motion of the rod permits the instrumentman to
read the rod when it reaches a vertical position
at the top of the arc and when the lowest reading
appears on the rod. Before or after the rod is in
this vertical position, the rod reading is greater.
5. Set the turning pin or pedestal firmly in
contact with the ground when setting a TP. Any
unfirm footing can sag under the weight of the
rod and result in incorrect readings between the
FS and BS. During freezing and thawing weather
conditions, the ground surface can heave in a
comparatively short time. Pins and pedestals can
be affected by the heave between the FS and the
following BS. For higher order of accuracy
surveys, you should be aware of this possibility
and select firm locations.
6. Extend the leveling rod to its maximum
length when the instrumentman calls for extending
the rod. The standard Philadelphia leveling rod
can be read to 7.100 ft or 2.164 meters when
collapsed and 13.000 ft or 3.962 meters when
extended. An extended leveling rod is called a
LONG ROD.
A leveling rod is a precision instrument and
has to be treated with care. Most rods are made
of carefully selected, kiln-dried, well-seasoned
hardwood and have metal scale faces on which
the scale graduations are painted. Unless a rod
is always handled with great care, the painted face
will become scratched, dented, or damaged in
other ways. Accurate readings on a rod that is
damaged are difficult.
Letting an extended rod close “on the run”
by allowing the extended upper section to drop
tends to damage both sections of the rod and to
displace the vernier. Always close an extended rod
by easing down the upper section.
A rod will read accurately only if it is perfectly
straight, so you must avoid anything that might
bend or warp the rod. Do not lay a rod down flat
unless it is supported throughout on a flat
surface. Do not use a rod as a support or as a
lever. Store the rod in a dry place to avoid any
possible warping and swelling from dampness,
and always wipe a wet rod dry before stowing it
away.
If there is mud on the rod, rinse it off, but
do not scrub it. If you have to use a soap solution
to remove grease, use a mild solution. Repeated
washings with strong soap solutions will eventually
cause the painted graduations to fade.
FIELD PROCEDURES FOR
DIFFERENTIAL LEVELING
Leveling operations require the teamwork of
both the instrumentman and the rodman to
achieve consistent results. The accuracy of the
survey depends upon the refinement with which
the line of sight can be made horizontal by the
instrumentman, the ability of the rodman to hold
the rod vertically, and the precision with which
the rod reading is made. Some of the basic
procedures and preparations applicable to direct
leveling are presented below.
Selecting Setup Points
Terrain and atmospheric conditions are the
main considerations affecting the selection of
setup points. It is essential that you select a point
from which you can best observe a rod reading
on the BS and FS points. In the interest of
balanced shots, a setup point should be about
equidistant from both BS and FS. In addition,
shorter setup distances will result in smaller
instrument errors caused by the atmospheric
refraction and curvature of the earth.
The average instrument height at any setup is
about 5 ft (1.5 m). On even downhill slopes, the
ground where the instrument is set up may not
be more than 3 to 5 ft below the TP for a level
BS. On the FS, the extended rod can be held on
the ground about 8 ft (2.5 m) below the
instrument ground level and still permit a reading
to be taken. This means that the tendency will be
to make FS distances longer going downhill and
to make BSs longer going uphill.
Therefore, it is necessary to conduct a
reconnaissance of the terrain before you start
leveling. You should note probable locations
of instrument setup and TPs. During the
reconnaissance, you should estimate the line of
sight by sighting through a hand level.
Setting Up a Level
In setting up the tripod, you first hold two
tripod legs with both hands and spread the tips
of these legs a convenient distance apart. Then
bring the third leg to a position that approximately
levels the top of the protector cap when the
tripod stands on all three legs. Then unscrew the
protector cap.
Next, you lift the instrument out of the
carrying case by the footplate or level bar, NOT
by the telescope, and set it gently and squarely
14-7
