- Drawing a circular curve begins with two tangents, or lines, that intersect. These lines represent two roadways and an intersection. The process begins with drawing a line between the two tangents. It starts at the point of curvature on one line and ends at the point of tangency on the other line. These points are the beginning and ending points of the circular curve, which is a half circle drawn between the lines toward the point of intersection.
- The center of the circular curve consists of a triangle that starts at the point of curvature and point of tangency. One side of the triangle is the line between the two points. The other two sides are lines drawn from the points that intersect a location that is on a perpendicular line from the point of intersection of the two tangents. The delta angle is the angle between these two sides of the triangle and is identical to the angle created at the point of intersection.
- The two tangents that intersect to form the delta angle carry the names back and forward tangent. The back tangent is the line with the point of curvature. When looking at the curve on a sheet of paper with two lines intersecting a point toward the top, the back tangent is the line on the left. The forward tangent contains the point of tangency and is the line on the right. If you extend the back tangent past the point of intersection, then the delta angle is the angle from the back tangent to the forward tangent.
- The line running between the point of curvature and the point of tangency is called the chord. The intersection of the chord with each tangent creates an angle known as the deflection angle. The deflection angle is equal to one-half of the delta angle. The deflection angle and delta angle provide the necessary data, along with the tangents, for surveyors to stake out the circular curve on the ground.
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