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Write short note on orientation in plane table surveying?
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The method of setting up the plane table at each of the successive station parallel to the position it occupied at the starting station is known as orientation. Orientation may be done by magnetic needle and Backsight.

Orientation by magnetic needle- This method is suitable when a local attraction is not suspected in an area.

• suppose Aand B are two stations. The plane table is set up a station A and levelled by spirit level. The centre is done by U fork and plumb bob so that point a is just over A. Then the through compass or circular box compass is placed on the right-hand top corner of the sheet in such a way that the needle consists with O-O mark. After this, a line represents the north line is drawn through the edge of the compass box the table is them clamped.

• With the alidade touch point, a ranging rod at A is bisected and ray is drawn. The distance AB is measured and plotted on any suitable scale.

• The table is sifted and centre over B, so that point b is just over B. the table is levelled now through compass, is exactly along the north line draw previously. The table is then turned clockwise or anticlockwise until the needle coincides exactly with the o-o mark of the compass. While turning the table care should be taken not to disturb the centring. In case it should be adjusted immediately.
• When the centring and levelling are perfect and the needle exactly at o-o the orientation should be perfect.

Orientation by Backsight- This method is always accurate and it is always preferred.

• Suppose A and B are the two station. The plane table is set up over A. the table is levelled by spirit level and centre by U fork so that point a is just over a station A. the north line mark on the right-hand top corner of the sheet by through compass.
• With the alidade touching a the ranging rod B is bisected and ray is drawn. The distance AB is measured and plotted on any suitable scale.so the point b represent station B.
• The table is shifted and set up over B . it is levelled and centre so that b is just over B. now the alidade is placed along the line ba and the ranging rod A is bisected by turning the table clockwise or anticlockwise. At this time centring may be disturbed and it should be adjusted immediately if required. When the centring, levelling and bisecting of ranging rod A is perfect then the orientation is said to be perfect.