Aquaculture Engineering

Orientation - Methods

Unit 1- Surveying

Plane table surveying
Working operations

Orientation
Orientation by means of trough compass
The plane table can be oriented by compass under the following conditions.

• When speed is more important than accuracy.
• When there is no second point available for orientation
• For approximate orientation prior to final adjustment.

For orientation, the compass is so placed on the plane table that the needle floats centrally and a fine pencil line in ruled against the long side of the box. At any other station where the table is to be oriented, the compass is placed against thin line and the table is oriented by turning it until the needle floats centrally. The table is then clamped is position.

Orientation by back sighting

To orient the table at the next station say B represented on the paper by a point “b” plotted by means of a line “ab” drawn from a previous station A, the alidade is kept on the line “ba” and the table is turned about its vertical axis in such a way that the line of sight passes through the ground station “A”. When this is achieved the plotted line “ab” will be co-insiding with the ground line AB and the table will be oriented. The table is then clamped line position.

Sighting the points
When once the table has been set, i.e., when levelling centring and orientation has been done, the points to be located are sighted through the alidade. The alidade is kept pivoted about the plotted location of the instrument station and is turned so that the line of sight passes or bisects the signal at the point to be plotted. A ray is then drawn from the instrument station along the edge of the alidade. Similarly the rays to other points to be sighted are drawn.