some line, such as the center line of a road, can
be used to determine the final grade or alignment
of that road, railroad, or sewer line. Profiles are
also used to compute volumes of earthwork.
Figure 14-16 shows a plotted profile of the
existing ground surface along a proposed highway
center line. Horizontally on the graph, you read
a succession of 100-ft stations, from 0 + 00 to
19 + 00. Vertically, you read elevations. Note
that, horizontally, the interval between adjacent
vertical grid lines represents 25 ft; but vertically
the interval between adjacent horizontal grid lines
represents 2.5 ft.
The profile was plotted through a succession
of points, each of which was obtained from a
profile elevation taken on the ground. Figure
14-17 shows field notes for the levels taken
from 0 + 00 through 10 + 00. The level was first
set up at a point about equidistant from station
0 + 00 and from a BM identified as National
Geodetic Survey Monument, Bradley, Missouri.
The elevation of the BM was 117.51 ft. The first
backsight reading on a rod held on the BM was
7.42 ft. The height of instrument (HI) was
therefore
117.51 + 7.42 = 124.93 ft.
You can see this entered in the “HI” column.
From the first instrument setup, FSs were
taken on station 0 + 00 and 1 + 00. The elevation
of the station in each case was determined by
subtracting the FS reading from the HI. Note that
the FS taken on station 1 + 00 is entered in a
column headed “FS, ” while the one taken on
station 0 + 00 is entered in a different column,
headed “IFS.” “ IFS” means intermediate FS, or
an FS taken on a point that is neither a BM nor
a TP, You can see that station 1 + 00 was used
as a TP in shifting the instrument ahead. Only
FSS taken on BMs or TPs are entered in the
column headed “FS.”
After an FS was taken on station 1 + 00, it
became necessary to shift the instrument ahead.
Station 1 + 00 was used as the TP. From the new
instrument setup, a BS was taken on a rod held
on 1 + 00. The new HI was found by adding the
BS reading to the previously determined elevation
of 1 + 00.
From the new setup, an FS was taken on
station 2 + 00; again, the elevation was found by
subtracting the FS reading from the HI. After this
sight was taken, the instrument was again shifted
ahead, probably because of the steepness of the
slope. This time, station 2 + 00 was used as the
TP2. From the new setup, a BS was taken on
station 2 + 00 and a new HI established. From
this setup, it was possible to take FSs on both
station 3 + 00 and station 4 + 00. Because station
3 + 00 was not used as a TP, the FS on it was
entered under IFS.
Apparently, the slope between station 4 + 00
and station 5 + 00 was so steep that sighting both
stations from the same setup with the rod being
used was impossible. Consequently, an inter-
mediate TP (TP4) was established at station 4 + 75
by determining the elevation of this station. The
instrument was shifted to a setup from which a
BS could be obtained on a rod held on this
station and from which FSs on stations 5 + 00,
6 + 00, 7 + 00, and 8 + 00 could be taken,
Station 8 + 00 was then used as a TP for the last
shift ahead. From this last setup, it was possible
to take FSs on stations 9 + 00 and 10 + 00.
As a check on the arithmetic, you customarily
check each page of level notes to check the
difference between the sum of the FSs and the sum
of the BSs against the difference in elevation
between the initial BM or TP and final BM or TP.
Obviously, only the BSs and FSs taken on BMs
and TPs are relevant to this check, This is the
reason why intermediate FSS not taken on BMs
or TPs are entered in a separate column.
If the arithmetic is correct, the two differences
will be the same. As you can see, the sum of the
relevant BSs in figure 14-17 is 39.63; the sum of
the FSs is 27.70; and the difference between the
two is 11.93. Note that from this difference, the
BS taken on TP5 is deducted. The reason is the
fact that this BS is not offset by a corresponding
FS on a BM or TP. With the BS taken on TP5
deducted, the difference between the sum of the
FSs and the sum of the BSs is 6.86. The difference
between the elevation of TP5 and the elevation
of the initial BM is 6.86, so the arithmetic checks.
Remember that this procedure provides a
check on the arithmetic only. If you have recorded
any incorrect values, the arithmetic will check out
just as well as when you have recorded the correct
values. The procedure is valuable, however, for
detecting two mistakes commonly made by
beginners. These are subtracting a BS from,
instead of adding it to, a BM elevation to get the
HI; and adding an FS to, instead of subtracting
it from, the HI to get an elevation.
14-17
