ELEMENTS OF A HORIZONTAL CURVE The elements of a circular curve are shown in figure
113. Each element is designated and explained as
follows:
PI POINT OF
INTERSECTION. The point of intersection
is the point where the back and forward tangents
intersect. Sometimes, the point of
intersection is designated as V
(vertex).
I INTERSECTING ANGLE.
The intersecting angle is
the deflection angle at the PI.
Its value is
either computed from the preliminary traverse
angles or measured in the field.
A CENTRAL ANGLE. The central angle is the angle
formed by two radii drawn from the
Figure 112.—Horizontal
curves.
Figure 113.—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.
R
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.
PC
POINT OF CURVATURE. The
point of curvature 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).
PT
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 POINT OF
CURVE. The point of curve is any point
along the curve.
L
LENGTH OF CURVE. The length of
curve is the distance from
the PC to the PT, measured
along the curve.
T 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.
LC LONG CHORD.
The long chord is the straightline
distance from the PC to the PT.
Other types of chords are
designated as follows:
C
The fullchord
distance between adjacent stations
(full, half, quarter, or onetenth stations)
along a curve.
c] The subchord
distance between the PC
and the first station on the curve.
c?
The subchord distance
between the last station on
the curve and the PT.
E EXTERNAL
DISTANCE. The external distance
(also called the external secant) is the distance
from the PI to the midpoint
of the curve. The external
distance bisects the interior angle
at the PI.
M MIDDLE
ORDINATE. The middle ordinate is the
distance from the midpoint of the curve to the
midpoint of the long chord. The extension of
the middle ordinate bisects the central angle.
D DEGREE OF
CURVE. The degree of curve defines
the sharpness or flatness of the curve.
