ORIENTATION OF PLANE TABLE

ORIENTATION

The method of setting up plane table at each of the successive stations parallel to the position it occupied at the starting station is known as orientation.

  Orientation must be done when the plane table is set up at more than one station. As already started, plane tabling is base on principle of parallelism. So, the relative position of the objects on the map will be accurate only if the orientation is proper. But orientation is not done, then the map will not represent the actual position of the objects.

  Orientation may be done by magnetic needle and back-sighting.

Orientation by magnetic needle: This method is suitable when the local attraction is not suspected in the area.

Procedure [a] Suppose A and B are two stations. The plane table is set up at station A and levelled by sprit level. The centring is done by U-fork and plumb bob so that the point a is just over station A. Then the tough compass or circular box compass is placed on the right-hand top corner of the sheet in such a way that the needle is coincides with ‘0-0’ mark. After this a line representing the north line is drawn through the edge of the compass box. The table is then clamped.

[b]. 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.    

[c]. The table is shifted and centred over B, so that the point b is just over B. The table is levelled. Now the trough compass is placed exactly along the north line drawn previously. The table is then turn clockwise or anticlockwise until the needle coincides exactly with the 0-0 mark of the compass.

  While turning the table, care should be taken not to disturb the centring. In case it is, it should be adjusted immediately.

[d]. When centring and leveling are perfect and the needle is exactly at 0-0, the orientation is said to be perfect (Fig. 8)

Orientation by back-sighting: This method is accurate and is always preferred.

Procedure [a] Suppose A and B are two stations. The plane table is set up at station A and levelled by sprit level. The centring is done by U-fork and plumb bob so that the point a is just over station A. Then the tough compass or circular box compass is placed on the right-hand top corner of the sheet in such a way that the needle is coincides with ‘0-0’ mark. After this a line representing the north line is drawn through the edge of the compass box. The table is then clamped.

[b]. 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.    

[c]. The table is shifted and centred over B, so that the point b is just over B. The table is levelled. 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 centring may be disturbed and should be adjusted immediately if required. When the centring, leveling and bisection of the ranging rod a A are perfected, then the orientation is said to be perfect (Fig. 9).

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