Magnetic Declination
Magnetic declination is the horizontal angle measured between true north (the direction of the geographic North Pole) and magnetic north (the direction indicated by a magnetic compass) at a specific location and time. This angle, known as declination, changes over time and varies depending on the position of the Earth’s magnetic meridian at the time of observation.
If the magnetic meridian lies to the east of the true meridian, the declination is termed eastern declination or positive declination. Conversely, if the magnetic meridian is west of the true meridian, the declination is referred to as western declination or negative declination. When the magnetic and true meridians align perfectly, the magnetic declination is zero.
On the Earth’s surface, locations with the same magnetic declination, whether positive or negative, can be connected by imaginary lines known as Isogonic lines. However, due to the irregular nature of Earth’s magnetic field, these isogonic lines do not form perfect circles but instead follow complex, irregular paths. The special isogonic lines where the declination is exactly zero are called Agonic lines.
In navigation, particularly for mariners, magnetic declination is often referred to as variation, reflecting its role in adjusting compass readings to find true direction.
Determination of Magnetic Declination
To determine magnetic declination:
- Establish true meridians at various locations in the survey area through astronomical observations, particularly by observing stars.
- Use a compass to take readings(Magnetic Meridians) along these true meridians.
- The angle between the true meridian and the magnetic meridian (indicated by the compass) is the magnetic declination at that location.
A key formula to remember is:
Magnetic declination = True bearing – Magnetic bearing
The sign of the difference indicates the direction of declination:
- Positive (+): Eastward declination
- Negative (-): Westward declination
Calculating True and Magnetic Bearings
True Bearing = Magnetic Bearing ± Magnetic Declination
Use (+) for eastern declination, (-) for western declination.
Magnetic Bearing = True Bearing ± Magnetic Declination
Use (-) for eastern declination, (+) for western declination.
Variation of Magnetic Declination
Magnetic declination at any given location is not fixed; it varies over time due to several natural factors. These variations are generally categorized into four main types:
- Secular Variation
- Annual Variation
- Diurnal Variation
- Irregular Variation
1.Secular Variation
Secular variation refers to the slow, long-term changes in magnetic declination, primarily caused by the shifting positions of the Earth’s magnetic poles in relation to the geographical poles. This gradual shift also affects the magnetic meridians, leading to a continual adjustment in declination at different locations. Over time, this variation follows a near-regular pattern, resembling a sine wave or harmonic motion, with declination values oscillating over centuries.
Secular variation is of particular importance to land surveyors due to the substantial impact it can have on the accuracy of magnetic bearings. The magnitude of this variation is not uniform across different regions, nor does it remain constant year to year. At certain locations, the change can range from as little as 0.02 minutes to as much as 12 minutes annually, depending on the geographical position of the site. Additionally, the rate of change tends to be greatest near the midpoint of a complete cycle of variation and least at the extremes.
Understanding and accounting for secular variation is crucial for converting magnetic bearings into true bearings accurately. Surveyors must therefore be aware of the precise declination values for a given time and place. Observations made over extended periods often reveal that magnetic meridians shift from one side of the true meridian to the other. This periodic movement must be carefully monitored to ensure the reliability of survey data.
2.Annual Variation
Annual variation refers to the change in magnetic declination at a specific location over the course of a single year. This variation arises due to the Earth’s dynamic magnetic field, which is influenced by various internal and external factors, including solar activity and the movement of molten iron within the Earth’s core.
Observations conducted over 12-month periods at different locations show that annual variation typically ranges between 1 and 2 minutes. However, the extent of this variation is not uniform and can differ based on the geographical position of the location, as well as its proximity to magnetic poles or regions of high magnetic intensity. For instance, areas closer to the magnetic poles tend to experience more pronounced annual variations.
Though smaller in magnitude compared to secular variation, annual variation is still significant for precise surveying and navigation. Surveyors must account for this variation when converting magnetic bearings to true bearings over the course of a year to maintain accuracy in their measurements.
3.Diurnal Variation
Diurnal variation refers to the fluctuation in magnetic declination from its average value over a 24-hour period at a given location. This short-term variation is driven by daily changes in the Earth’s magnetic field, which are primarily influenced by the Sun’s activity and its interaction with the Earth’s atmosphere.
The extent of diurnal variation depends on several key factors:
(a) Geographical Location:
Diurnal variation tends to be more pronounced in regions located at higher latitudes, where the Earth’s magnetic field is more susceptible to daily fluctuations. In contrast, locations near the equator experience smaller variations.(b) Seasonal Changes:
The time of year significantly influences diurnal variation. During the summer months, the variation is generally more pronounced compared to winter. This seasonal effect is likely due to the increased solar activity and longer daylight hours during summer.(c) Time of Day:
Diurnal variation follows a predictable pattern throughout the day. It tends to be more substantial during daylight hours, particularly around noon when solar radiation is strongest. At night, when solar influences are reduced, the variation diminishes.(d) Secular Cycle Influence:
The year within the broader cycle of secular variation also affects diurnal changes. The amount and pattern of diurnal variation can differ from year to year as part of the longer secular cycle, which causes slow shifts in the Earth’s magnetic field.
4.Irregular Variation
Irregular variation refers to sudden, unpredictable changes in magnetic declination caused by external disturbances. Unlike the more systematic secular, annual, or diurnal variations, irregular variations are abrupt and are typically triggered by phenomena such as magnetic storms, earthquakes, and other solar influences (such as solar flares or geomagnetic activity).
These disturbances can occur unexpectedly at any time and in any location, making them difficult to anticipate. Magnetic storms, for instance, are caused by fluctuations in the Earth’s magnetic field due to interactions with charged particles emitted by the Sun, especially during periods of heightened solar activity. Earthquakes can also disrupt local magnetic fields, leading to sudden changes in declination.
The magnitude of these irregular variations can range significantly, with the displacement of a magnetic needle sometimes shifting by as much as 1° to 2°. Although these variations are typically short-lived, they can have a considerable impact on activities reliant on precise magnetic readings, such as surveying and navigation.
Due to their unpredictable nature, surveyors must be aware of the possibility of irregular variations, especially during periods of heightened solar activity or in regions prone to seismic disturbances. While these variations cannot be forecasted, it is important to recognize their potential influence on magnetic readings to ensure that data affected by these events are handled appropriately.
