# Describe any one method of orientation of plane table surveying

Describe any one method of orientation of plane table surveying

Orientation by back sighting method:

This method is accurate and is always preferred.

Procedure:

1.Suppose A and B are two stations.The plane table is set up over A. The table is leveled by spirit level and centerd over station A so that point ‘a’ is just over station

A. The north line is marked on the right hand top corner of the sheet by trough compass.

2. With alidade touching a, the ranging rod at B is bisected and a ray is drawn. The distance AB is measured and plotted to any suitable scale .so, the point b represents Station B.

3.The table is shifted and set up over B. It is leveled and centered so that ‘b’ is just over B. Now the alidade is placed along the line ba, and the ranging rod at A is bisected by turning the table clockwise or anticlockwise . At this time the centering may be disturbed, and should be adjusted immediately if required. When the centering ,leveling and bisection of ranging rod at A are perfect ,then the orientation is said to be perfect.

OR
2.Orientation by magnetic needle:
Procedure:
This method is suitable when local attraction is not suspected in the area.
1. Suppose A and B are two stations. The plane table is set up at station A. The centering is done by U-frame so that the point ‘a’ is just over A. Then the trough compass is placed on right hand top corner of the sheet in a such way that the needle coincides 0-0 mark. After this a line representing the north line is drawn through edge of the compass box.
2. With the alidade touching the point ‘a’ ,the ranging rod at B is bisected and a ray is drawn. The distance AB is measured and plotted to any suitable scale.
3. The table is shifted and centered over B, so that the point b is just over B.
The table is leveled .Now the trough compass is placed exactly along the north line drawn previously. The table is turned clockwise or anticlockwise until the needle coincides exactly with the 0-0 mark of the compass.
4. While turning the table ,care should be taken not to disturb centering. In case it is ,it should be adjusted immediately.
5. While the centering and leveling are perfect and needle is exactly at 0-0 , the orientation is said to be perfect.

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Last Answer : 1) It is used for sighting the objects to be located.  2) It is used for drawing the rays.

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Last Answer : (C) Both levelling and centering are done simultaneously

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Last Answer : (C) Given points are inaccessible

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Last Answer : (A) 1, 3, 2, 4, 5

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Last Answer : (B) (i), (iii), (ii)

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Last Answer : c) Both leveling and centering are done simultaneously *

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Last Answer : (C) Plan table surveying

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