. Railroad construction. Theory and practice . cular curve located as usual,its central angle being A—2(j). A similar operation will locateQ from Z\ To locate points on the spiral. Set up at Q, with the plates 54 RAILROAD CONSTRUCTION. §50. reading 0° when the telescope sights along VQ. Set off fromQ the deflections given in Table IV for the instrument at Q,using a chord length of 25 feet, the process being like the methodfor simple curves except that the deflections are irregular. Ifa full station-point occurs within the spiral, interpolate betweenthe deflections for the adjacent spiral-point
. Railroad construction. Theory and practice . cular curve located as usual,its central angle being A—2(j). A similar operation will locateQ from Z\ To locate points on the spiral. Set up at Q, with the plates 54 RAILROAD CONSTRUCTION. §50. reading 0° when the telescope sights along VQ. Set off fromQ the deflections given in Table IV for the instrument at Q,using a chord length of 25 feet, the process being like the methodfor simple curves except that the deflections are irregular. Ifa full station-point occurs within the spiral, interpolate betweenthe deflections for the adjacent spiral-points. For example,a spiral begins at Sta. 56 + 15. Sta. 57 comes 10 feet beyondthe third spiral point. The deflection for the third point is350; for the fourth it is 56 15. ^ of the difference(21 15) is 8 30; the deflection for Sta. 57 is therefore 43 method is not theoretically accurate, but the error is at Z, the forward ahgnment may be obtained by sight-ing back at Q (or at any other point) with the given deflection. for that point from the station occupied. Then when the platesread 0° the telescope will be tangent to the spiral and to thesucceeding curve. All rear points should be checked from Z,If it is necessary to occupy an intermediate station, use the de-flections given for that station, orienting as just explained for Z, § 51. ALIGNMENT. 55 checking the back points and locating all forward points up to Zif possible. After the center curve has been located and Z is reached, theother spiral must be located but in reverse order, , the sharpcu. vature of the spiral is at Z *and the curvature decreasestoward Q\ 51. To replace a simple curve by a curve with spirals. Thismay be done by the method of § 49, but it invohes shifting thewhole track a distance m, which in the given example foot. Besides this the track is appreciably shortened,which would require rail-cutting But the track may be keptat practically the same length and the l
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