marks. When you cannot see the next lower foot mark
through the level instrument, you signal or ask the
rodman to raise for red. The rodman should slowly
raise the rod until the next lower red number comes
NOTE: The feet measurements on the Philadel-
phia rod are in red.
Conditions that hinder direct reading, such as poor
visibility, long sights, and partially obstructed sights,
as through brush or leaves, sometimes make it
necessary to use targets. The target is also used to mark
a rod reading when numerous points are set to the same
elevation from one instrument setup.
Targets (fig. 15-45) for the Philadelphia rod are
usually oval, with the long axis at right angles to the
rod, and the quadrants of the target painted alternately
red and white. The target is held in place by a C-clamp
and a thumbscrew. A lever on the face of the target is
used for fine adjustment of the target to the line of
sight of the level. The targets have rectangular
openings approximately the width of the rod and
0.15 feet high through which the face of the rod may
be seen. A linear vernier scale is mounted on the edge
of the opening with the zero on the horizontal line of
the target for reading to thousandths of a foot. When
the target is used, the rodman takes the rod reading.
When sighting through the level instrument, the
levelman motions either up or down so that the rodman
can place the horizontal separation of the target in line
with the horizontal cross hair of the instrument. When
the horizontal separation and the horizontal cross hair
coincide, the levelman waves ALL RIGHT.
After the levelman signals the all right, the rodman
tightens the target clamp. Then the rodman holds the
rod on the point again to ensure the target has not
slipped and waves the rod by pushing it about a foot
away from and towards his body to see if the rod was
initially held in an absolutely vertical position. The
levelman should recheck the target reading. If the
horizontal cross hairs do not coincide, the target must
Figure 15-46.Zero elevation point.