to arrive at this by changing the ordinate by one unit, either plus or minus, at one or more stations.
3.24.
ADJUSTING FINAL THROW
If the ordinate at any station deviates by one-eighth inch from a smooth curve, the riding quality of the
track is not affected because the variation is too slight to be noticed. This fact makes it possible to adjust a curve
slightly to make it come out right. Often you measure and calculate a curve, only to find that a throw is required
at the last station. Since the rule says that there can be no throw there, you have to adjust your proposed curve.
You do this by changing the proposed ordinates at two of the preceding stations, one +1 and one -1, so that the
total still equals the sum of the measured ordinates.
Here are the calculations for the last few stations on a spiral of a typical curve. At the last station--Sta.
22, there is a half throw of +3, as shown in part A of table VI in which the proposed ordinates for stations 16 and
19 are circled. It is these two that you are to change. Since the last throw is plus, you increase the proposed
ordinate at Sta. 16 by 1 and decrease that at Sta. 19 by 1. Note that the stations chosen are three stations apart;
this is because the final half throw is 3. Also note that the proposed ordinate at the earlier or lower numbered
station was increased, because the final half throw is plus. Now the proposed ordinate at Sta. 16 is 10, and that at
Sta. 19 is 2, as shown in part B of table VI in which all the corrected numbers are outlined in heavy black lines.
Note particularly that all full throws from Sta. 17 through Sta. 22 decreased.
The rules to follow in adjusting a final half throw are given below. However, remember this one
precaution: it is usually easier and produces a better curve if the stations chosen are on the circular part of the
curve rather than on a spiral.
(1) Choose a pair of stations on the curve, whose numbers differ by the amount of the final half
throw.
(2) If the final half throw is plus, add 1 to the proposed ordinate at the earlier station, and take 1 away
at the later station.
(3) If the final half throw is minus, take 1 away from the proposed ordinate at the earlier station, and
add 1 at the later.
(4) Recalculate the curve from the earlier station.
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