Figure 5-2. The geodimeter (AISI)
c. Levels. Discussed in the following paragraphs are the different types of levels that may be
used in construction surveys.
(1) Dumpy Level. The dumpy level is a very sturdy and reliable instrument that was used
extensively for leveling operations until it was replaced by more modern equipment (see Figure 5-3). Its
sighting device is a 28 variable-power telescope with a maximum length of 18 inches and an erecting
eyepiece that changes the inverted/upside-down image so that it can be seen right-side up. The focusing
knob is normally external. Rotating the focusing knob brings the target into clear focus. The reticle has
two crosshairs at right angles to each other, and some models have stadia hairs for distance measurement
to the nearest foot or meter. Rotating the eyepiece brings the crosshairs into focus. The telescope and
level-bar assembly are mounted on a spindle that permits the unit to be moved only in a horizontal
plane. The telescope and level bar cannot be elevated or depressed. The dumpy level's telescope is
rigidly attached to the level bar that holds an adjustable, highly sensitive level vial. The azimuth clamp
and azimuth tangent screw allow for slow motion of the telescope for accurate centering on a target.
The spindle mounts in a four-screw leveling head that rests on a footplate. The footplate screws onto the
threads of the tripod. When the instrument is properly leveled and adjusted, the horizontal line of site,
which is defined by the horizontal crosshair, forms a horizontal plane.
(2) Automatic Level. The automatic level (also called the auto level) is a self leveling level
that has become the most popular, standard-type level used in construction surveys (see Figure 5-4). It
is very easy to use and can be set up quickly. The automatic level has upgraded surveying operations by
taking the place of the dumpy level, which requires a great amount of time to center its bubble and reset
its position. The automatic level has a small circular level called a bull's-eye level and only three
leveling screws. The leveling screws are on a triangular footplate and are used to center the bubble in
the bulls-eye level. The line of sight automatically becomes horizontal and remains horizontal as long
as the bubble stays centered. Inside the automatic level, a gravity-suspended prism (called a
compensator) is hung on fine, nonmagnetic wires. The action of gravity on the compensator causes the
EN0591
5-4
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