What is the correct order for determining the radius of a curve?

What is the correct order for determining the radius of a curve?

• A. Measure the chord, find the midpoint, then use the radius formula
• B. Use the radius formula first, then measure the chord
• C. Find the midpoint, then measure the chord, then use the radius formula
• D. None of the above

Answer: (A)

To find the radius from a curve, you need two measurements: the chord length across the curve and the sagitta, which is how far the arc bulges above the chord measured at the chord’s midpoint. With those two values, you can use the standard formula R = (c^2)/(8h) + h/2, where c is the chord length and h is the sagitta. So you start by measuring the chord to get c, then locate the chord’s midpoint and measure the sagitta h, and finally plug both into the formula to find the radius. This order matters because the radius formula requires both c and h; you can’t compute R without knowing both.

To find the radius from a curve, you need two measurements: the chord length across the curve and the sagitta, which is how far the arc bulges above the chord measured at the chord’s midpoint. With those two values, you can use the standard formula R = (c^2)/(8h) + h/2, where c is the chord length and h is the sagitta.

So you start by measuring the chord to get c, then locate the chord’s midpoint and measure the sagitta h, and finally plug both into the formula to find the radius. This order matters because the radius formula requires both c and h; you can’t compute R without knowing both.