What is Compass Surveying? Principle & Bearings

Compass surveying is a type of land surveying technique used to determine the location of various points on the ground by measuring the angles using a compass and linear measurements by using a chain.

Compass survey is employed for small-scale surveys like property surveys or land subdivisions where the accuracy demanded is not high compared to large-scale engineering surveys. 

This article explores the basic principle followed in compass surveying, the measurements, and their applications. 

What is the principle of Compass Surveying?

The principle of compass surveying is traversing. Traversing is the method of locating a series of survey lines in the form of a network by placing survey stations one after another. The previously surveyed station is used as the base to measure the next point.
The traverse can be either open traverse or closed traverse.

Unit of Measurement in a Compass

The units of angle measurement are performed by:
  1. Sexagesimal System 
  2. Centesimal System
  3. Hours System
The most commonly used system is Sexagesimal System. The centesimal system is used in Europe. The hour's system is used for astronomical purposes.

In Sexagesimal System, 1 Circumference is 360 degrees of arc, with 1 degree=60' (minutes of arc) and 1 minute = 60'' (seconds of arc).

Types of Compass in Compass Survey

The two main types of compass used in compass surveying are the Prismatic compass and Surveyor's Compass. 
  1. Prismatic Compass: This is a simple compass that consists of a magnetic needle attached to a graduated circle. It is used for measuring horizontal angles.
  2. Surveyor's Compass: This is a more advanced version of the prismatic compass, and it has a sighting device that allows the user to sight a target while taking a reading.
  3. Vernier Compass: This compass has a graduated circle with a vernier scale, which allows for more precise readings than the prismatic compass.

Bearings in Compass Survey

The bearings can be represented by:
  1. Whole circle bearing system (W.C.B) or Azimuthal System 
  2. Quadrantal Bearing System (Q.B) or Reduced Bearing

1. Whole circle bearing system (W.C.B) or Azimuthal System

In the WCB system:
  1. Bearings are measured from the North magnetic meridian only.
  2. Measured in the clockwise direction.
  3. W.C.B of any line lies between 0 to 360 degrees
  4. It is implemented in Prismatic Compass

Whole circle bearing system (W.C.B) or Azimuthal System 

2. Quadrantal Bearing System (Q.B) or Reduced Bearing

Quadrantal Bearing System (Q.B) or Reduced Bearing

In the Q.B system:
  1. Bearings can be measured either from North or south whichever is near.
  2. Measured clockwise or anti-clockwise
  3. The bearings obtained lie between 0 and 90 degrees
  4. It is implemented in Surveyor's Compass

Applications of Compass Survey

The main applications of compass survey are:
  • Land surveying: For small-scale surveys like property surveys, boundary surveys, topographic surveys, etc.
  • Engineering surveys: Construction of roads, bridges, and highways. etc.
  • Mining
  • Navigation
  • Military
  • Archaeology survey
  • Forestry

Benefits of Compass Survey

  1. Portability: Compasses are small and portable, making them easy to transport and use in the field. This makes them an ideal tool for small-scale surveys or surveys in remote locations.
  2. Low Cost: Compasses are relatively inexpensive compared to other surveying equipment, making them a cost-effective option for small-scale surveys.
  3. Easy to Use: Compasses are simple to use and do not require extensive training. This makes them a good option for novice surveyors or those with limited experience.
  4. Quick Measurements: Compass surveying is a quick method for measuring horizontal angles and distances between points. This can be useful for surveys that require a quick turnaround time.
  5. Good for Rough Terrain: Compass surveying can be useful in areas with dense vegetation or rough terrain where other surveying equipment may not be practical.
  6. Accurate Enough for Some Applications: While compass surveying may not be as accurate as other surveying methods, it is accurate enough for many small-scale surveys, such as property surveys or land subdivisions.
  7. Useful in Combination with Other Methods: Compass surveying can be used in conjunction with other surveying methods, such as theodolite surveying or GPS surveying, to increase accuracy and ensure reliable results.

Limitations of Compass Survey

  1. Magnetic Interference: Compasses can be affected by magnetic interference from nearby metallic objects, power lines, or other sources. This interference can cause the compass needle to deviate from the true north, resulting in inaccurate readings.
  2. Inclination Error: Compass surveying assumes that the Earth's magnetic field is horizontal, but in some areas, such as near the poles, the magnetic field can be inclined. This can cause errors in the compass reading.
  3. Limited Accuracy: Compass surveying is less accurate than other surveying methods, such as theodolite surveying or GPS surveying. This is because compass surveying relies on manual measurements and does not account for factors such as curvature of the Earth or atmospheric refraction.
  4. Limited Range: Compasses have a limited range of measurement and are best suited for small-scale surveys. For large-scale surveys, other surveying methods may be more appropriate.
  5. Difficulty in taking vertical angles: Compass surveying is primarily used for measuring horizontal angles, and it is difficult to take vertical angles with a compass. Theodolite surveying is more suitable for vertical angle measurement.

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