Plane Tables

Definition

A plane table is a surveying instrument consisting of a drawing board mounted on a tripod, used to plot map details and features directly in the field. Unlike other surveying methods that require recording measurements in a field book, plane tabling allows the surveyor to draw the map simultaneously while observing the ground.

Main Content

1. Components of Plane Table

• The Drawing Board: A flat, wooden table usually made of well-seasoned wood to prevent warping, providing a smooth surface for drawing.
• The Tripod: A stable, three-legged stand that supports the board, allowing for height adjustment and leveling.
• The Alidade: A sighting instrument consisting of a metal or wooden ruler with sighting vanes (or a telescope) used for drawing lines of sight toward points.

2. Principles of Plane Tabling

• Parallelism: The fundamental principle is that the lines drawn on the paper are parallel to the actual lines on the ground.
• Ray Intersection: The instrument relies on the geometric principle of intersection, where the position of an object is determined by the intersection of lines drawn from two different station points.

3. Methods of Plane Tabling

• Radiation: Suitable for small areas where all points are visible from a single station; lines are drawn from the station to the objects.
• Intersection: Used when points are inaccessible (like a river or a mountain peak); lines are drawn from two different stations, and the intersection point on the map marks the object's location.
• Traversing: Used for connecting a series of points to form a survey line, similar to a chain survey.

Working / Process

1. Centering

• The table must be positioned directly over the ground station point using a U-fork and plumb bob.
• This ensures that the point plotted on the paper corresponds exactly to the station on the ground.

2. Leveling

• The table is leveled using a spirit level placed on the board in two directions at right angles.
• Proper leveling is essential to ensure that the alidade rotates on a true horizontal plane.

3. Orientation

• This is the process of setting the table so that all lines on the paper are parallel to the corresponding lines on the ground.
• It is commonly achieved using the "Backsighting" method, ensuring the table points in the same direction at every station.

       [  Plane Table Board  ]
             |     |
             |     | (Alidade)
       ______|_____|______
      /      |     |      \
     /       |     |       \  (Tripod Legs)
    /        |     |        \
   O         O     O
(Station Point on Ground)

(Diagram: Basic assembly of a plane table setup over a ground station)

Advantages / Applications

• Instant Mapping: It is the fastest method of surveying because plotting occurs simultaneously with field observations, reducing the risk of errors.
• No Field Book Required: Because the map is drawn on-site, there is no need to maintain a separate field book or interpret measurements later in an office.
• Ideal for Topography: It is extensively used in landscape surveying, coastal mapping, and areas where graphical representation is more important than extreme numerical precision.

Summary

Plane tabling is a graphical surveying technique where the map is created directly in the field using a drawing board, tripod, and alidade. It is highly efficient for topographic mapping as it eliminates the need for field books and complex calculations.

Important terms to remember: - Alidade: The sighting tool used for drawing lines. - Orientation: Setting the board parallel to the ground objects. - Radiation: A method for mapping points visible from one station. - Centering: Aligning the table over the specific ground mark.