Understanding Simple Curve Setting Checks

When setting out horizontal curves in surveying, it's essential to verify the accuracy of the work. This ensures that the curve has been laid out correctly according to the design specifications. Several methods can be used for setting out curves, such as using total stations with deflection angles or chain and offset methods. After the curve is set out, a check is performed to confirm its precision.

Verifying Simple Curve Accuracy

The most common and reliable check for the accuracy of setting out a simple curve involves verifying the final located point. This check confirms that the endpoint of the curve, known as the Point of Tangency (PT), has been positioned correctly.

The process involves an independent measurement or calculation related to the curve's geometry. Specifically, the location of the Point of Tangency (PT) is determined through measurements originating from the Point of Intersection (PI). The PI is the vertex where the two tangent lines (the straight sections of the road or alignment before and after the curve) meet.

The check described in the correct option states:

• The last point so located: This refers to the final point that has been pegged out on the ground while setting the curve, typically the Point of Tangency (PT).
• Must coincide with the point of tangency: The pegged point must accurately represent the true PT of the designed curve.
• Fixed independently by measurements from the point of intersection: The position of the PT is re-established or verified using a separate method, starting from the known PI location and using the calculated curve elements (like tangent length). This independent verification ensures that no cumulative errors have occurred during the setting-out process.

By measuring from the PI to locate the PT independently, surveyors can compare this independently determined PT with the PT arrived at through the primary setting-out method (e.g., by accumulating deflection angles). If these two points match, it confirms the accuracy of the entire curve setting.

Why Other Options Are Incorrect

• Option 1 mentions verifying against the "point of curve" (PC) from the intersection angle. While the PC is crucial, the final check usually verifies the end point (PT).
• Option 3 suggests measurements from the "point of tangency" itself to verify the "forward tangency". This isn't an independent check originating from a known reference point like the PI.
• Option 4 mentions verifying the "point of curve" from the "point of tangency", which reverses the typical flow and purpose of a final check. The independence usually comes from a reference point external to the curve itself, like the PI.