Magnetic Declination and True Bearing
In surveying, accurate direction measurement is essential for preparing correct maps and layouts. The directions measured using a compass are affected by the difference between the magnetic north and the true north. This difference is known as magnetic declination. Understanding magnetic declination and true bearing is important in ITI Draughtsman Civil for accurate compass surveying.
1. True North and Magnetic North
- True North: The direction of the geographic North Pole.
- Magnetic North: The direction indicated by the magnetic needle of a compass.
These two directions are not the same, and the angle between them is called magnetic declination.
2. Magnetic Declination
Definition
Magnetic declination is the horizontal angle between the true meridian (true north) and the magnetic meridian (magnetic north).
Types of Declination
- East Declination: When magnetic north lies to the east of true north.
- West Declination: When magnetic north lies to the west of true north.
Variation of Declination
- Declination varies from place to place.
- It also changes with time due to movement of magnetic poles.
3. True Bearing
Definition
True bearing is the horizontal angle measured between a survey line and the true north direction.
Bearings measured using a compass are magnetic bearings. To obtain true bearings, correction for magnetic declination is required.
4. Relationship Between Magnetic Bearing and True Bearing
The relationship between magnetic bearing (MB), true bearing (TB), and declination (D) is:
- TB = MB + Declination (for east declination)
- TB = MB – Declination (for west declination)
Proper application of these formulas helps in converting magnetic bearings into true bearings.
5. Importance of Magnetic Declination
- Ensures accurate direction measurement.
- Helps in converting magnetic bearings to true bearings.
- Important for preparing maps and plans.
- Essential for large-scale and precise surveys.
6. Practical Applications
- Used in compass surveying.
- Important in navigation and mapping.
- Helps in aligning roads, canals, and buildings.
Precautions
- Always check the value of declination for the area.
- Apply correct sign (east or west) while calculation.
- Update declination values for time variation.
Role of Draughtsman Civil
A Draughtsman Civil uses true bearings for accurate plotting of survey lines. Proper understanding of magnetic declination ensures correct alignment and positioning in construction drawings.
Conclusion
Magnetic declination is the angle between true north and magnetic north, and it affects compass readings. True bearing is the correct direction with respect to true north.
For ITI Draughtsman Civil students, understanding these concepts is essential for accurate surveying, mapping, and construction planning.