straight stretches are referred to as tangents; the arcs, as curves. By definition, a tangent is a
straight line that touches the circumference of a circle at only point, the point of tangency. A
radius of the circle drawn to that point of tangency is perpendicular to the tangent. The radius
and the tangent form a right, or 90-degree, angle. An arc is any part of the circumference of a
circle and is formed by drawing a nontangent line other than the diameter through two points on
the circumference. Tangent and nontangent lines and an arc are shown in figure 1.2; tangents
and curves, in figure 1.3. Curvature, easement, and right of way are three terms with which you
should be familiar in studying alinement.
a. Curvature. The measure or amount of curving is known as curvature. It tells you
whether or not a curve is sharp. The sharpness of a curve depends upon the length of the radius
of the circle of which it is an arc. If the radius is long, the curve has a light curvature--it is not
sharp. Conversely, if the radius is short, the curve is sharp. In some countries, the sharpness of
curves is measured in terms of the radius. In the United States, curvature is measured in degrees.
The geometry of this measurement is discussed in Railway Track Maintenance II. It is sufficient
for now that you know that the larger the number of degrees of curvature, the
Figure 1.2. Tangent and Nontangent Lines.