Angles Azimuths and Meridians

Introduction

  • Finding the locations of points and orientations of lines depends on measurements of angles and directions.
  • In surveying, directions are given by azimuths and bearings.
  • Angels measured in surveying are classified as
    • Horizontal angels
    • Vertical angles
  • Total station instruments are used to measure angels in the field.

Three basic requirements determining an angle:

    • Reference or starting line,
    • Direction of turning, and
    • Angular distance (value of the angel)

Units of Angel Measurement

In the United States and many other countries:

  • The sexagesimal system: degrees, minutes, and seconds with the last unit further divided decimally. (The circumference of circles is divided into 360 parts of degrees; each degree is further divided into minutes and seconds)
  • In Europe
    • Centesimal system: The circumference of circles is divided into 400 parts called gon (previously called grads)
  • Digital computers
    • Radians in computations: There are 2π radians in a circle (1 radian = 57.30°)
  • Mil – The circumference of a circle is divided into 6400 parts (used in military science)

Kinds of Horizontal Angles

  • The most commonly measured horizontal angles in surveying:
    1. Interior angles,
    2. Angles to the right, and
    3. Deflection angles
  • Because they differ considerably, the kind used must be clearly identified in field notes.

Interior Angles

  • It is measured on the inside of a closed polygon (traverse) or open as for a highway.
  • Polygon: closed traverse used for boundary survey.
  • A check can be made because the sum of all angles in any polygon must equal
  • (n-2)180° where n is the number of angles.

Clockwise interior angles (angles to the right). Counterclockwise interior angles (angles to the left).

Exterior Angles

  • Located outside a closed polygon.
  • The advantage to be gained by measuring this angle is their use as another check.
  • Interior Angel + Exterior Angle = 360°

Angels to the Right

  • The measured clockwise angle between the preceding line and the next line of the a traverse (clockwise from the rear to the forward station).
  • As a survey progresses, stations are identified by consecutive alphabetic letters.

  • Most automatic data collectors require that angles to the right be measured in the field.
  • Turned counterclockwise from the rear station.
  • A serious mistake occurs if counterclockwise angles are measured and recorded or assumed to be clockwise.
  • To avoid this confusion, always measure angel to the right and note the direction of turning in the filed book with a sketch.

Deflection Angles

  • Measured from an extension of the back line, to the forward station.
  • Used principally on the long linear alignments of route surveys.

  • Deflection angles may be measured to the right (clockwise) or to the left (counterclockwise) depending upon the direction of the route.

  • Clockwise (+) and Counterclockwise (-)
  • Deflection angles are always < 180°
  • The direction of turning is identified by appending an R or L to the numerical value.

Azimuths

  • Azimuths are horizontal angles measured clockwise from any reference meridian.
  • In plane surveying, azimuths are generally measured from north.
  • Azimuths are used advantageously in boundary, topographic, control, and other kinds of surveys, as well as in computations.
  • Every line has two azimuths (forward and back) and their values differ by 180°
  • Azimuth are referred to astronomic, magnetic, or assumed meridian
  • For example: the forward azimuth of line AB is 50° – the back azimuth or azimuth of BA is 230° North North

Meridians

  • There are three types of meridians
    • Astronomic – direction determined from the shape of the earth and gravity; also called geodetic north
    • Magnetic – direction taken by a magnetic needle at observer’s position
    • Assumed – subjective direction taken for convenience

Bearing

  • Bearings are another systems for designating directions of lines.
  • The bearing of a line is defined as the acute horizontal angle between a reference meridian and the line.
  • Measured from either the north or south toward the east or west, to give a reading smaller than 90°.
  • For example; N70°E, N30°W, S35°E, and S55°W

  • Assume that total station is set up at points A, B, C, and D; bearings read on lines AB, BA, BC, CB, CD, and DC.
  • AB, BC, and CD are Forward bearings
  • BA, CB, and DC are Back bearings
  • Back bearings should have the same numerical values as forward bearings but opposite letters.
  • Bearing AB = N44°E, bearing BA =S44°W

Azimuths and Bearings

  • If the bearing or azimuth of one side of traverse has been determined and the angles between the sides have been measured, the bearings or azimuths of the other sides can be computed
  • One technique to solve most of these problems is to use the deflection angles

 

 

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