. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . ° 22 co-log sin 33° 40 a7= AF= 0^= sin 15° 18 ab ^ sin 33° 40 log sin 15° 18 co-log sin 33° 40 67= ^7= feJ5= b. When the point of curve (or point of tangency) is inacces-sible. At some distance (As, Fig. 23) an unobstructed line pnmay be run parallel with AV. nv==pnf==As=R vers a. vers a = As-7-/2. ns=/)s=i2 sin 0!. §63, 71 At 2/, which is at a distance ps back from the
. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . ° 22 co-log sin 33° 40 a7= AF= 0^= sin 15° 18 ab ^ sin 33° 40 log sin 15° 18 co-log sin 33° 40 67= ^7= feJ5= b. When the point of curve (or point of tangency) is inacces-sible. At some distance (As, Fig. 23) an unobstructed line pnmay be run parallel with AV. nv==pnf==As=R vers a. vers a = As-7-/2. ns=/)s=i2 sin 0!. §63, 71 At 2/, which is at a distance ps back from the computed posi-tion of A, make an offset sAto p. Run p?! parallel to thetangent. A tangent to thecurve at n makes an angle of awith np. From n the curve isrun in as usual If the point of tangency isobstructed, a similar process,somewhat reversed, may beused. ^ is that portion of J stillto be laid off when m is sin ^. mz=tB=lx=Rvers ^. c. When the central part of the curve is obstructed, a is the central angle between two points of the curve between which a may equal any angle, but it is prefer-. Fig. a chord may be runable that a should be a multipleof D, the degree of curve, and thatthe points m and n should be oneven stations, mn =2R sin ^a. Apoint s may be located by an offsetks from the chord mn by a similarmethod to that outlined in § 60. The device of introducing thedotted curve mn having the sameradius of curvature as the other,although neither necessary noradvisable in the case shown inFig. 24, is sometimes the bestmethod of surveying around anobstacle. The offset from any point on the dotted curve tothe corresponding point on the true curve is twice the ordinateto the long cord/^ as computed in § 60. 63, Modifications of location. The following methods maybe used in allowing for the discrepancies between the paperlocation ^ based on a more or less rough prehminary survey andthe more accurate instrumental location, (See § 18.) They
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