Compass Surveying

What is compass surveying?

Compass surveying is an important branch of surveying that is usually adopted in determining the position of an object using both linear and angular measurements.

The linear measurement can be measured with the help of a chain or tape and the angular measurement can be measured with the help of a compass.

What is the need for compass surveying?

Compass surveying can be done under the following circumstances,

• Chain surveying can be done if the surveying area is fairly small and flat. However, for larger areas, it is convenient to use compass surveying rather than chain surveying.
• If the surveying area has numerous numbers of obstacles or several undulations, it makes the chaining process more difficult. In that case, compass surveying can be used.
• If there is any time constraint, it is convenient to use compass surveying because some compasses like a prismatic compass can be handheld for rough survey work.

Principle of compass surveying

The principle of compass surveying is Traversing.

Traversing is the type of survey in which a various number of survey lines forms a framework by connecting the adjacent survey stations.

The orientation and the length of survey lines are determined using a compass and a chain or tape.

• When the survey lines form the closed circuit, then it is called a Closed traverse.
• When the survey lines didn’t form the closed circuit and if one end ends somewhere else, then it is called an Open traverse.

What is a compass?

We know that in traversing both orientation and length of the survey line have to be measured.

The length of the survey line can be measured using chain surveying. The orientation of a survey line is measured by using a compass.

So, Let’s learn more about the compass.

The earth acts as a powerful magnet and produces a strong magnetic field around it. The magnetic field originates from the south pole and ended up at the north pole of the planet.

If any magnetic bar is suspended freely, it aligns itself along the direction of the magnetic field of the earth. The same principle was used in the compass.

Meridian, Bearings, and Angles

The orientation of the survey line can be established in 3 ways,

• With respect to True meridian
• With respect to Magnetic meridian
• With respect to any other arbitrary meridian

1. Meridian

These are the imaginary lines that run around the earth vertically up and down and join together at the north and south poles. There are 3 types of meridians available, they are,

• True meridian

The true meridian is the imaginary line that passes through the True north (also known as Geographical north) and True south (Geographical south) of the earth. It can be found using astronomical observation and it is constant.

• Magnetic meridian

The magnetic meridian is the imaginary line that passes through the magnetic north and magnetic south of the earth.

The magnetic north is the direction that a freely suspended magnetic bar aligns. It can be found by using a compass and it may vary according to time and place.

The angle between the magnetic meridian and the true meridian is known as Magnetic declination.

• Arbitrary meridian

An arbitrary meridian is any convenient direction toward a well-defined permanent object like a building, church, well, etc. These meridians are used to determine the relative directions of other lines in a small area.

2. Bearing

The bearing of a line is its orientation relative to the given meridian. In other words, the bearing of the line is nothing but the angle of the line with respect to its meridian.

• True bearing

True bearing is the horizontal angle of a survey line measured from the true meridian. Since the true meridian is always constant, the true bearing of a line is also constant.

• Magnetic bearing

A magnetic bearing is the horizontal angle of a survey line measured from the magnetic meridian. The magnetic bearing is calculated using a magnetic compass.

• Arbitrary bearing

Arbitrary bearing is the horizontal angle of a survey line measured from the arbitrary meridian.

On each station in compass surveying, two bearings should be noted. They are,

• Fore bearing – If the bearing of line AB is measured from A to B, then it is called a Forward or Fore Bearing (F.B).
• Back bearing – If the bearing of line AB is measured from B to A, then it is called a Backward or Back Bearing (B.B) since it is measured in the backward direction.

3. Angles

Knowing the bearings of two lines, the angle between them can be calculated easily as shown in the following figure. We can also measure angles directly using theodolite or sextant, but these types of equipment require high-quality of work and should always be fixed with the tripod.

Designation of Bearings

The common systems of notation of bearing are,

• Whole Circle Bearing (WCB)
• Reduced Bearing (RB)

1. Whole Circle Bearing (or) Azimuthal system

• Point of reference – In this system, the bearing of the line is always measured from Magnetic North in the clockwise direction.
• The value of bearing varies from 0° to 360° i.e., 0° or 360° at the north, 90° at the East, 180° at the south, and 270° at the west.
• The Prismatic compass is graduated in this system.
• Notation – It is just mentioned by the angle of the survey line. For example. 60°.

2. Reduced Bearing (or) Quadrantal Bearing system

• Point of reference – In this system, the bearing of the line is measured from either the North or the South in the clockwise or anticlockwise direction depending upon the position of the survey line.
• The value of bearing varies from 0° to 90°. 0° at the North and the south and 90° at the east and the west.
• The surveyor compass is graduated in this system.
• Notation – The reference and rotation directions are mentioned in the reading. For example, N45°E.

Also, Read Types of compasses used in compass surveying.

Limitations of compass survey

Sometimes the magnetic needle was attracted and deflected by some of the magnetic sources found in the local region such as wire carrying electricity, too much iron ore in the ground, presence of more metals. Due to these attractions, the magnetic needle is deflected and doesn’t show the true magnetic meridian. This type of error is called Local Attraction.

If the difference between FB and BB of the line doesn’t show exactly 180°. We can say that station is affected by Local attraction.

We can eliminate this error by determining the station which was not affected by Local attraction and we can nullify the error from there.

Extra Points

• Isogonic line – The line connecting the points which are having the same declination.
• Agonic line – The line connecting the points which are having zero declination.