Angles and Directions
4.1
Angles can be measured by transit/theodolite or total station.
(Clinometers, sextant (hydrographic) or compasses)
4.2
Reference directions for vertical angles.
1.
2.
3.
4.3
Reference to horizontal (+ or -)
Zenith (above observer)
Nadir (below observer)
Meridians
Geographic Meridian:
and South Poles
Line on the mean surface of earth joining North
Intersection of each surface with plane including the axis
of rotation of earth (fixed)
Magnetic Meridian:
Lines parallel to directions taken by freely moving
magnetized needless (compass) – (vary with time and
location)
Grid meridian:
// to central meridian
All horizontal directions referenced to meridians
4.4
Horizontal Angles
Usually measured by theodolite or TS to 1” – 20” precision
Closed Traverse
Closed Polygon
n side
Σ Interior Angles =
Σ Exterior Angles =
(n – 2) 180
(n + 2) 180
Check on accuracy
Open Traverse
Deflection angle
Right of or left of the prolongation of the back line to the
forward line.
4.5
Azimuths
Direction of a line given by an angle measured clock-wise from North
end of Meridian 0 – 360o
4.6
Bearing
Direction of a line given by acute angle between the line and a
meridian.
Clockwise or counter clockwise
NE, NW, SE, SW
Due West, Due North, ……. etc.
4.7
Relationship between bearings and azimuths
See Page 105
4.8
Reverse Direction
To get Back Azimuth of line AB Add or Subtract 180 o
To get Back Bearing of line AB Reverse the direction letters.
4.9
Azimuth Computations
Counterclockwise:
Add the interior angle to the back
azimuth of the previous course.
Clockwise:Subtract the interior angle from the back azimuth
of the previous course.
4.10 Bearing
Computations
Neat Well labeled diagram for each Computation
4.11 Comments on Bearing and Azimuth
See page 113
4.12 Magnetic Direction
Magnetic North Pole is some distance from geographic North
Pole
Magnetic Declination is the angle between geographic
North Pole and Magnetic North Pole direction.
Isogonic Chart: lines on earth surface that have same
Magnetic Declination