How is orientation done in plane table surveying?

Short Answer:

Orientation in plane table surveying is the process of aligning the drawing sheet on the table so that it matches the actual directions of the ground. It helps ensure that the direction of lines drawn on the sheet corresponds exactly to the directions of the objects in the field.

Orientation is done by using a compass or by back-sighting a previously plotted point with an alidade. Once properly oriented, the plane table can be used to sight and plot new points accurately without shifting the directions of existing features.

Detailed Explanation:

Orientation in plane table surveying

Orientation is a very important step in plane table surveying. It ensures that the drawing sheet fixed on the plane table is set in such a way that the direction of lines on the sheet corresponds exactly to the actual directions in the field. If orientation is done properly, every ray or line drawn from a station will point toward the correct ground feature, keeping the entire map accurate.

When surveying multiple stations or returning to a previous station, correct orientation ensures that the already drawn features on the sheet remain aligned with the real-world directions. Without proper orientation, all plotted points may be wrongly placed, and the survey will be incorrect.

How orientation is done

There are mainly two methods of orientation in plane table surveying:

1. Orientation by back-sighting (alidade method)

This is the most commonly used and most accurate method.

Steps:

  • First, set up the plane table at the new station and fix the drawing sheet.
  • Rotate the table roughly in the direction of the previous station.
  • Place the alidade on the line already drawn between the two stations on the sheet.
  • Sight back to the previous station through the alidade.
  • Rotate the entire table (not the alidade) until the object is properly sighted along the drawn line.
  • Once the previous point is aligned, clamp the table to lock its position. The table is now oriented.

Advantage: This method gives the best orientation accuracy because it uses actual plotted lines and sights.

2. Orientation by compass

This is a quicker but less accurate method, used when time is limited or when the previous point is not visible.

Steps:

  • Fix the plane table and drawing sheet at the new station.
  • Place a magnetic compass on the table.
  • Align the north of the drawing with the magnetic north shown on the compass.
  • Rotate the table until both match and then clamp the table.

Limitation: Compass readings may be affected by nearby metal objects or magnetic materials, leading to small errors.

Importance of orientation

  • Ensures consistency in direction across multiple stations.
  • Allows proper plotting of all points on the same sheet.
  • Avoids errors caused by wrongly placed features.
  • Saves time by preventing rework or corrections.
  • Keeps the drawn map aligned with actual terrain.

Tips for good orientation

  • Always use back-sighting when possible.
  • Use a compass only if the previous station is not visible.
  • Double-check orientation before drawing any new rays.
  • Avoid rotating the drawing sheet after orientation.
Conclusion

Orientation in plane table surveying is the method of aligning the drawing sheet with the actual directions on the ground. It can be done by back-sighting using an alidade or with a compass. Proper orientation is key to accurate plotting and must be done carefully to ensure correct survey results.