Orientation

Orientation is the operation of revolving a plane table about its vertical axis so that all the lines on the sheet become parallel to the corresponding lines on the ground. It is an essential step in the process of conducting a survey using a plane table.

Importance of Orientation:
Orientation is crucial in surveying because it aligns the plane table with the true north-south line, allowing accurate measurements and mapping. It ensures that the lines on the sheet correspond to the actual features on the ground, leading to precise and reliable survey results.

Procedure for Orientation:
The process of orientation involves the following steps:

1. Placing the Plane Table: The plane table is set up on a tripod at the desired location for the survey.

2. Leveling: The plane table is leveled using a spirit level to ensure that it is horizontal. This step is necessary to prevent any errors caused by an uneven surface.

3. Centering: The table is then centered over the survey point or station by sighting through the alidade. This ensures that the center of the table aligns with the survey point.

4. Rough Alignment: The table is roughly aligned by rotating it until the line of sight of the alidade is parallel to the north-south line of the survey area.

5. Sighting: Once the rough alignment is achieved, the surveyor sights a known object, such as a distant landmark or a survey station, through the alidade. The table is then rotated until the line of sight aligns with the object.

6. Final Alignment: The final alignment is achieved when the lines on the table become parallel to the corresponding features on the ground. This is done by rotating the table about its vertical axis.

7. Locking the Table: Once the orientation is complete, the plane table is locked in position to prevent any accidental movement during the survey.

Conclusion:
Orientation is a critical step in conducting a survey using a plane table. It ensures that the lines on the sheet accurately represent the features on the ground. By revolving the plane table about its vertical axis, all the lines become parallel to the corresponding lines on the ground, leading to accurate measurements and mapping.