Question 1

What do T, L, LC, E and M mean on a horizontal curve?

Accepted Answer

T is the tangent (from the PC to the PI), L is the arc length between PC and PT, LC is the long chord (the straight PC→PT distance), E is the external distance (PI to the mid-curve), and M is the middle ordinate (the largest gap between the arc and its chord). All follow from the radius R and the deflection angle Δ.

Question 2

What is the difference between degree of curve and radius?

Accepted Answer

The radius R is a direct geometric measure in metres. The degree of curve D (arc definition) is the central angle subtended by a 100 m arc, related to the radius by R = 18000 / (π·D). A smaller D means a larger radius and a flatter curve. The tool accepts either value as input.

Question 3

How do I use the deflection-angle table in the field?

Accepted Answer

Set the instrument on the PC, sight the PI as zero, then for each station turn the horizontal angle to the listed deflection and measure the chord from the previous point (or from the PC) to fix the point. The first table row is the PC and the last is the exact PT.

Question 4

What does the K value mean on a vertical curve?

Accepted Answer

K = L / |A|, where L is the curve length and A = g2 − g1 is the algebraic grade change in percent. It is the horizontal length per 1% of grade change — a measure of flatness used in sight-distance and comfort criteria; a larger K means a gentler curve.

Question 5

How do I tell a crest from a sag high/low point?

Accepted Answer

If A = g2 − g1 is negative (an upgrade followed by a downgrade) the curve is a crest and its turning point is the highest elevation. If A is positive (a downgrade followed by an upgrade) it is a sag and the turning point is the lowest elevation. The point is shown only when it falls within the curve.

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Curve calculator