The theodolite is an instrument used to measure horizontal and vertical angles that is more precise than a magnetic compass. It can measure angles to an accuracy of 10-20 seconds whereas a compass is only accurate to 30 minutes. The theodolite is used to measure horizontal and vertical angles when objects are at a distance or elevation where more precise measurements are needed. The method of surveying that uses a theodolite to measure angles is called theodolite surveying. The theodolite can be used to measure angles, bearings, distances, elevations, set out curves, and for mapping and construction applications.

1. THEODOLITE
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2. Theodolite surveying
• The theodolite is the most precise instrument used for
measurement of horizontal and vertical angles than compass.
• When the objects are at a considerable distance or situated at a
considerable elevation or depression ,it becomes necessary to
measure horizontal and vertical angles more precisely. So these
measurements are taken by an instrument known as a
Theodolite.
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3. • It is more precise than magnetic compass.
• Magnetic compass measures the angle up to an accuracy of
30’.
• However a vernier theodolite measures the angles up to and
accuracy of 10’’, 20”.
• The system of surveying in which the angles are measured with
the help of a theodolite, is called theodolite surveying.
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4. Uses of theodolite
• Measurement of Horizontal and Vertical angle
• Measurement of magnetic bearing of lines
• Locating points on line
• Prolonging survey lines
• Establishing grades
• Measuring the horizontal distance between two points
• Determining difference in elevation
• Setting out curves
• Mapping applications and in the construction industry, Aligning
tunnels, Mining works etc.
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5. Theodolites may be classified as ;
A. Primary
i) Transit Theodolite.
ii) Non Transit Theodolite.
B. Secondary
i) Vernier Theodolites.
ii) Micrometer Theodolites.
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6. i) Transit Theodolite: or simply transit is one in which the line of
sight can be reversed by revolving the telescope through 180º
about its horizontal axis in the vertical plane, i.e it can revolve
through a complete revolution about its horizontal axis in the
vertical plane.
ii)Non-Transit theodolite/plane theodolite / Y-theodolites the
telescope cannot be transited. They are inferior in utility and
have now become obsolete.
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7. i)Vernier Theodolite: For reading the graduated circle if verniers
are used ,the theodolite is called as a Vernier Theodolite.
ii)Whereas, if a micrometer is provided to read the graduated
circle the same is called as a Micrometer Theodolite.
Vernier type theodolites are commonly used.
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Transit Theodolite Non- Transit Theodolite Micrometer TheodolitesVernier Theodolite

9. Essential parts of a Transit Theodolite
I. Telescope
II. The vertical circle
III. The index frame (or T-
frame or Vernier
Frame)
IV. The standards or A-
frame
V. The levelling head
VI. The two spindles (or
Axes or Centres)
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VII. The lower plate or
Scale Plate
VIII.The upper plate or
vernier plate
IX. The plate levels
X. Tripod
XI. The plumb bob
XII. The compass
XIII.Striding level

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13. DEFENITIONS AND TERMS
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