Parts of the Instrument and Levelling Head
The aim of this tutorial is to introduce you to the automatic level and levelling head on the instrument. The three screw levelling head is found on most modern survey instruments including levels, theodolites, and EDM systems.
Typical parts of an automatic level
The instrument is secured to the tripod head using the tripod screw.
Right
side of Level
The levelling
head has three parts:
A top plate or tribrach which carries a spirit level and the instrument
Three levelling, or foot, screws
A foot plate or trivet that attaches to the tripod head
Also labeled in this picture:
The horizontal circle, which allows the instrument to be used to measure horizontal angles to an accuracy of 1° (Not found on all instruments)
The object focusing screw, which is used to bring the staff or other image in to focus.
Top
of Level
This picture shows the instrument controls:
Spherical level is a bubble spirit level attached to the tribrach and referenced to the axis of the telescope. In use the bubble must be within the circle for the instrument to give a horizontal sight line (Collimation).
The eye piece is adjustable and should be set for each observer to bring the cross hairs in to sharp focus.
The instrument can be rotated by hand, using the 'gun sight' on top of the telescope to find the staff.
Tangent screws (one on each side) allow fine adjustment when aligning the telescope on the staff, or setting out a horizontal angle using the horizontal circle.
The Telescope is focussed using the object focus screw on the right side of the instrument.
Left
side of level
The only new component in this view is the mirror over the spherical level. This mirror allows the observer to see the bubble and confirm that the instrument is correctly levelled before taking a reading. Not all instruments will have a mirror.
Linear bubble level
Not all instruments have exactly the same features. This automatic level has a linear bubble level. It only has one tangent screw and the horizontal circle is replaced by marks at 90° intervals to allow setting out of right angles.
Eye piece focusing of the cross hairs and an object focus screw are usually provided on all instruments.
Now check your knowledge:
