Circular Curves Setting Out Methods

Setting out circular curves is a fundamental task in surveying, particularly in road, railway, and canal construction. It involves transferring the curve design from paper to the ground. Various methods are employed for this purpose, each with its own advantages and suitability.

Common Methods for Setting Out Circular Curves

Several well-established methods are used for setting out circular curves:

• Rankine’s method of tangential angle: This is a very common and widely used method. It involves calculating angles (tangential angles) from a tangent line at the point of commencement of the curve to various points along the curve, and measuring corresponding chord lengths. It typically requires a theodolite and measuring tape.
• Two theodolite method: This method is known for its accuracy and speed, especially when there are obstructions along the curve. It involves setting up two theodolites, usually one at the point of commencement and the other at the point of tangency. Angles are turned simultaneously from both stations to locate points on the curve by the intersection of lines of sight.
• Tacheometric method: This method uses a tacheometer (a type of theodolite that measures distances trigonometrically using stadia hairs) to measure both angles and distances simultaneously. While less precise than the two-theodolite method, it can be faster in rough terrain or where chaining is difficult. Points on the curve are located by turning the calculated angle from the tangent and measuring the required tacheometric distance.

Method NOT Commonly Used for Circular Curves

Now let's consider the Plane table method:

• Plane table method: The plane table is a surveying instrument used for plotting features directly in the field on a drawing sheet. It is primarily used for topographical mapping and plane table surveying of relatively small areas or where high precision is not required. While you can theoretically plot points of a curve on a plane table map, using the plane table itself for precisely setting out (marking on the ground) points of a pre-designed circular curve is not a standard or commonly used method. The other methods, especially those involving a theodolite, offer much greater accuracy and efficiency for laying out predefined geometric shapes like circular curves on the ground.

Therefore, out of the given options, the Plane table method is the one NOT commonly used for setting out circular curves.