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Which of the following is not a function of levelling head?
To support the main part of the instrument
To attach the theodolite to the trip
To provide a mean for levelling the theodolite
To provide the exact centering over the station mark
To provide the exact centering over the station mark
The levelling head of a theodolite consists of two parallel triangular plates and three foot screws ┬╖ Its primary functions are to support the instrument, facilitate attachment to the tripod, and perform the levelling process; however, it does not perform lateral shift for centering.
The levelling head of a theodolite consists of two parallel triangular plates and three foot screws ┬╖ Its primary functions are to support the instrument, facilitate attachment to the tripod, and perform the levelling process; however, it does not perform lateral shift for centering.
L=тИСi=1nтАЛsiтАЛтЛЕfiтАЛ тАФ Representation of tilt adjustment via foot screws
╬Фx,╬Фy тАФ Represent lateral displacement which is NOT handled by the levelling head
Levelling is achieved by manipulating the foot screws to tilt the tribrach until the plate bubble is centered ┬╖ Centering is a distinct operation usually performed using a shifting head (on the tripod or instrument) or an optical plummet/plumb bob, which allows for small lateral adjustments relative to the station mark.
Levelling head consists of a tribrach and three foot screws.
The primary goal of levelling is to make the vertical axis of the instrument truly vertical.
Centering is the process of setting the instrument exactly over a survey station.
Shifting heads are typically provided between the tripod head and the tribrach for centering.
Provides precise horizontal orientation
Allows smooth adjustment of tilt
Does not inherently correct for longitudinal or lateral positional errors
Requires a separate plumb bob or optical plummet for true verticality
Surveying theodolite setup
Total station mounting
Centering is often facilitated by a sliding mechanism called a 'shifting head' which allows the instrument to move on the tripod head without rotating.
Option D is the correct answer because levelling heads lack the mechanism for lateral shift required to align the instrument's center with a station point precisely.
D is correct тАФ The levelling head is designed for angular adjustment (leveling) and mechanical support, not for the lateral translation required for precise centering.
Always distinguish between 'Levelling' (adjusting the vertical axis) and 'Centering' (adjusting the planimetric position) when analyzing surveying instrument setup procedures.