Orientation in plane table surveying is the process of putting the plane table into some fixed direction so that line representing a certain direction on the plan is parallel to that direction on the ground. This is an essential condition to be fulfilled when more than one instrument station is to be used.
If the orientation is not done the table will not be parallel to itself at different positions resulting in an overall distortion of the map. The processes of centering and orientation are dependent on each other. For orientation, the table will have to be rotated about its vertical axis, thus disturbing the centering.
There are two main methods of orientation in plane table surveying.
- Orientation by through compass
- Orientation by means of back sighing.
1.Orientation by trough compass
The compass, though less accurate, often proves a valuable adjunct in enabling the rapid approximate orientation to be made prior to the final adjustment. The plane table can be oriented by compass under the following conditions final adjustment. 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
- When the traverse is so long that accumulated errors in carrying the azimuth forward might be greater than the orientation of the compass
- For approximate orientation prior to final adjustment.
2. Orientation by backsighting
Orientation in plane table surveying can be done precisely by sighting the points already plotted on the sheet. Two cases may arise
- When it is possible to set the plane table on the point already plotted on the sheet by way of observation from the previous section.
- When it is not possible to set the plane table on the point.
To orient the table at the next station, say B, represented on the paper by a point b plotted by means of line ab drawn from previous section A, 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 coinciding with the ground line AB (provided the centering is perfect) and the table will be oriented. The table then is clamped in the positions.
This method is equivalent to that employed in azimuth traversing with the transit. Greater precision is obtainable than with the compass, but an error in direction of a line is transferred to succeeding lines.