Defining Angular Measurements and Directions

• Level surface - a curved surface every element of which is normal to a line that is directed exactly toward the earth's center of gravity (plumbline)
• Zenith - a direction away from the center of the earth and above the observer's head
• Nadir - direction towards the center of the earth
• Horizontal angle - formed by the intersection of two lines in a horizontal plane
• Horizontal line - a tangent to a level surface; considered straigth
• Horizontal plane - plane tangent to a level surface at a particular point
• Vertical lines - line perpendicular to the horizontal plane
• Vertical angle - angle between two intersecting lines in a vertical plane
• vertical plane - a plane in which a vertical line is an element
• Components of an angle

  1. reference line
  2. Direction of the turn
  3. Magnitude of the angle itselg
• Zenith Angle - measured from a line directed towards the zenith
• Vertical angle - angle measured from the horizontal
• Nadir angle - angle measured from a line towards the nadir
• Meridian

  1. True meridian
  2. Magnetic meridian
  3. Grid meridian
  4. Assumed meridian
• True meridian - north-south passing through the geographic poles of the earth
• Magnetic Meridian: Lies parallel to the magnetic lines of force of the earth and is indicated by the direction of the magnetized needle (compass)
• Grid meridian: Line parallel to the central true meridian
• Assumed meridian: Arbitrarily chosen for convenience
• Bearing - direction of any line with respect to the given meridian
• Azimuth - direction as given by the angle between the meridian (n or S) and the line measured in a clockwise direction
• Interior angle - Angle between adjacent lines inside a polygon
• Deflection angle - angle between the line and the prolongation of the proceeding line (either to the left or to the right)
• Angle to the right - measured clockwise from the preceding to the following line