EMDc?The subchord distance between the laststation on the curve and the PT.EXTERNAL DISTANCE. The externaldistance (also called the external secant) is thedistance from the PI to the midpoint of thecurve. The external distance bisects the interiorangle at the PI.MIDDLE ORDINATE. The middle ordinate isthe distance from the midpoint of the curve tothe midpoint of the long chord. The extensionof the middle ordinate bisects the central angle.DEGREE OF CURVE. The degree of curvedefines the sharpness or flatness of the curve.DEGREE OF CURVATUREThe last of the elements listed above (degree ofcurve) deserves special attention. Curvature may beexpressed by simply stating the length of the radius ofthe curve. That was done earlier in the chapter whentypical radii for various roads were cited. Stating theradius is a common practice in land surveying and inthe design of urban roads. For highway and railwaywork, however, curvature is expressed by the degreeof curve. Two definitions are used for the degree ofcurve. These definitions are discussed in the followingsections.Degree of Curve (Arc Definition)The arc definition is most frequently used in high-way design. This definition, illustrated in figure 11-4,states that the degree of curve is the central angleformed by two radii that extend from the center of acircle to the ends of an arc measuring 100 feet long(or 100 meters long if you are using metric units).Therefore, if you take a sharp curve, mark off a portionso that the distance along the arc is exactly 100 feet,and determine that the central angle is 12°, then youhave a curve for which the degree of curvature is 12°;it is referred to as a 12° curve.Figure 11-4.—Degree of curve (arc definition).11-4

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