Table of Contents
Understanding Magnetic Bearings Surveying
This guide covers essential surveying techniques, focusing on traversing, magnetic bearings, and meridians. Understanding Magnetic Bearings Surveying It explains concepts like true, magnetic, arbitrary, and grid meridians, as well as methods to handle magnetic declination, local attraction, and error corrections in survey readings. Ideal for understanding the foundational principles of compass traversing and surveying accuracy.
Introduction And Purpose
In chain surveying, the region to be surveyed is divided into multiple triangles. This technique is ideal for relatively flat terrain and small-scale areas. when the area is large, undulating and crowded with many details, triangulation (which is the principle of chain survey) is not possible. In such an area, the method of traversing is adopted Understanding Magnetic Bearings Surveying.
In traversing, the framework is made up of several connected lines. The lengths are measured using a chain or tape, and the directions are determined by measuring angles.- measuring instruments. In one of the methods, the angle measuring instrument used is the compass.
Hence, this method is referred to as compass traversing.
Note: Considering the traverse in an anticlockwise direction is generally more convenient when running the survey lines.
Definitions
1. True Meridian
The line or plane that extends through the geographical North and South Poles, intersecting any given point on the Earth’s surface, is known as the “true meridian” or “geodetic meridian.” This meridian at a particular location remains constant. While true meridians through different surface points are not parallel, they converge towards the poles. However, in surveys covering small areas, these meridians are often treated as parallel Understanding Magnetic Bearings Surveying.
The angle formed between the true meridian and a specific line is called the “true bearing” of the line.
2. Magnetic Meridian
When a magnetic needle is suspended freely and balanced properly, unaffected by magnetic substances, it indicates a direction. This direction is referred to as the ‘magnetic meridian.’ The angle between the magnetic meridian and a line is called the ‘magnetic bearing’ or simply the ‘bearing’ of the line Understanding Magnetic Bearings Surveying.
3. Arbitrary Meridian
In some cases, for surveying a small area, a convenient direction is assumed as a meridian, which is known as the “arbitrary meridian.” Often, the starting line of a survey is selected as the arbitrary meridian.
The angle between the arbitrary meridian and a particular line is referred to as the “arbitrary bearing” of the line.
4. Grid Meridian
Sometimes, for preparing a map, some state agencies assume several lines parallel to the true meridian for a particular zone. These lines are termed grid lines and the central line the ‘grid meridian’. The bearing of a line in relation to the grid meridian is called the “grid bearing” of the line, Understanding Magnetic Bearings Surveying.
5. Designation of Magnetic Bearing
Magnetic bearings are designated by two systems:
- Whole circle bearing (WCB), and
- Quadrantal bearing (QB).
1. Whole Circle Bearing (WCB)
The magnetic bearing of a line, measured clockwise from the north pole to the line, is referred to as the “whole circle bearing” of that line.Such a bearing may have any value between 0° and 360°. The whole circle bearing of a line is obtained by prismatic compass. Understanding Magnetic Bearings Surveying.
2. Quadrantal bearing (QB)
The line or plane that passes through the geographical North Pole, South Pole, and a specific point on the Earth’s surface is called the “true meridian” or “geodetic meridian.”
The true meridian at a particular station is fixed and does not change.
True meridians passing through different points on the Earth’s surface are not parallel; instead, they converge toward the poles. but the quadrants should always be mentioned. Quadrantal bearings are obtained by the surveyor’s compass Understanding Magnetic Bearings Surveying.
6. Reduced Bearing (RB)
When the whole circle bearing of a line is converted to a quadrantal bearing, it is referred to as the ‘reduced bearing.’ The reduced bearing is similar to the quadrantal bearing, with its value ranging between 0° and 90°. However, the quadrants must be specified for proper designation, Understanding Magnetic Bearings Surveying.
The following table should be remembered for conversion of WCB to RB:
| WCB Between | Corresponding RB | Quadrant |
| 0° and 90° | RB = WCB | NE |
| 90° and 180° | RB = 180° – WCB | SE |
| 180° and 270° | RB = WCB – 180° | SW |
| 270° and 360° | RB = 360° – WCB | NW |
7. Fore and Back Bearing
Every line has two bearings: one observed along the direction of the survey or forward direction, called the ‘fore bearing,’ and the second observed in the reverse or opposite direction, called the ‘back bearing.’
We consider the direction of the meridian as upward, and the bearing is measured clockwise from the meridian. The bearing measured at point A along the progress of the survey from A to B is 8°.So the angle or bearing theta is the fore bearing of the line overline AB.
Similarly, the bearing measured at point B in the opposite direction of the survey progress from A to B, along the clockwise direction, is called beta. The bearing beta is the back bearing of the line AB. It is evident that the fore bearing and back bearing of a line differ exactly by 180°, Understanding Magnetic Bearings Surveying.
A positive sign is used when the fore bearing is less than 180°, and a negative sign is used when it is greater than 180°.
In the quadrantal bearing system, the numerical value of the fore bearing and back bearing is the same, but the quadrants are opposite. For instance, if the fore bearing is N30°E, its back bearing would be S30°W. In this case, the fore bearing of line AB is equal to the back bearing of line BA, i.e., the opposite direction of the survey progress, Understanding Magnetic Bearings Surveying.
8. Magnetic Declination
The horizontal angle between the magnetic meridian and the true meridian is known as ‘magnetic declination.’
When the north end of the magnetic needle points towards the west side of the true meridian, the position is termed Declination West (W).
When the north end of the magnetic needle points towards the east side of the true meridian, the position is termed Declination East (E), Understanding Magnetic Bearings Surveying.
9. Isogonic and Agonic Lines
Lines that connect points of equal magnetic declination are called ‘isogonic’ lines.
The line that passes through points with zero magnetic declination is known as the ‘agonic’ line.
The Survey of India has created a detailed map of India showing isogonic and agonic lines, providing a valuable guideline for conducting compass surveys in various regions of the country.
10. Variation of Magnetic Declination
The magnetic declination at a place is not constant. It varies due to the following reasons:
(a) Secular Variation: The magnetic meridian behaves like a pendulum relative to the true meridian. Approximately every 100 years, it swings from one direction to the opposite, causing changes in declination. This phenomenon is referred to as ‘secular variation, Understanding Magnetic Bearings Surveying.
(b) Annual Variation: Magnetic declination fluctuates yearly due to the Earth’s elliptical orbit around the sun and its axis inclination. This change, called ‘annual variation,’ amounts to about 1 to 2 minutes.
(c) Diurnal Variation: Magnetic declination also changes daily as the Earth rotates on its axis within 24 hours. This is known as ‘diurnal variation,’ with variations ranging from 3 to 12 minutes.
(d) Irregular Variation: Sudden changes in magnetic declination occur due to natural phenomena like earthquakes, volcanic eruptions, or solar storms. These unexpected changes are termed ‘irregular variation.’
11. Dip of the Magnetic Needle
If a needle is perfectly balanced before magnetization, it no longer remains in a balanced position after being magnetized due to the Earth’s magnetic influence. The needle becomes inclined towards the pole, and this inclination with respect to the horizontal is referred to as the ‘dip of the magnetic needle.’
In the northern hemisphere, the north end of the magnetic needle is deflected downward, while in the southern hemisphere, the south end dips downward. At the equator, the needle remains perfectly horizontal. To counterbalance the dip of the needle, a rider (typically made of brass or silver) is provided. This rider is placed on the needle at an appropriate position to ensure it stays horizontal, Understanding Magnetic Bearings Surveying.
12. Local Attraction
A magnetic needle freely suspended or pivoted usually indicates the north direction. However, when it is near magnetic substances like iron ore, steel structures, or electric cables carrying current, the needle gets deflected from its true direction and fails to indicate the actual north. This disturbing influence caused by magnetic substances is termed ‘local attraction.’
To identify the presence of local attraction, both the fore bearing (FB) and back bearing (BB) of a line must be observed. If the difference between the FB and BB is exactly 180°, it indicates the absence of local attraction.
However, if the FB and BB do not differ by 180°, the magnetic needle is considered to be influenced by local attraction, assuming there is no instrumental error, Understanding Magnetic Bearings Surveying.
To compensate for the effect of local attraction, the error is calculated and equally distributed between the fore bearing (FB) and back bearing (BB) of the affected line to correct the readings.
13. Method of Application of Correction
(a) First Method:
In this method, the interior angles of a traverse are calculated from the observed bearings. An angular check is then applied, ensuring that the sum of the interior angles equals (2n – 4) x 90, where (n is the number of sides in the traverse).
If the sum is incorrect, the total error is distributed among all the angles of the traverse. Afterward, starting from the unaffected line, the bearings of all other lines are corrected using the adjusted interior angles.
This method, however, is time-consuming and therefore is not commonly used.
(b) Second Method:
In this method, the interior angles are not calculated. The first step is to identify the unaffected line using the provided data. Then, starting from this unaffected line, the bearings of the other affected lines are corrected by determining the necessary correction at each station, Understanding Magnetic Bearings Surveying.
This method is considered easier and is widely used due to its simplicity and practicality.
Note: If all the lines in a traverse are affected by local attraction, the line with the minimum error is first identified. The fore bearing (FB) and back bearing (BB) of this line are then adjusted by equally distributing the total error.
After adjusting the bearings of this line, the corrections are applied to the fore and back bearings of the other lines, starting from the corrected line. This method ensures consistency in the entire traverse, Understanding Magnetic Bearings Surveying.
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