instrument (H.I.) and the base of the rod. On a windy
day, the rodman may have difficulty holding the rod
In this case, the levelman can have the
rodman wave the rod back and forth, allowing the
levelman to read the lowest reading touched on the
engineers level cross hairs.
The use of a rod level ensures a vertical rod. A
bulls-eye rod level is shown in figure 5-14. When it
is held as shown (on a part of the rod where readings
are not being taken to avoid interference with the
instrumentmans view of the scale) and the bubble is
centered, the rod is plumb. A vial rod level has two
spirit vials, each of which is mounted on the upper
edge of one of a pair of hinged metal leaves. The vial
level is used like the bull s-eye level, except that two
bubbles must be watched instead of one.
Care of Leveling Rods
A leveling rod is a precision instrument and must
be treated as such. Most rods are made of carefully
selected, kiln-dried, well-seasoned hardwood. Scale
graduations and numerals on some are painted
directly on the wood; however, on most reds they are
painted on a metal strip attached to the wood. Unless
a rod is handled at all times with great care, the
painted scale will soon become scratched, dented,
worn, or otherwise marked and obscured. Accurate
readings on a scale in this condition are difficult.
Allowing an extended sliding-section rod to close
on the run, by permitting the upper section to drop,
may jar the vernier scale out of position or otherwise
damage the rod.
Always close an extended rod by
easing the upper section down gradually.
A rod will read accurately only if it is perfectly
straight. It follows, then, that anything that might
bend or warp the rod must be avoided. Do not lay a
rod down flat unless it is supported throughout, and
never use a rod for a seat, a lever, or a pole vault. In
short, never use a rod for any purpose except the one
for which it is designed.
Store a rod not in use in a dry place to avoid
warping and swelling caused by dampness. AI ways
wipe off a wet rod before putting it away. If there is
dirt on the rod, rinse it off, but do not scrub it off. If a
soap solution must be used (to remove grease, for
example), make it a very mild one. The use of a
strong soap solution will soon cause the paint on the
rod to degenerate.
Figure 5-14.Bulls-eye rod level.
Protect a rod as much as possible against pro-
longed exposure to strong sunlight. Such exposure
causes paint to chalk (that is, degenerate into a
chalk-like substance that flakes from the surface).
LEARNING OBJECTIVE: Upon completing
this section, you should be able to determine
elevations in the field to locate points at
The most common procedure for determining
elevations in the field, or for locating points at
specified elevations, is known as differential leveling.
This procedure, as its name implies, is nothing more
than finding the vertical difference between the
known or assumed elevation of a bench mark and the
elevation of the point in question. Once the
difference is measured, it can (depending on the
circumstances) be added to or subtracted from the
bench mark elevation to determine the elevation of
the new point.
ELEVATION AND REFERENCE
The elevation of any object is its vertical distance
above or below an established height on the earths
This established height is referred to as
either a reference plane or simple reference. The
most commonly used reference plane for elevations is
mean (or average) sea level, which has been assigned
an assumed elevation of 000.0 feet. However, the
reference plane for a construction project is usually
the height of some permanent or semipermanent