4-5. Precise Leveling. The instrument used
in first- and
is a precise level. The
instrument is constructed of materials that have a low coefficient of thermal expansion. It is of the
tilting type and is equipped with three horizontal hairs (often referred to as threads or wires) for
determining the rod reading and length of sights.
a. A graduated micrometer is built into the instrument to allow readings to the nearest 0.001 of a
unit. The sensitivity of the tubular vial, the prismatic device for centering the bubble; the telescopic
power; the focusing distance; and the size of the objective lens are factors in determining the precision
of the level. These factors may vary individually, but when one factor is weakened, other factors must
be strengthened to maintain accuracy. Levels are tested and rated according to their ability to maintain
the specified order of accuracy. Only those rated as precise geodetic levels may be used for first- and
second-order work. There are various levels available which are rated as precise-leveling instruments.
One of these, the wild N-3 precision level, is shown in Figure 4-2, page 4-6. Refer to this figure to
identify instrument parts.
(1) Three foot screws (7) rest on a triangular base plate (15), which eliminates the necessity of
inserting slots in the tripod head and permits instrument use with a theodolite tripod. You can regulate
the motion of the foot screws on their threads using an adjusting screw (14). The lug (8) secures the
instrument to its case.
(2) After loosening the azimuth clamp (13), the instrument can swing completely around on its
vertical axis. After tightening the screw, fine movements can be made using the azimuth tangent screw
(12). By turning the tilting screw (4), the telescope can be inclined to a small degree. A precise setting
is assured by transmission through a special level system. The tilting screw is connected to a drum scale
(5) bearing a graduation, each interval of which corresponds to a tilt of 0.01 of the distance between the
instrument and the leveling rod. The tilting screw can be regulated using the adjusting nut (6). A
similar adjusting nut is placed behind the knob of the horizontal azimuth tangent screw.
(3) The micrometer knob (11) controls the optical micrometer. The most important feature of
the optical micrometer is its plane-parallel glass plate, which is placed in front of the objective lens and
turns on a horizontal axis. The movement of the glass plate causes upward or downward displacement
of the telescope's line of sight. The amount of displacement can be read directly on a scale. Since the
total possible displacement amounts to 1 centimeter, it is possible to bring the middle thread of the
reticule into coincidence with the centimeter line of the leveling rod.
(4) The micrometer knob (11) is fitted to a horizontal shaft attached to a cog, which engages the
teeth of a horizontal guide rod. One end of this rod fits, with a ball, into the socket of a lever attached to
the glass plate. The other end bears a fine glass scale divided into 100 intervals. This scale can be read
using the eyepiece (17), which is movable and can be adjusted to the correct focus. One interval on the
scale corresponds to a vertical displacement of 0.1 millimeter on the line of sight. Estimation readings
can be made to an accuracy of 0.01 millimeter.