1.14.4. Magnetic declination

The magnetic meridian and the true meridian at any place are not same but different, the horizontal angle between these is known as magnetic declination.

Magnetic declination refers to the angle between two types of north directions: the magnetic north and the true north. The magnetic north is the direction that a compass points to, while true north is the direction along the Earth's surface towards the North Pole. Since these two directions do not align, magnetic declination is the angle that reflects this difference. It varies based on geographical location and changes over time due to shifts in the Earth's magnetic field.

Imagine you are on a road trip using a compass to guide you north and a map that shows true north. Sometimes, you may notice that your compass points slightly to the left or right of where your map indicates north. This discrepancy is similar to how magnetic declination works—it shows you the difference between what your compass reads and the actual direction to the North Pole.

The magnetic north at a place may be either towards east or west of true north (Figure 1.16). If it is towards east, it is known as eastern or +ve declination, and if towards west, it is called as western declination or –ve declination.

Magnetic declination can be categorized into two types based on its direction: eastern declination and western declination. If the magnetic north is located east of the true north, it is referred to as eastern or positive (+ve) declination. Conversely, if it is west of true north, it is termed western or negative (–ve) declination. This distinction is important because it guides how adjustments are made when using a compass for navigation.

Think of magnetic declination like two friends pointing to a cafe from different locations. One friend (magnetic north) might be pointing slightly to the right (east) of the actual cafe (true north), while another friend could be pointing slightly to the left (west). Depending on which direction your friends are pointing, you'll need to adjust your approach to reach the cafe, just like you adjust your bearings based on the type of magnetic declination.

It means, eastern declination is to be added and western to be subtracted to the observed magnetic bearings to obtain true meridian.

To accurately determine the true direction when using a compass, you need to consider magnetic declination. If you encounter eastern declination, you add that angle to your magnetic bearing to calculate the true bearing. Conversely, for western declination, you subtract the angle from the magnetic bearing to find the true direction. This adjustment is crucial for precise navigation and surveying.

Imagine you're using a compass to find a treasure buried in the woods. Your compass points you in a direction, but you also need to know the declination. If the treasure is located just to the east of where your compass indicates, you would add that east angle to your direction to ensure you dig in the right spot. Conversely, if it’s west, you'll want to subtract that angle to avoid missing the treasure.

To know the magnetic declination at a location, true meridian is established from astronomical observations (Sun or Stars) and magnetic meridian is determined by a compass.

To find the magnetic declination specific to a location, two measurements must be made: the true meridian and the magnetic meridian. The true meridian can be established through astronomical observations, usually involving the position of the Sun or stars. Once the true meridian is known, it can be compared to the direction indicated by a magnetic compass to determine the magnetic declination. This process helps navigators and surveyors understand how much their compass readings deviate from true north.

Think about this like navigating using the stars. Imagine you’re on a boat at night—by observing the stars, you can find true north. However, since your compass doesn't point in the same direction, you need to compare the compass direction with your star-derived true north to see how far off you are, allowing you to adjust your course correctly.

Engineering maps are made with respect to magnetic meridians, while maps used by army are made with respect to true meridian.

Understanding and correctly applying magnetic declination is vital in surveying and navigation. For instance, engineering maps often rely on magnetic meridians for their bearings, while military maps typically use true meridians. This difference influences how various professions approach their tasks and guides them in ensuring their navigation and mapping are as accurate as possible, which is crucial in fields like construction, navigation, and military strategy.

When planning a construction project, engineers must ensure their maps align with the correct form of north. It’s like baking a cake where the ingredients must be precise. If you substitute baking powder (true north) for baking soda (magnetic north), the cake might not rise properly. Similarly, without accurate bearings, any surveying or navigation could lead to significant errors in projects.

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