TBM 16. The BS reading of +6.659 is added to
the elevation of BM 35 and gives the resulting HI
(139.822). The rod is moved to Peg 16 (which later
becomes TBM 16). The FS reading of –4.971 is
subtracted from the HI to get the elevation of Peg
16. Note that the distance (220 ft each way) is also
recorded for balancing. The process continues
until BM 19 is reached.
LEVEL COMPUTATIONS.— ln making level
computations, you should be sure to check on the
notes for a level run by verifying the beginning
BM; that is, by determining that you used the
correct BM and recorded its correct elevation, as
required.
Then, you should check on the arithmetical
accuracy with which you added BSs and subtracted
FSs. The difference between the sum of the BSs
taken on BMs or TPs and the sum of the FSs
taken on BMs or TPs should equal the difference
in elevation between the initial BM or TP and the
final BM or TP.
Balanced BS and FS distances are shown in
figure 14-14. The distance used for the first
instrument setup was 220 ft. The first BS (rod
reading on El 35) was 6,659 ft. The first FS (rod
reading on 0 16) was 4.971. Notice that the plus
sign (+) appears at the top of the BS column and
that the minus sign (–) appears at the top of the
FS column in the field notebook. This helps you
to remember that BSs are added and FSs are
subtracted as you compute the new elevations.
The BS taken on a point added to the elevation
of the point gives the HI. This establishes the
elevation of the line of sight so that an FS can
then be taken on any point (BM, TBM, or TP).
The level line is extended as far as desired with
as many instrument setups as may be necessary
by a repetition of the process used in the first
setup.
The elevation of El 35 is 133.163 ft. The first
HI is
133.163 + 6,659 = 139.822 ft.
The FS subtracted from the HI,
139.822 – 4.971 = 134.851 ft,
gives the elevation of 0, the first established.
Following through with a similar computation for
each setup, notice that the elevation of El 19 was
found to be 136.457 ft.
Look now at the notes in figure 14-14. The
sum of all the BSs is 24.620 ft. The sum of all
the FSs is 21.326 ft. The difference between the
sum of the BSs and the sum of the FSs is
24.620 – 21.326 = 3.294 ft.
This difference should agree with the difference
between the actual elevation of BM 35 and the
elevation already found for BM 19; that is,
136.457 – 133.163 = 3.294 ft.
This provides a check on the step-by-step
computation of elevations.
ADJUSTMENT OF INTERMEDIATE BENCH
MARK ELEVATIONS.— Level lines that begin
and end on points that have fixed elevations, such
as BMs, are often called level circuits. When
leveling is accomplished between two previously
established BMs or over a loop that closes back
on the starting point, the elevation determined for
the final BM will seldom be equal to its previously
established elevation. The difference between
these two elevations for the same BM is known
as the ERROR OF CLOSURE. The Remarks
column of figure 14-14 indicates that the actual
elevation of BM 19 is known to be 136.442 ft. The
elevation found through differential leveling was
136.457 ft. The error of closure of the level
circuit is
136.457 – 136.442 = 0.015 ft.
It is assumed that errors have occurred
progressively along the line over which the leveling
was done so that adjustments for these errors are
distributed proportionally along the line as shown
by the following example: Referring to figure
14-14, you will notice that the total distance
between BM 35 and BM 19, over which the line
of levels was run, was 2,140 ft. The elevation on
the closing BM 19 was found to be 0.015 ft greater
than its known elevation. You must therefore
adjust the elevations found for the intermediate
TBMs 16, 17, and 18.
The amount of correction is calculated as
follows:
TBM 16 is 440 ft from the starting BM. The total
length distance between the starting and closing
BMs is 2,140 ft. The error of closure is 0.015 ft.
14-14
