Plane Table Survey Orientation Basics

The plane table survey is a graphical method of surveying where field observations and plotting are done simultaneously. It is particularly useful for small-scale mapping and detailed plotting. A crucial step in plane table survey is the proper orientation of the plane table at each station. Orientation refers to setting the table in such a way that the lines on the drawing paper are parallel to the corresponding lines on the ground. This ensures that the map being created maintains its correct alignment with the actual features on the ground.

Orientation Methods in Plane Table Survey

There are several methods for orienting a plane table, each suited for different field conditions and requirements. These methods include:

• Orientation by compass
• Orientation by back-sighting
• Orientation by resection

The question specifically mentions 'three point problem' and 'two point problem' as orientation methods. Both of these are advanced techniques used for orientation and position fixing, and they fall under the broader category of resection.

Resection Method Explained

Resection is a method of orientation and plotting the position of the plane table station on the map, when the table is not set up at a known point. In the resection method, the position of the instrument station (the plane table station) is determined by drawing rays from the station towards three or two well-defined points whose positions are already plotted on the drawing sheet. The intersection of these rays helps locate the instrument station.

• Three Point Problem: This is a method of resection used to determine the exact location of the plane table station by taking sights to three well-defined points whose positions are already plotted on the map. This method is considered more accurate and provides a unique solution, eliminating potential ambiguity. Various methods exist to solve the three point problem, such as Bessel's graphical method, Lehmann's method, or the tracing paper method.
• Two Point Problem: This is another method of resection used to determine the exact location of the plane table station when only two well-defined points are available whose positions are already plotted on the map. While solvable, the two point problem can be more challenging and might require trial-and-error or specific geometric constructions to achieve a precise orientation and location. It's often used when a third known point is not visible or accessible.

Both the 'three point problem' and 'two point problem' are fundamentally techniques of resection because they involve locating the unknown position of the plane table by observing known points.

Analyzing Other Options

• Radiation: This is a method of plotting details on the map, not an orientation method. In radiation, the plane table is set up at a known station, and rays are drawn from this station to various points in the field, and distances are measured and plotted. It's a method of detail plotting, not table orientation.
• Levelling: This is the process of setting the plane table exactly horizontal using a spirit level. It is a preliminary step before any surveying work begins, ensuring the accuracy of plotting, but it is not an orientation method itself.
• Centring: This is the process of placing the plane table exactly over the ground station mark. It is another preliminary step, essential for accuracy, but like levelling, it is not an orientation method. Centring ensures that the plotted position on the sheet corresponds to the actual position on the ground.

Therefore, the 'three point problem' and 'two point problem' are directly connected to the resection method for orientation in plane table survey.