Angular Measurement
Angular Measurement
Angular Measurement
• Horizontal and vertical angles are fundamental
measurements in surveying
• Theodolite is the instrument used to perform
accurate angular measurements in surveying
• Vertical angle is the angle of
elevation or depression
between the line of collimation
and the horizontal plane which
passes through the horizontal
axis of the theodolite
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The geometrical relationships of a
properly adjusted instrument
(1) the vertical axis should be ⊥ to
the plate bubble axis
(2) the horizontal axis should be ⊥
to the vertical axis
(3) Line of collimation should be ⊥
to the horizontal axis
(4) vertical axis reads 90° or 270 °
when the telescope is placed at
level position
(5) The optical plummet coincides
with the line of gravity and
passes through the vertical axis
(6) The crosshairs are in focus so
that no parallax exists
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• Horizontal angle is the
difference between 2
intersecting lines when they are
projected onto the datum plane
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Station Adjustment or Temporary
Adjustment
• A theodolite must be accurately adjusted to
the correct position and is levelled in every
set-up.
• This adjustment includes:
– Setting over the station (centering);
– Levelling up; and
– Focusing and Elimination of Parallax.
( Please refers to the extra notes provided on the
procedures of temporary adjustment )
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Angular Measurement
Setting Up – rough centering
(1) Setting up
(1) Establish the
tripod roughly over
the survey point
(2) Examine and memorise
the position of the
instrument in the box
(3) Attach the
theodolite to the
tripod
(4) Using the footscrews to incline the line of
sight through the optical plummet, centre
the plummet exactly on the survey point
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Rough & Fine Levelling
• slide the legs in or out until the
circular bubble is exactly centre.
• although the tripod movement may
be excessive, the plummet will still
be on the survey point.
• instrument is approx. centred and
levelled
Total Station Instrument
Vertical tangent screw
0°
Focusing ring
Vertical Clamp screw
90
Horizontal tangent screw
Eyepiece
•precisely level the instrument using the plate
bubble
•Unclamp and move the whole instrument
over the tripod until the plummet crosshair is
exactly over the survey point
•repeat these 2 steps until the instrument is
exactly centred and levelled
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Horizontal clamp screw
Display Panel
ON/OFF switch
Distance Measurement button
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Angular Measurement
Eyepiece
Focusing ring
Display Panel
Horizontal clamp screw
Distance
Measurement
button
Horizontal
tangent screw
ON/OFF switch
Optical plummet
footscrew
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Errors in Angular measurement
• Instrumental errors; Human errors; Natural errors
Instrumental Errors
• systematic errors and can be corrected through permanent adjustment of the
theodolite
• 5 types: Vertical axis error; Horizontal axis error; Horizontal collimation error;
Vertical collimation error; Optical plummet error
Natural Errors
• They are:
1. Unequal atmospheric refraction (choose cool days or night time);
2. Differential expansion in certain of the theodolite (insulation);
3. Vibration of the theodolite due to strong wind
4. Improper settlement of the tripod (pushing tripod legs firmly into the grounds)
5. Limitations of the theodolite reading systems and human eyesight
6. Heat shimmer
(Note: To minimize time spent and movements while using the theodolite in the
observations )
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Face of theodolite and compensated
measurement
• Position of the vertical circle relative to the
observer
• Face Left (F.L.) – normal observing position
where the vertical circle is on the left of the
observer
• Face Right (F. R.) – vertical circle is to the right
side of the observer
• To change from F.L. to F.R., the telescope is
transited and then followed by 180° turn in the
horizontal plane
• The telescope will than be pointing at the target
and with the vertical circle to the right of the
observer
• All angles should be measured once with F.L.
and once with F.R. and the results be averaged to
eliminate most of the instrumental error
face left
face right
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Errors in Angular measurement
Human Errors
•
Mistakes caused by poor observational techniques or carelessness.
•
They are serious and significant as it is impossible to correct or make
adjustments
•
They can be avoided if proper field procedure is adopted such as
observing more than one round of observations
•
The errors are:
1. Set up the theodolite on a wrong station
2. Sights a wrong target
3. Fails to recognize the settlement of the tripod
4. Transcribes errors and interchanges digit in booking
5. Reads the wrong circle in the reading system
6. Ignores the movement of the plate bubble during observation
7. Fails to adjust the eyepiece to eliminate parallax completely
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Angular Measurement
Horizontal Angle
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Verticals Angle or Zenith Angle
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Verticals Angle or Zenith Angle
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Angular Measurement
Face Left (FL) Position
Face Right (FR) Position
Focusing ring
Eyepiece
Vertical tangent screw
Optical
Theodolite
Vertical
Clamp screw
face
left
face right
Horizontal
clamp screw
Horizontal tangent screw
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