There are three situations in which target reading,
rather than direct reading, is done on the face of the
When the rod is too far from the level to be
read directly through the telescope:
When a reading to the nearest 0.001 foot, rather
than to the nearest 0.01 foot, is desired (a
vernier on the target or on the back of the rod
makes this possible;
When the instrumentman desires to ensure
against the possibility of reading the wrong
foot (large red letter) designation on the rod.
For target readings up to 7.000 feet, the rod is
used fully closed, and the rodman, on signals from the
instrumentman, sets the target at the point where its
horizontal axis is intercepted by the cross hair, as seen
through the telescope. When the target is located, it is
clamped in place with the target screw clamp, as
shown in figure 5-9.
When a reading to only the
nearest 0.01 foot is desired, the graduation indicated
by the targets horizontal axis is read; in figure 5-9,
this reading is 5.84 feet.
If reading to the nearest 0.001 foot is desired, the
rodman reads the vernier (small scale running from 0
to 10) on the target. The 0 on the vernier indicates
that the reading lies between 5.840 feet and 5.850
feet. To determine how many thousandths of a foot
over 5.840 feet, you examine the graduations on the
vernier to determine which one is most exactly in line
with a graduation (top or bottom of a black dash) on
the rod. In figure 5-9, this graduation on the vernier is
the 3; therefore, the reading to the nearest 0.001 foot
is 5.843 feet.
For target readings of more than 7.000 feet, the
procedure is a little different. If you look at the
left-hand view of figure 5-8 (showing the back of the
rod), you will see that only the back of the upper
section is graduated, and that it is graduated
downward from 7.000 feet at the top to 13.09 feet at
the bottom. You can also see there is a rod vernier
fixed to the top of the lower section of the rod. This
vernier is read against the graduations on the back of
the upper section.
For a target reading of more than 7.000 feet, the
rodman first clamps the target at the upper section of
the rod. Then, on signals from the instrumentman, the
rodman extends the rod upward to the point where the
horizontal axis of the target is intercepted by the cross
hair. The rodman then clamps the rod, using the rod
clamp screw shown in figure 5-13, and reads the
vernier on the back of the rod, also shown in that
figure. In this case, the 0 on the vernier indicates a
certain number of thousandths more than 7.100 feet.
Remember, that in this case, you read the rod and the
vernier down from the top, not up from the bottom.
To determine the thousandths, determine which
vernier graduation lines up most exactly with a
graduation on the rod.
In this case, it is the 7;
therefore, the rod reading is 7.107 feet.
A rod reading is accurate only if the rod is
perfectly plumb (vertical) at the time of the reading.
If the rod is out of plumb, the reading will be greater
than the actual vertical distance between the height of
Figure 5-13.Philadelphia rod target reading of more than