(11) The optical micrometer can be used for all rod readings. Turn the micrometer knob (11) until
the wedge lines of the reticule enclose an interval line of the rod (as shown in Figure 4-4). The interval
gives the reading in centimeters. Observe the micrometer reading through the micrometer scale
eyepiece (17). As shown on the lower left in Figure 4-4, the scale runs from left to right. (In older N-3
models, the scale runs from right to left.) The figures on it represent millimeters, while the smaller
intervals represent tenths of millimeters. Estimations may be made to hundredths of a millimeter. The
reading obtained on the micrometer is added directly as a decimal fraction to the rod reading. The
middle thread reading in Figure 4-4 is 77.653 centimeters (0.77653 meter) and would be recorded as
b. Most surveying organizations within the Army and the majority of other well-known federal
surveying agencies lock the micrometer with a micrometer setting lock. When installed, this lock is
located just forward and in from the front end of the reflector. The rod is read directly to the nearest
centimeter, and the reading is estimated and recorded to the nearest millimeter. When reading the rod
by this direct method, the readings of the three crosshairs (as shown in Figure 4-4) are as follows: Read
the bottom hair first (0.807 millimeter), and record it as 0807 at the top of the page in the field book.
Read the middle hair (0.770 millimeter), and record it as 0770. Read the top hair last (0.733 millimeter),
and record it as 0733. This procedure speeds the leveling process and provides required accuracies and
checks with minimum effort by the observer and recorder.
4-6. Precise Rods.
A precise rod
in first- and
second-order leveling. This rod
consists of a
graduated metal strip, 25 millimeters wide by 1 millimeter thick, with a low coefficient of thermal
expansion. The strip is rigidly attached to a metal foot piece. The foot piece is fastened to a wooden
backing which supports the strip. The backing is about 2.8 centimeters thick by 7.6 centimeters wide,
with an overall length of 3.3 meters.
a. The front of the rod is graduated in meters and decimeters. The Invar strip is graduated in
centimeters. The rear of the rod is graduated in feet and tenths of feet. Mounted on the back of the rod
is a circular or bull's-eye level and thermometer. The bull's-eye bubble is used to plumb the rod. The
thermometer, located above the bull's-eye bubble, is used to obtain the rod temperature to correct for the
expansion or contraction of the metal strip.
b. Rods are standardized by the NBS (or their foreign counterpart), and their index and length
corrections are determined. The rods are paired and marked according to similar characteristics and
their convenience in applying various corrections. The Invar strips require periodic checks against the
standard to determine any changes that may affect the accuracy of readings.