Understanding Plane Table Surveying Instruments

Plane table surveying is a graphical method of surveying where the field work and plotting are done simultaneously. It's particularly useful for small-scale maps and details. A key part of this process involves accurately sighting objects in the field and transferring their directions onto the map being drawn on the plane table.

Instrument for Sighting Objects in Plane Table Survey

To accurately sight objects and draw lines representing their direction on the plane table, a specific instrument is used. Let's examine the options provided:

• Alidade: An alidade is a straight edge ruler fitted with sighting vanes or a telescope. It is the primary instrument used with a plane table to sight objects. One edge of the alidade is placed along the station point plotted on the map, and the alidade is then rotated until the object in the field is visible through the sighting vanes or telescope. A line (ray) is then drawn along the edge of the alidade from the station point on the map towards the sighted object.
• Clinometers: A clinometer is an instrument used for measuring angles of slope or elevation/depression. While useful in surveying for determining gradients or heights, it is not used for the primary purpose of sighting the horizontal direction of objects for plotting on a plane table map.
• Cross staff: A cross staff is a simple instrument used primarily for setting out right angles (90 degrees) on the ground. It is not used for sighting arbitrary objects and drawing direction lines in the context of plane table mapping.
• Prism square: A prism square is also an optical instrument used for setting out or checking right angles on the ground, similar to a cross staff but often more accurate. It is not used for sighting objects in various directions for plotting on a plane table.

Based on the function of each instrument in surveying, the instrument specifically designed for sighting objects and drawing direction lines in a plane table survey is the Alidade.

Detailed Explanation of Alidade in Plane Table Survey

The alidade is essential for plane table surveying. There are two main types:

• Plain Alidade: This is a simple brass or wooden ruler with two sighting vanes at the ends. One vane has a narrow slit, and the other has a horsehair or thin wire. The surveyor looks through the slit and aligns the wire with the object. The straight edge of the ruler is then used to draw the ray on the paper.
• Telescopic Alidade: This type replaces the sighting vanes with a telescope, similar to those used in a theodolite or dumpy level. This allows for more accurate sighting of distant objects and can also incorporate a vertical circle for measuring vertical angles.

Regardless of the type, the fundamental principle is the same: aligning the straight edge with the object being sighted so that a corresponding direction line can be accurately drawn from the instrument station point on the map.

Revision Table: Key Instruments for Plane Table Survey

Additional Information on Surveying Techniques

Plane table surveying employs several methods for mapping, all of which rely on the alidade for sighting:

• Radiation: The plane table is set up at a single station, and rays are drawn to various objects. Distances are measured (often by chaining or taping) and plotted to scale along the rays.
• Intersection: Objects are sighted from two different plane table stations, and the intersection of the corresponding rays determines the object's position on the map.
• Traversing: A traverse line is run, with the plane table being moved from one station to the next, orienting at each station and sighting the previous and next stations.
• Resection: The plane table is set up at an unknown location, and the position of the station is determined by sighting known objects whose positions are already plotted on the map.

In all these methods, the process of sighting the field object and drawing the direction line on the paper is performed using the alidade, making it an indispensable tool for plane table surveying.