8. Plane Table Surveying

 

Plane Table Surveying 

1. Definition

Plane table surveying is a graphical method of surveying in which field observations and plotting are done simultaneously on a drawing sheet fixed to a plane table.

Instead of recording measurements in a field book and plotting later, the map is drawn directly in the field.

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2. Instruments Used

• Plane table with tripod

• Alidade (plain or telescopic)

• Spirit level

• Plumbing fork with plumb bob

• Drawing sheet and accessories

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3. Principle

The principle of plane table surveying is parallelism:
Lines drawn on the sheet are kept parallel to the corresponding lines on the ground.

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4. Methods of Plane Table Surveying

(a) Radiation

All details are located from one station by drawing rays.

(b) Intersection

Points are fixed by intersection of rays from two stations.

(c) Traversing

Used when the survey area is large. Stations are connected like a traverse.

(d) Resection

Used to find the position of the plane table station using known points.

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5. Advantages

• Fast and economical for small areas

• No need for field book

• Errors can be detected immediately

• Best for rough, hilly terrain

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6. Limitations

• Not suitable for large or very accurate surveys

• Affected by wind and rain

• Difficult in congested areas

• Accuracy depends on drawing skill

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7. Numerical Example (Radiation Method)

Problem

From a plane table station O, three points A, B, and C are located by radiation.
The following field measurements are taken:

Point	Distance from O (m)	Bearing
A	40 m	30°
B	60 m	110°
C	50 m	210°

Scale: 1 cm = 10 m

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Step-by-Step Solution

Step 1: Convert ground distances to map distances

Point	Ground Distance	Map Distance
A	40 m	4 cm
B	60 m	6 cm
C	50 m	5 cm

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Step 2: Plotting procedure

1. Fix point o on the sheet as station O.

2. Using alidade:

   • Draw a ray at 30° → mark A at 4 cm

   • Draw a ray at 110° → mark B at 6 cm

   • Draw a ray at 210° → mark C at 5 cm

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Final Result

The triangle ABC is plotted directly on the sheet with correct scale and orientation.

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8. Short Numerical (Resection – Two Point Method)

Problem

Two known points A and B are already plotted on the map.
Ground angles measured at unknown station P are:

$$\angle APB = 60^\circ$$

Distance on map:

$$AB = 8 \text{ cm}$$

Find position of P graphically.

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Solution (Graphical)

1. Join A–B on sheet.

2. At A, draw a ray making angle equal to angle PAB.

3. At B, draw a ray making angle equal to angle PBA.

4. Intersection of these two rays gives point P.

Thus, station P is fixed by resection.