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Horizontal Angle Measurement
Theodolites or Transits can Measure:
• horizontal angles
• vertical angles
• distances (stadia)
• elevations
Components of a Theodolite
Vertical circle
Horizontal axis
Plate bubble
Horizontal circle
Tribrach
Micrometer
Optical Plummet
Vertical axis
COMPONENTS OF A TYPICAL ‘OPTO-MECHANICAL’ THEODOLITE
Vertical Circle
Micrometer for
Circle Readings
Objective of
Telescope
Fine Vertical
Adjustment for
Telescope
Alignment
(Tangent Screw)
HORIZONTAL AXIS
Optical Switch for
Horizontal/ Vertical
Circle Readings
TURN
Standard
Fine Horizontal
Adjustment for Telescope
Alignment (Tangent
Screw)
Precise Hor. Bubble
(Plate Bubble)
Horizontal Circle
Adjustment
For Orientation
(Orientation Driver)
Eye piece of
optical plummet
Circular Bubble
Vertical Axis
of Rotation
Horizontal circle
housing
Tribrach (detachable)
Foot Screw
Basic Components of an Angle
• reference or starting line (A)
• direction of turn (B)
• angular value (C) usually in DMS
A
B
C
Kinds of Angles
1. Clockwise Interior
2. Counter-clockwise Interior
3. Deflection Angles
R
L
L
Measuring Angles
B
C
1130
A
Azimuth
FIRST
QUAD
2ND
QUAD
4TH
QUAD
3RD
QUAD
Measuring Azimuths
N
B
3100
C
0
490
45
W
270
90
A
135
180
S
E
“CLOSING THE HORIZON”
B
A
D
Measure all the angles around a point
C
Units
• Generally angles/azimuths measured in degrees, mins, secs
• 2 PI Radians = 3600
90
90
100
100
1600
1600
90
90
100
100
1600
1600
DMS
(sexagesimal)
Grads/Gons
Mils
(Russia uses 6000)
Converting Angular Values
Convert a = 390 41’ 54” to radians
2 p radians = 3600 OR p radians = 1800
a = (39 + 41/60 +54/3600) degs / (180/p) ……. radians
DMS must usually be converted to D.DD before they can be
operated on in a calculator or computer. In some cases (e.g.
Excel) angular values must be converted to radians for trig
functions.
Most calculators will have a hard-wired function to go between
DMS (HMS) and D.DD (H) and vice versa.