1.14.4 - Magnetic declination
Enroll to start learning
You’ve not yet enrolled in this course. Please enroll for free to listen to audio lessons, classroom podcasts and take practice test.
Interactive Audio Lesson
Listen to a student-teacher conversation explaining the topic in a relatable way.
Definition of Magnetic Declination
🔒 Unlock Audio Lesson
Sign up and enroll to listen to this audio lesson
Today, we're discussing magnetic declination, which is the angle between the magnetic meridian and true meridian at a location. Can anyone tell me why understanding this angle is important?
Is it because it helps us to know the difference between the true north and magnetic north?
Exactly, Student_1! This distinction is crucial for accurate navigation and surveying.
So, if the magnetic north is east of true north, we add the declination, right?
That's correct, Student_2. We refer to this as eastern declination. Can someone explain what happens in the case of western declination?
We subtract the declination value from the magnetic bearing for western declination!
Well done, Student_3! Remember: eastern declination means addition, and western declination means subtraction.
In summary, magnetic declination helps us to adjust our measurements accurately for the true north, which is essential in various applications like map reading and navigation.
Applications of Magnetic Declination
🔒 Unlock Audio Lesson
Sign up and enroll to listen to this audio lesson
Now let's discuss the applications of magnetic declination in surveying. Why do you think it's critical to consider declination when making measurements?
If we ignore it, we might end up with inaccurate maps or navigation paths!
Exactly, Student_4! Maps used by the army and other entities often rely on true meridians, which are established through astronomical observations.
So, when we draw engineering maps, do we rely more on magnetic meridians?
That's a good observation, Student_2. However, inaccuracies can arise from local attractions as well. Can anyone think of what could cause these local attractions?
Metal objects and structures can interfere with the magnetic needle!
Great job, Student_1! These local attractions can lead to deviations in readings, which is why knowing the declination is vital for correction.
Establishing Magnetic Declination
🔒 Unlock Audio Lesson
Sign up and enroll to listen to this audio lesson
Next, let's talk about how we establish magnetic declination. What tools do we use for this?
We can use a compass to determine the magnetic meridian!
That's right! And how do we find the true meridian?
Using astronomical observations of the sun or stars!
Excellent, Student_4. By comparing the two, we can find the magnetic declination at that location.
So, does this declination change over time?
Yes, Student_2, it can change based on various geological and geomagnetic factors. It's essential to stay updated for accurate navigation.
To summarize, establishing both true and magnetic meridians is vital to compute the magnetic declination effectively.
Introduction & Overview
Read summaries of the section's main ideas at different levels of detail.
Quick Overview
Standard
This section details the concept of magnetic declination, which is the angle between the true north and magnetic north. It explains how to adjust magnetic bearings to find true bearings by either adding or subtracting the declination value, and highlights the significance of local attractions affecting magnetic readings.
Detailed
Magnetic Declination
Magnetic declination is defined as the angle between the magnetic meridian and the true meridian at a specific location on the Earth. This angle can be oriented either east or west.
- Eastern Declination: This occurs when the magnetic north is located to the east of the true north, marked as positive (+) declination. It is added to the observed magnetic bearings to determine the true bearing.
- Western Declination: In contrast, when the magnetic north is to the west of the true north, it is denoted as negative (-) declination and subtracted from the magnetic bearings.
The magnetic declination does not remain constant; it can change depending on the local geomagnetic field and the observer's geographical location.
To compute the true bearing, the relationship is given as:
True bearing = magnetic bearing ± magnetic declination (E or W)
By establishing both the true meridian using astronomical observations and the magnetic meridian through compass usage, one can find the degree of magnetic declination. This knowledge is critical for engineering and navigation, as accurate maps used by the army are typically created with respect to true meridian, while engineering maps might reference magnetic meridian.
Audio Book
Dive deep into the subject with an immersive audiobook experience.
Definition of Magnetic Declination
Chapter 1 of 5
🔒 Unlock Audio Chapter
Sign up and enroll to access the full audio experience
Chapter Content
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.
Detailed Explanation
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.
Examples & Analogies
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.
Types of Declination
Chapter 2 of 5
🔒 Unlock Audio Chapter
Sign up and enroll to access the full audio experience
Chapter Content
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.
Detailed Explanation
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.
Examples & Analogies
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.
Correcting for Declination
Chapter 3 of 5
🔒 Unlock Audio Chapter
Sign up and enroll to access the full audio experience
Chapter Content
It means, eastern declination is to be added and western to be subtracted to the observed magnetic bearings to obtain true meridian.
Detailed Explanation
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.
Examples & Analogies
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.
Determining Magnetic Declination
Chapter 4 of 5
🔒 Unlock Audio Chapter
Sign up and enroll to access the full audio experience
Chapter Content
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.
Detailed Explanation
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.
Examples & Analogies
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.
Importance of Magnetic Declination
Chapter 5 of 5
🔒 Unlock Audio Chapter
Sign up and enroll to access the full audio experience
Chapter Content
Engineering maps are made with respect to magnetic meridians, while maps used by army are made with respect to true meridian.
Detailed Explanation
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.
Examples & Analogies
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.
Key Concepts
-
Magnetic declination: The angle that adjusts magnetic readings to true north.
-
Eastern declination: Positive value indicating magnetic north is east of true north.
-
Western declination: Negative value indicating magnetic north is west of true north.
-
Local attraction: Interference affecting the readings of a magnetic compass.
Examples & Applications
Example of eastern declination: If the magnetic bearing of a line is 30° and the magnetic declination is +5°, the true bearing will be 30° + 5° = 35°.
Example of western declination: If the magnetic bearing is 75° and the magnetic declination is -3°, the true bearing will be 75° - 3° = 72°.
Memory Aids
Interactive tools to help you remember key concepts
Rhymes
North to the east, rise and beam, magnetic declination is key to the theme.
Stories
Imagine a navigator lost at sea, who relies on both true north and magnetic north to guide him home. One day, he discovers that the magnetic north has shifted due to local attractions, leading him into dangerous waters. Through his perseverance, he learns about magnetic declination, helping him adjust his course and find safety.
Memory Tools
E.W. (East = add, West = subtract) to remember how to adjust magnetic bearings.
Acronyms
D.E.A.D. (Declination, East is Add, West is Deduct) helps remember how to adjust for declinations.
Flash Cards
Glossary
- Magnetic Declination
The angle between the magnetic meridian and true meridian at a location, which can be either positive (eastern) or negative (western).
- True Meridian
The direction towards the geographic North Pole, determined by astronomical observations.
- Magnetic Meridian
The direction indicated by a magnetic compass at a specific location.
- Eastern Declination
Occurrence when magnetic north is east of true north, indicated by a positive value.
- Western Declination
Occurrence when magnetic north is west of true north, indicated by a negative value.
- Local Attraction
The magnetic interference caused by nearby metallic objects affecting the readings of magnetic compasses.
Reference links
Supplementary resources to enhance your learning experience.