. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . the book. In Fig. 14both notations are given, the A. A. notations beinggiven in parentheses. Both notations are also shown inFig. 36, which illustrates a transition curve or spiral. Itshould be noted that some of the notations coincide for someof the elements. 51. Elements of a curve. Considering the line as runningfrom A toward By the beginning of the curve, at A, is calledthe point of curve (PC). The other end of the curvej at By iscalled the point of tangency (PT). The
. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . the book. In Fig. 14both notations are given, the A. A. notations beinggiven in parentheses. Both notations are also shown inFig. 36, which illustrates a transition curve or spiral. Itshould be noted that some of the notations coincide for someof the elements. 51. Elements of a curve. Considering the line as runningfrom A toward By the beginning of the curve, at A, is calledthe point of curve (PC). The other end of the curvej at By iscalled the point of tangency (PT). The intersection of thetangents is called the vertex (V), The angle made by the 60 HAILROAD CONSTRUCTION. §52. tangents at F, which equals the angle made by the radii tothe extremities of the curve, is called the central angle (A). AVand BV, the two equal tangents from the vertex to the PCand PT, are called the tangent distances (T). The chordAB is called the long chord {LC). The intercept HG fromthe middle of the long chord to the middle of the arc is calledthe middle ordinate (M). That part of the secant GV from. Fig. 14. the middle of the arc to the vertex is called the external distance(E). From the figure it is very easy to derive ^the followingfrequently used relations: T=R ta^niA (4) LC = 2EsiniA (5) M^RversiA (6) E = R exsec |A. ....... (7) 52. Relation between T, E, and A. Join A and G in Fig. 14. The angle VAG = iAj since it is measured by one half of thearc AG between the secant and tangent. AGO = 90-iA.
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