Unit-5 : Compass Surveying

 

# Traversing: 

Surveying, which involves a series of interconnected lines, is known as traversing. The side of a traverse is referred to as a traverse leg. In traversing, the length of a line is measured with a chain tape, and angles are measured using a compass or theodolite. There are several methods of traversing:

1. Chain traversing.

2. Compass traversing.

3. Plane table traversing.

4. Theodolite traversing.

Alternatively, traverses may be categorized into two types:

 Open traverse: When a sequence of connected survey lines extends along the length of an object and does not return to the starting point, it is known as an open traverse or unclosed traverse.

In the case of an open traverse, represented by the sequence ABCDF, these traverses are well-suited for surveys involving features such as rivers, roads, railways, transmission lines, etc.

                                    Fig:- Open Traverse
 

Closed Traverses: A closed traverse occurs when a series of connected lines form a closed circuit, signifying that the finishing point coincides with the starting point of the survey.

Closed Traverse Example: Illustrated in the diagram labeled "ABCD," the points form a closed traverse. The designation "Pond" within the traverse indicates that this example involves surveying around a pond. Closed traverses are particularly applicable for surveying forested areas, ponds, and other point project areas.

# Compass Traversing 

It is the process of measuring distances in which linear measurements are taken through a chaining tape, and angular measurements are taken from a compass. Compass traversing is recommended when:
1. The area to be surveyed is large.
2. The area to be surveyed has many details and triangulation is not possible.

Compass surveying is not recommended for areas where local attraction is suspected. Local attraction may be caused by the presence of magnetic substances like steel structures, iron ore, and electrical cables carrying current.

The principle of compass surveying is traversing, which involves a series of connected lines. The bearings of the lines are measured by a compass, and distances are measured by a chain or tape.

Some definitions:

1. True Meridian and True Bearing - The true meridian is the line or plane passing through the geographical North Pole and the geographical South Pole and lying on the surface of the Earth is known as the true meridian. The angle between the true meridian and a line is known as the true bearing.

2. Magnetic Meridian and Magnetic Bearing - The magnetic meridian, indicated by a freely suspended magnetic needle, is known as the magnetic meridian. The angle between the magnetic meridian and a line is known as the magnetic bearing.

3. Arbitrary Meridian - For the survey of small areas, a convenient direction assumed as a meridian is called an arbitrary meridian. The angle between the arbitrary meridian and true north is known as arbitrary declination.

# Designation of Magnetic Bearings:

Magnetic bearings can be designated by two systems:

1. Whole Circle Bearing

The magnetic bearing is measured in a clockwise direction from the north toward the line is known as Whole Circle Bearing. The value of whole circle bearing lies between 0 to 360°. Whole Circle Bearing is measured by a prismatic compass.

 

2. Quadrantal Bearing or Reduced Bearing

The magnetic bearing of a line measured clockwise or anticlockwise from the north or south pole (whichever is nearer the line) toward the east or west is known as quadrantal bearing of a line. The system consist of a four quadrants, NE, NW, SE, SW. The value of quadrantal bearing lies between 0° to 90° but quadrant should always be mentioned. Quadrantal bearing are obtained by surveyors compass. 

 

*Bearing of OA = N θ1 E

 *Bearing of OB = S θ4 E

 *Bearing of OC = S θ3 W

 *Bearing of OD = S θ2 W

# Fore Bearing and Back Bearing:

Each line in a survey has two bearings. The first is calculated along the direction of survey progress (forward) and is known as the fore bearing. The second bearing is expressed in the reversed or opposite direction and is called the back bearing.

Let’s denote:
Θ as the fore bearing of AB
Ф as the back bearing of AB (or the bearing of BA)
Consider the direction of the meridian as upward, and bearings are measured clockwise from the meridian. If the bearing measured at point A along the survey progress from A to B Then the fore bearing of line is Θ. If the angle or bearing measured from B to A Then the back bearing of the line is Ф. 

# Detection of Local Attraction

To identify the presence of local attraction, measure both the fore bearing and back bearing of a line. If the difference between the fore bearing and back bearing is exactly 180°, the line is free from local attraction. Otherwise, it is affected by local attraction. To compensate for this effect, determine the amount of error and distribute it equally between the fore bearing and back bearing.

# Problems Related to Magnetic Declination:

True Bearing = Magnetic Bearing – Declination (if declination is westward).

True Bearing = Magnetic Bearing + Declination (if declination is Eastward).
Remember that accurate navigation and mapping require accounting for these factors. 🌐🧭