What is a bearing in compass surveying?

Short Answer:

A bearing in compass surveying is the horizontal angle measured between a survey line and a fixed reference direction, usually magnetic north. It helps in describing the direction of one point from another on the ground.

Bearings are measured in degrees and can be either in whole-circle (0° to 360°) or quadrantal (0° to 90° from a cardinal direction) format. They are essential in preparing survey maps, calculating angles, and plotting land boundaries accurately during compass-based surveys.

Detailed Explanation:

Bearing in compass surveying

In compass surveying, a bearing is the direction of a line relative to a fixed reference point, commonly magnetic north. It is measured as an angle formed between the survey line and the reference meridian. Bearings are crucial because they help surveyors describe and record the orientation of survey lines, making it easier to draw maps, calculate angles, and understand the layout of an area.

Bearings are one of the basic tools in surveying and navigation. They allow surveyors to establish directions between known points and help in completing traverses (a connected series of lines forming a survey path). Without bearings, it would be difficult to determine the accurate shape and orientation of a land parcel.

  1. Types of bearings

There are mainly two types of bearings used in compass surveying:

  • Whole-circle bearing (WCB): This is measured clockwise from the magnetic north. It ranges from 0° to 360°. For example, a line pointing directly east has a bearing of 90°, south is 180°, and west is 270°. This system is used mostly in prismatic compasses.
  • Quadrantal bearing (QB): This system uses the four main directions—north, south, east, and west—and measures angles within 90° from either north or south towards east or west. It is expressed in forms like N 30° E or S 45° W. This format is often used in surveyor’s compasses.

Both types of bearings are useful depending on the method and instrument being used. However, whole-circle bearings are simpler for calculations and widely used in modern surveys.

  1. Fore bearing and back bearing
  • Fore bearing (FB): The bearing taken in the forward direction of a survey line.
  • Back bearing (BB): The bearing taken in the reverse direction of the same line, usually differs by 180°.

For example, if the fore bearing is 70°, the back bearing would be 70° + 180° = 250°, and vice versa. These help in error checking and balancing survey measurements.

  1. Measurement process

To measure a bearing:

  1. Set up the compass over a known point using a tripod or stand.
  2. Sight the next point along the survey line through the instrument’s sighting device.
  3. Read the angle (bearing) shown on the compass scale.
  4. Record the bearing in a field book along with the line’s length.

This bearing will then be used in calculations to determine relative positions of points and to create maps.

  1. Importance of bearings

Bearings help in:

  • Fixing directions between survey points
  • Drawing accurate survey maps
  • Calculating included angles in closed traverses
  • Setting out roads, buildings, and boundaries
  • Navigating routes in forest, hills, or remote areas

Surveyors rely on correct bearings to ensure the layout on ground matches the design or plan accurately.

  1. Errors affecting bearings

Bearings can be affected by local attraction, which is caused by nearby magnetic objects like electric poles, iron fences, or vehicles. To avoid errors:

  • Check the difference between fore and back bearings.
  • Avoid working near metallic objects.
  • Apply corrections when needed.

Proper care during measurement ensures that bearings remain reliable and survey results are accurate.

Conclusion:

A bearing in compass surveying is the angle measured from a reference direction (usually magnetic north) to a survey line. It is used to define the direction of lines during a survey and is essential for drawing maps and calculating land boundaries. By using whole-circle or quadrantal systems, surveyors can accurately describe directions between points. Accurate bearing measurement ensures precise, useful, and reliable survey results in civil engineering work.