Setting Up a Plane Table: Understanding the Correct Order

Plane table surveying is a graphical method of surveying in which field work and plotting are done simultaneously. The plane table is an instrument used for this purpose. Before any surveying work can begin using a plane table, it needs to be properly set up at a station. This setup involves several steps, and the order in which these steps are performed is critical for accurate results.

The Essential Steps for Setting Up a Plane Table

The primary steps involved in setting up a plane table at a station are:

1. Levelling
2. Centering
3. Orientation

Let's understand each step and why this specific order is correct.

1. Levelling the Plane Table

The first step is to make the surface of the plane table perfectly horizontal. This is usually done using a spirit level. If the table is not level, measured angles and distances projected onto the drawing sheet will not be accurate representations of the horizontal distances and angles on the ground. A level table ensures that the drawing sheet represents a true horizontal plane.

2. Centering the Plane Table

After levelling, the table needs to be centered. This means positioning the drawing sheet attached to the table exactly over the ground station mark. A plumbing fork and plumb bob are commonly used for this purpose. Centering ensures that the point on the drawing sheet corresponding to the instrument station is directly above the actual station mark on the ground. Accurate centering is essential for plotting points correctly relative to the station.

It's important to level the table *before* centering because if the table is not level, the plumb bob suspended from the centering fork will not hang vertically from the point directly above the station mark on the table, leading to incorrect centering.

3. Orienting the Plane Table

The final step in setting up is orientation. Orientation involves setting the plane table such that the lines on the drawing sheet are parallel to the corresponding lines on the ground. In simpler terms, it means ensuring that the north direction on your map aligns with the north direction on the ground, or that a line drawn between two points on your map is parallel to the actual line between those two points on the ground. Orientation is crucial for maintaining consistency and direction throughout the survey.

Orientation is done after levelling and centering because you need a level and correctly positioned table before you can accurately align your survey lines with the ground features.

Correct Sequence Explained

Considering the function and dependency of each step, the logical and correct order for setting up a plane table is:

• First, Levelling to make the table horizontal.
• Second, Centering to position the sheet over the ground station.
• Third, Orientation to align the map lines with ground features.

Performing these steps in this sequence ensures that the drawing sheet is a true-to-scale representation of the horizontal ground features relative to the station.

Why Other Orders Are Incorrect

• Centering before Levelling: If centered on an unleveled table, the plumb bob won't hang correctly from the point above the station, leading to centering errors once leveled.
• Orientation before Levelling or Centering: Orientation requires a level table and correct centering to be meaningful and accurate relative to other points being surveyed.

Therefore, the sequence Levelling, Centering, and Orientation is the standard and correct method for setting up a plane table for surveying.

Revision Table: Steps for Plane Table Setup

Additional Information on Plane Table Surveying

Understanding the tools and methods related to each step provides a deeper insight into plane table surveying.

• Spirit Level: A small accessory level is placed on the table surface in two perpendicular directions to ensure it's horizontal.
• Plumbing Fork and Plumb Bob: The fork has two equal arms, one placed on the table with its tip pointing to the station point on the map, and the plumb bob suspended from the other arm hanging below. When the plumb bob is exactly over the ground station peg, the table is centered.
• Alidade: An alidade is a straight edge rule with sights. It is used for sighting objects and drawing lines on the plane table.
• Orientation Methods:
  • By Trough Compass: A trough compass is used to draw a magnetic meridian on the sheet. At subsequent stations, the table is oriented by aligning the magnetic needle in the trough compass along the previously drawn magnetic meridian line.
  • By Back Sighting: This is a more accurate method. The table at the new station is oriented by sighting back to the previous station with the alidade placed along the line connecting the points of the two stations on the map.

Mastering these setup steps is fundamental for conducting accurate plane table surveys.