Lec9 : Direction of Lines

Angles (or Angles measured)

Are classified as either horizontal or vertical.

Directions

- In surveying, are given by azimuths and bearings.

Sexagesimal System / English System

Based on degrees, minutes, and seconds

Centesimal System / French System

Based on gradians

International System (SI)

Based on radians

Angular Mil

Based on milliradians

Direction of Lines

1rev = 360° = 2π = 400grads = 6400mils

Meridian

The direction of a line is defined by a horizontal angle between the line and an arbitrary chosen reference line

Types of meridians

1. Geographic (True)

2. Magnetic

3. Grid

4. Assumed

Geographic Meridian

North-south reference line that passes through a mean position of the Earth’s geographic poles.

Magnetic Meridian

Defined by a freely suspended needle that is only influenced by the Earth’s magnetic field.

Grid Meridian

A fixed line of reference parallel to the central meridian of a system of plane rectangular coordinates.

Assumed Meridian

Established by merely assigning any arbitrary direction.

Geographic and Magnetic Meridian

Grid and Assumed Meridian

True North

North point of the true meridian.

Magnetic North

Established by means of a magnetic compass needle when there are no local attractions affecting it.

Grid North

North point which is established by lines on map which are parallel to a selected central meridian.

Assumed North

Used to portray the location of any arbitrary chosen north point.

Bearings

- The acute horizontal angle between a reference meridian and the line

- Observed from either north or south toward the east or west

Bearings

Azimuths

- Horizontal angles observed clockwise from any reference meridian

- Observed from either north or south toward the east or west

- To avoid confusion, it is necessary to state in the field notes what reference meridian applies for them and whether they are observed from north or south

Azimuths