R
PC
PT
Figure 11-3.—Elements of a horizontal curve.
center of the circle (O) to the PC and PT. The
value of the central angle is equal to the I angle.
Some authorities call both the intersecting
angle and central angle either I or A.
RADIUS. The radius of the circle of which the
curve is an arc, or segment. The radius is
always perpendicular to back and forward
tangents.
POINT OF CURVATURE. The point of curva-
ture is the point on the back tangent where the
circular curve begins. It is sometimes
designated as BC (beginning of curve) or TC
(tangent to curve).
POINT OF TANGENCY, The point of
tangency is the point on the forward tangent
where the curve ends. It is sometimes
designated as EC (end of curve) or CT (curve
to tangent).
POC
L
T
LC
11-3
POINT OF CURVE. The point of curve is any
point along the curve.
LENGTH OF CURVE. The length of curve is
the distance from the PC to the PT, measured
along the curve.
TANGENT DISTANCE. The tangent distance
is the distance along the tangents from the PI
to the PC or the PT. These distances are equal
on a simple curve.
LONG CHORD. The long chord is the
straight-line distance from the PC to the PT.
Other types of chords are designated as
follows:
C
The full-chord distance between adja-
cent stations (full, half, quarter, or one-
tenth stations) along a curve.
c]
The subchord distance between the PC
and the first station on the curve.
