defines a horizontal sight.
is maintained despite a
This horizontal line of sight
slight out of level of the
telescope or even when a slight disturbance occurs on
the instrument.
HAND LEVEL
The hand level, like all surveying levels, is an
instrument that combines a level vial and a sighting
device. It is generally used for rough leveling work. In
a cross-sectional work, for example, terrain
irregularities may cause elevations to go beyond the
instrument range from a setup. A hand level is useful
for extending approximate elevations off the control
survey line beyond the limits of the instruments.
Figure 11-28, view A, shows a LOCKE HAND
LEVEL; view B shows an ABNEY HAND LEVEL.
For greater stability, both hand levels may be rested
against a tree, rod, range pole, or on top of a staff. A
horizontal line, called an index line, is provided in the
sight tube as a reference line. The level vial is mounted
atop a slot in the sight tube in which a reflector is set at
a 45° angle. This permits the observer, while sighting
through the tube, to see the landscape or object, the
position of the bubble in the vial, and the index line at
the same time.
The distances over which a hand level is sighted are
comparatively short; therefore, no magnification is
provided for the sighting.
The Abney hand level is more specialized than the
Locke type. It has a clinometer for measuring the
vertical angle and the percent of grade. The clinometer
has a reversible graduated arc assembly mounted on one
side. The lower side of the arc is graduated in degrees,
and the upper side, in percent of slope. The level vial
is attached to the axis of rotation at the index arm.
When the index arm is set at zero, the clinometer is
used like a plain hand level. The bubble is centered by
moving the arc and not the sighting tube as is the case
in the plain hand level. Thus, the difference between
the line of sight and the level bubble axis can be read in
degrees or percent of slope from the position of the
index arm of the arc. The 45° reflector and the sighting
principle with its view of the landscape, bubble, and
index line are the same as in the plain hand level.
45.749B
Figure 11-28.-Types of hand levels.
11-32
