 m or 2 m).

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Advantages

• Ideal for hilly terrains where radial access is more practical.

• Requires only one instrument setup at the center, saving time.

• Direct plotting on a plane table avoids the need for post-processing data.

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Applications

• Roadway alignment in mountainous regions

• Site selection for dam construction

• Topographic studies and environmental assessments

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Conclusion

The radial method of contour mapping using a levelling machine and plane table is a simple yet powerful technique in topographical surveying. It offers a balance of field efficiency and mapping accuracy, especially for circular or uneven terrains. Proper execution ensures detailed and precise contour representation, critical for planning and civil engineering projects.

             PRACTICAL

Here is a practical field report format with step-by-step procedure, observations, and result format for performing Contour Mapping by Radial Method using Levelling Machine and Plane Table — suitable for engineering lab/fieldwork documentation.

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🧭 Practical: Contour Mapping by Radial Method using Levelling Machine and Plane Table

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🎯 Aim:

To prepare a contour map of a given area using the Radial Method with the help of a levelling instrument and a plane table.

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🧰 Apparatus Required:

1. Dumpy Level / Auto Level

2. Levelling Staff

3. Plane Table with Drawing Sheet

4. Alidade

5. Tripod Stand

6. Ranging Rods

7. Compass (optional)

8. Pegs and Measuring Tape

9. Field Book / Observation Sheet

10. Plumb Bob and Spirit Level

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📍Principle:

In the radial method, a central station is selected, and lines (rays) are drawn radially outward. Elevations are recorded at set distances along each ray using the levelling instrument. These values are plotted directly on the plane table to generate contour lines.

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📌 Procedure:

A. Reconnaissance:

1. Inspect and select a central station (preferably a high point or centrally located spot).

2. Clear obstacles from the area and mark radial directions using ranging rods.

B. Plane Table Setup:

1. Set up the plane table at the central station.

2. Clamp the drawing sheet, and fix the table horizontally using the spirit level.

3. Mark the central station as point 'O' on the sheet.

4. Orient the table with the help of alidade and compass (or back sighting).

C. Drawing Radial Lines:

1. Divide the area using radial lines at uniform angular intervals (e.g., every 30°).

2. Label each radial line (e.g., Line A, B, C, etc.) on the sheet.

D. Levelling Observations:

1. Along each radial line, mark points at equal distances (e.g., every 10 m).

2. Place the levelling staff at each point.

3. Use the levelling machine (dumpy/auto level) to take staff readings.

4. Record Back Sight (BS) at central point and Intermediate Sight (IS) at each radial point.

5. Calculate the Reduced Level (RL) of each point using:

   RL = RL of central point – (Staff Reading at point – BS)

6. Record all readings in the observation table.

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📒 Sample Observation Table:

Radial Line	Distance (m)	Staff Reading	RL (m)	Remarks
A	0 (center)	1.355	100.00	Central Point
A	10	1.865	99.49
A	20	2.355	98.99	Slope detected
B	0 (center)	1.355	100.00	Central Point
B	10	1.600	99.75	Gentle slope

(Continue for all radial lines)

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📐 Plotting & Drawing Contours:

1. Plot the observed points using distances along radial lines on the drawing sheet.

2. Mark their RLs.

3. Interpolate between points to draw contours at uniform intervals (e.g., every 0.5 m or 1 m).

4. Use smooth curves to connect points of equal RL.

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📝 Result:

• A contour map of the surveyed area was prepared using the radial method.

• The land slopes downward from the central station.

• Contour intervals used: 1.0 m

• Central Station RL: 100.00 m

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⚠️ Precautions:

• Ensure the levelling instrument is properly calibrated and leveled.

• Always read staff vertically to avoid parallax errors.

• Fix plane table rigidly and check orientation periodically.

• Maintain consistent spacing on radial lines for better contour accuracy.

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✅ Conclusion:

The radial method using levelling machine and plane table successfully provided a practical and visual understanding of terrain profile. It is especially useful when working from a single point with a wide visible area, such as hills or valleys.

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