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A dumpy level, builder's auto
level, leveling instrument, or automatic level is an optical instrument
used in surveying and building to transfer, measure, or set horizontal
levels.
The level instrument is set up on a tripod and, depending on the type,
either roughly or accurately set to a leveled condition using footscrews
(levelling screws). The operator looks through the eyepiece of the telescope
while an assistant holds a tape measure or graduated staff vertical
at the point under measurement. The instrument and staff are used to
gather and/or transfer elevations (levels) during site surveys or building
construction. Measurement generally starts from a benchmark with known
height determined by a previous survey, or an arbitrary point with an
assumed height.
A dumpy level is an older-style instrument that requires skilled use
to set accurately. The instrument requires to be set level (see spirit
level) in each quadrant, to ensure it is accurate through a full 360°
traverse. Some dumpy levels will have a bubble level insuring an accurate
level.
A variation on the dumpy and one that was often used by surveyors, where
greater accuracy and error checking was required, is a tilting level.
This instrument allows the telescope to be effectively flipped through
180°, without rotating the head. The telescope is hinged to one side
of the instrument's axis; flipping it involves lifting to the other
side of the central axis (thereby inverting the telescope). This action
effectively cancels out any errors introduced by poor setup procedure
or errors in the instrument's adjustment. As an example, the identical
effect can be had with a standard builder's level by rotating it through
180° and comparing the difference between spirit level bubble positions.
An automatic level uses a swinging prism to compensate for small inclination
deviations.An automatic level, self-levelling level or builder's auto
level, includes an internal compensator mechanism (a swinging prism)
that, when set close to level, automatically removes any remaining variation
from level. This reduces the need to set the instrument truly level,
as with a dumpy or tilting level. Self-levelling instruments are the
preferred instrument on building sites, construction and surveying due
to ease of use and rapid setup time.
A digital electronic level is also set level on a tripod and reads a
bar-coded staff using electronic laser methods. The height of the staff
where the level beam crosses the staff is shown on a digital display.
This type of level removes interpolation of graduation by a person,
thus removing a source of error and increasing accuracy.
The term dumpy level endures despite the evolution in design.
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