proposed ordinate is larger than the measured ordinate, the error is entered as minus, such as that at Sta. 7. If the
measured ordinate is the larger of the two, the error is plus, as at Sta. 5. If the measured ordinate is minus, the
error is the sum of the measured and proposed ordinates, and is minus, as at Sta. 1. Calculate the error at each
station and enter it in column 4. The sum of the entries in this column must be zero.
3.22.
CALCULATIONS
Making the calculations for columns 5, 6, and 7 of figure 3.5 gives you the distance the track must be
moved or "thrown" at each station. They consist of rounding out the ordinate readings by borrowing from the
high ones and lending to the low. Properly done, the calculations provide you with uniform curvature, the result
you wish to obtain. The remainder of the paragraph takes you through the steps.
You first figure out the sum of errors up to and including each station, and enter it in column 5. At Sta. 0
where there is no error and no previous error, the sum of errors is 0. At Sta. 1, the error is -3, but there is no
previous error. As the zigzag arrow shows, 0 is added to -3, and the result is -3. At Sta. 2, previous errors add up
to -3, and the error is +1; -3 +1 equals -2. This calculation is made at each station to the end of the curve. The
total of this column must also be zero. This leads you to where you can figure half of the necessary throw.
For each station, you add the sum of errors at the previous station--column 5, and the half throw there--
column 6, as the arrows between those columns indicate. At Sta. 0, there is no previous error and no previous half
throw so that the half throw is 0. The same thing is true at Sta. 1, but at Sta. 1 there is an error and this will affect
Sta. 2. To find the half throw at Sta. 2, add the sum of the errors at Sta. 1 to the half throw there. Here -3 +0
equals -3, so that figure is entered. At Sta. 3, the figures for Sta. 2 are added; -2 and -3 equal -5, which is the half
throw for the station.
This process is repeated at each station to the end of the curve. Note that the figures in this column need
not add up to 0; however, the last entry in the column, the half throw at the last station, must be 0. If it is not, it
must be adjusted by the method discussed in paragraph 3.24.
The half throw at each station is doubled and entered in column 7. This is the full throw, the amount the
track must be
80
Previous Page
