Magnetic Declination and Local Attraction and Types of Compass

Magnetic Declination and Local Attraction and Types of Compass

Magnetic Declination and Local Attraction and Types of Compass, Navigating the Earth's magnetic field unveils a fascinating interplay of forces, where magnetic declination, local attraction, and the diverse types of compasses become our guides through uncharted territories. Join us on this magnetic journey, as we dissect the nuances of orientation, addressing questions of precision and accuracy that resonate with every explorer and surveyor preparing for challenges like those posed in GATE examinations.

Magnetic Declination

In general, magnetic meridian and true meridian at a place do not coincide with each other.

What is Magnetic declination ?

Magnetic declination: The horizontal angle between the magnetic meridian and true meridian is termed as magnetic declination or declination at that place.

Magnetic declination, also referred to as magnetic variation, is the angle between magnetic north (the direction a magnetic compass points) and true north (the direction towards the geographic North Pole). This angle is not constant and varies depending on your location on the Earth's surface.

Earth's magnetic field is not aligned perfectly with its rotational axis, and it is generated by the movement of molten iron and nickel in its outer core. As a result, the magnetic north pole and the geographic north pole are not at the same location.

Magnetic declination is expressed in degrees east or west, depending on whether magnetic north is east or west of true north. For example, if the magnetic declination is 5 degrees west, it means that a compass needle, when pointing to magnetic north, would be 5 degrees to the west of true north.

Knowing the magnetic declination for a specific location is important for accurate navigation with a magnetic compass. Many maps and navigation tools provide information about the local magnetic declination to help users correct their compass readings and navigate more accurately.

• If magnetic north (MN) is on the west side of true north, then declination is termed as declination west or negative, and if it is on the east side of true north (TN) it is termed as declination east or positive.
• TN can be established by astronomical observations and MN can be established with the help of a compass, and the difference between the two is declination.
• Declination can also be established by astronomical observation.
• The declination changes from one place to another and also varies at a place from time to time.

What are isogonic and agonic lines?

The lines passing through the points on the earth surface at which the declination is same are termed as Isogonic lines.
Agonic lines are those isogonic lines which pass through the point of zero declination (i.e., at these points TN and MN coincides).

1. Isogonic Lines:
   • Isogonic lines connect points on the Earth's surface that have the same magnetic declination. In other words, all locations along a particular isogonic line will have the same angle between magnetic north and true north.
   • On maps, isogonic lines are often depicted with curves connecting points of equal magnetic declination. These lines help users understand how magnetic declination changes across different regions.
2. Agonic Line:
   • The agonic line is a special case of an isogonic line where the magnetic declination is zero. Along the agonic line, magnetic north coincides with true north, meaning that a magnetic compass needle points directly to the geographic North Pole.
   • In areas near the agonic line, there is minimal or no need for correction when using a magnetic compass for navigation because the magnetic declination is essentially zero.

Understanding isogonic and agonic lines is crucial for navigational purposes, as it allows navigators to correct their compass readings based on the local magnetic declination. Nautical and aeronautical charts often include information about these lines to aid in accurate navigation.

Variation of magnetic declination:

Declination at a point is not constant and varies from time to time due to different reasons according to which variations are classified as follows:

Secular variation

• It occurs continuously over a long period of a time in between (100 - 350 years) to change the direction.
• The annual rate of change of this variation is not same and is in between 5 - 10 minutes.

annual variation

• It is the change in the declination at a place over a period of 1 year.
• It is caused due to the rotation of earth around the sun.
• The annual rate of change of this variation is 1 - 2 min.

diurnal variation

• It is the change in the declination at a place in 24 hours.
• It is due to rotation of the earth about its own axis.
• The amount of variation, in this case, is fraction of a minute to over 12'.
• It also depends upon the following factors:
  • (a) Geographical position of the place - It is less near the equator and more near the poles.
  • (b) Time of day - More in day and less in night.
  • (c) Season of the year - More in summer and less in winters
  • (d) The year of the cycle of secular variation.

irregular varition

• Magnetic disturbances in the earth's magnetic field lead to the irregular variation.
• Such type of variations are uncertain, random and unpredictable.
• It is due to natural phenomenon earthquake, volcanic eruption, tsunami's, cyclone etc.

Determination of true bearing:

The determination of true bearing involves correcting the magnetic bearing obtained from a magnetic compass for the local magnetic declination. True bearing is the direction of an object or location measured in relation to true north, which is the geographic North Pole. Here's a step-by-step guide on how to determine true bearing:

1. Obtain Magnetic Bearing:
   • Use a magnetic compass to determine the magnetic bearing to the object or location of interest. The magnetic bearing is the angle measured clockwise from magnetic north.
2. Determine Local Magnetic Declination:
   • Find the local magnetic declination for your specific location and date. This information is crucial for correcting the magnetic bearing to obtain the true bearing. You can find the declination on maps, charts, or online resources specific to your region.
3. Apply Magnetic Declination Correction:
   • Correct the magnetic bearing by adding (if east) or subtracting (if west) the magnetic declination. The formula is: True Bearing=Magnetic Bearing±Magnetic Declination.True Bearing=Magnetic Bearing±Magnetic Declination.
   • If the magnetic declination is east, add it to the magnetic bearing. If it's west, subtract it.
4. Consider Other Corrections (Optional):
   • In addition to magnetic declination, there might be other corrections needed based on your specific navigation scenario. These could include corrections for variation over time, compass deviation caused by local magnetic influences on the vessel or equipment, and so on.
5. Use True Bearing for Navigation:
   • The true bearing obtained after correcting for magnetic declination is the direction in reference to true north. This is the value you would use for navigation or plotting on maps and charts.

It's important to note that the terms and procedures may vary slightly depending on the specific navigation tools or charts being used. Always refer to the specific guidelines and information provided for your navigation equipment or resources. Additionally, the use of electronic navigation tools and GPS has become prevalent, and they often provide true bearing information directly without the need for manual correction.

Note: The above analysis is valid, if bearing is in WCB system. If bearings are in the quadrantal bearing system, then either convert it in WCB system and do the above analysis or find the relation for each quadrant between TB & MB.

important question in magnetic declination

Q 1) In a region with magnetic declination of 2°E, the magnetic fore bearing (FB) of a line AB was measured as N79°50'E. There was local attraction at A. To determine the correct magnetic bearing of the line, a point 0 was selected at which there was no local attraction. The magnetic FB of line AO and 0A was observed to be S52°40'E and N50°20°W, respectively. What is the true FB of line AB?

a) N77°50'E
b) N84°10'E
c) N81°50'E
d) N82°10'E

(GATE-2015, SET-I)

Q 2) A line AB was drawn to have a MB of 25°20' in an old map, at that time the declination was 2°20'E. if the present declination is 4°30' W, then the MB of the line will be:

a) 32°10'
b) 45°20'
c) 30°
d) 35°2'

Q 3) A line AB had the MB 44°20' in 1910, when the declination was 4°20'W. Determine the MB of the same line in 1990 if the annual declination change is observed as 6' eastward?

Running a compass traverse:

Running a compass traverse involves using a compass to navigate and measure direction and distance between points along a route. This technique is commonly used in orienteering, land surveying, and other outdoor activities. Here is a step-by-step guide on how to run a compass traverse:

Preparation:

1. Map Familiarization:
   • Study the map of the area where you'll be running the traverse. Identify key landmarks, features, and the intended route.
2. Select Control Points:
   • Choose specific control points or waypoints along your route. These points should be easily identifiable on the map and in the field.

Equipment:

1. Compass:
   • Ensure your compass is in good working condition. Familiarize yourself with its features, including the direction-of-travel arrow, the magnetic needle, and the rotating bezel with degree markings.
2. Map:
   • Bring a detailed map of the area. Make sure the map and compass are compatible (i.e., both using the same reference for North).
3. Writing Tools:
   • Carry a notebook and pencil for recording bearings, distances, and other relevant information.

Execution:

1. Orient the Map:
   • Align the map with the actual terrain by matching features on the map with what you see around you. Ensure that the map's north is aligned with true north.
2. Set Compass Declination:
   • If the map uses true north and your compass uses magnetic north, set the declination on your compass accordingly.
3. Take a Bearing:
   • Select a control point in the distance, sight it through the direction-of-travel arrow on your compass, and rotate the bezel until the magnetic needle aligns with the orienting arrow. The degree reading is your bearing.
4. Follow the Bearing:
   • Walk in the direction of the bearing, regularly checking your compass to stay on course. Note the distance traveled.
5. Arrive at Control Point:
   • Reach the first control point. Confirm your location by identifying features around you and on the map.
6. Repeat the Process:
   • Choose the next control point, take a new bearing, and proceed. Continue this process until you complete the traverse.
7. Record Information:
   • Document bearings, distances, and any other observations in your notebook. This information can be crucial for retracing your steps or for future reference.
8. Adjust for Variations:
   • If you encounter obstacles or terrain changes, adjust your route as needed, and make note of any deviations from the original plan.
9. Complete the Traverse:
   • Arrive at the final destination, confirming your location and completing the traverse.

Running a compass traverse requires practice, and it's essential to stay aware of your surroundings and continuously refer to the map and compass for accurate navigation.

local attraction:

"Local attraction" in the context of navigation refers to the influence of nearby magnetic or ferrous objects on a magnetic compass. These objects can cause the compass needle to deviate from its true magnetic north reading. Local attraction is a form of compass error that navigators need to be aware of and account for, especially when using magnetic compasses for navigation.

• It is the attraction of the magnetic needle to the local magnetic field other than the earth's magnetic field.
• It is caused by iron fences, steel pipes, electric wires, watch, pen, vehicles, railroad rails etc.
• A freely suspended magnetic needle takes the direction of the earth's magnetic field only if there is no local attraction present in the area.
• The magnetic needle deviates from the magnetic meridian under local magnetic force, thereby affecting the magnetic bearing of the traverse line.
• Local attraction can be identified by noting the FB and BB of a line.
• If the difference between FB and BB of a line is not 180° in that case either there is an observational error in FB or BB or both or there is local attraction at one or both stations of that line.

Here are some key points about local attraction:

1. Causes of Local Attraction:
   • Local attraction can be caused by nearby metallic objects, electronic equipment, or any other materials that can influence the Earth's magnetic field around the compass.
2. Identifying Local Attraction:
   • Navigators can identify local attraction by comparing the readings of a magnetic compass with known bearings or by observing discrepancies in readings when the compass is moved around the area.
3. Minimizing Local Attraction:
   • To minimize the effects of local attraction, navigators should keep the compass away from metal objects, electronic devices, and other sources of magnetic interference. This might involve physically moving away from potential sources of attraction.
4. Compensating for Local Attraction:
   • If local attraction is known or suspected, navigators can apply a correction to the compass readings. This correction involves mentally or physically adjusting the compass reading based on the estimated or measured local attraction.
5. Adjusting for Deviation:
   • Deviation is the term used to describe the error introduced by local attraction. When plotting courses on a nautical chart or orienteering map, navigators often apply a deviation correction to ensure that their intended course aligns with the compass reading.
6. Regular Checks:
   • It's good practice to regularly check for local attraction, especially when navigating in areas with a high likelihood of magnetic interference. Regular checks help ensure that the compass readings remain accurate throughout the journey.
7. Using Multiple Bearings:
   • Taking multiple bearings to the same object from different locations can help confirm the accuracy of compass readings and identify any consistent errors caused by local attraction.

Elimination of local attraction:

Eliminating local attraction in the context of navigation often involves identifying and mitigating the effects of nearby magnetic or ferrous objects that can influence the accuracy of a magnetic compass. While complete elimination may not always be possible, minimizing the impact of local attraction is essential for accurate navigation. Here are some steps to help reduce local attraction:

1. Identification of Local Attraction:
   • Before attempting to eliminate local attraction, it's crucial to identify the sources. This can be done by comparing compass readings with known bearings, both on the map and in the field. Look for discrepancies and anomalies in readings.
2. Move Away from Magnetic Interference:
   • Physically move away from potential sources of magnetic interference. Metallic objects, electronic devices, and other materials can create magnetic fields that affect the compass needle. Creating distance from these objects reduces their influence.
3. Keep Electronics Away:
   • Turn off or move electronic devices, such as radios, GPS units, and smartphones, away from the compass. These devices can produce magnetic fields that interfere with the compass reading.
4. Use Non-Magnetic Materials:
   • When setting up navigation equipment or plotting courses, use non-magnetic materials. For example, avoid metal clips or rulers, as they can introduce local attraction.
5. Check for Vehicle Interference:
   • In vehicles, especially boats and aircraft, check for metallic objects or equipment that might influence the compass. Adjust the location of such objects or add compensation magnets if necessary.
6. Adjust the Compass:
   • Some compasses allow for compensation adjustments. Follow the manufacturer's instructions to adjust the compass to minimize local attraction. This might involve turning adjustment screws or rotating compensating magnets.
7. Periodic Checks:
   • Regularly check for local attraction, especially when moving to a new location or when conditions change. Periodic checks help ensure that the compass remains accurate during the journey.
8. Take Multiple Bearings:
   • Confirm the accuracy of compass readings by taking multiple bearings to the same object from different locations. Consistent errors in readings may indicate the presence of local attraction.
9. Create a Deviation Card:
   • If local attraction is persistent and cannot be completely eliminated, create a deviation card. This is a record of the compass errors introduced by local attraction for different compass headings. Refer to the deviation card to apply corrections when navigating.

dips:

In the context of magnetism, "DIPs" refers to the inclination or angle between the magnetic field lines and the horizontal plane at a specific location. Magnetic dip is measured with a dip needle or dip circle. At the magnetic equator, the dip is close to zero, while at the magnetic poles, the dip is 90 degrees.

• As the earth has its own magnetic field, lines of forces are created by this and are directed towards the north and south magnetic poles.
• A freely suspended magnetic needle aligns itself with the lines of magnetic forces of the earth.
• The angle made by the lines of magnetic field with the earth's surface is called as 'Dip'.
• Dip is zero at the equator and 90° at the poles.
• At other places, the value of the dip varies between (0 - 90°).

types of compass:

Magnetic Compass used in compass traversing are generally of Two types of compass:

A) Surveyor Compass

A surveyor compass is a specialized compass used in land surveying and geodetic work for measuring horizontal angles. It is a precise instrument designed for accurate angle measurements, often featuring additional tools and accessories to aid surveyors in their work. Here are some key characteristics and features of a surveyor compass:

1. Alidade or Sighting Mechanism:
   • A surveyor compass typically includes an alidade, which is a straightedge or sighting mechanism used for aligning the compass with distant points or survey lines. This allows surveyors to precisely aim the compass at the target.
2. Clinometer:
   • Some surveyor compasses come equipped with a clinometer, which measures the angle of inclination or slope. This feature is useful for determining elevations and vertical angles in surveying.
3. Magnetic Needle and Azimuth Ring:
   • The compass needle in a surveyor compass points to magnetic north. An azimuth ring around the compass housing allows surveyors to read and set the azimuth or horizontal angle of the compass.
4. Bubble Level:
   • A bubble level is often integrated into the compass to ensure that it is held horizontally. This is crucial for accurate angle measurements.
5. Adjustment Screws:
   • Surveyor compasses are designed to be adjustable. They may include screws or mechanisms that allow surveyors to calibrate or adjust the instrument for accuracy.
6. Tripod Mount:
   • Many surveyor compasses are designed to be mounted on a tripod. This helps stabilize the instrument, especially during prolonged surveying tasks, and allows for precise measurements.
7. Plumb Bob:
   • Some surveyor compasses have a plumb bob attached to the bottom. This helps in aligning the instrument vertically over a survey point.
8. Optical Peep Sight:
   • An optical peep sight may be included to provide a clear line of sight to the target. This enhances accuracy when aiming the compass at specific survey points.
9. Magnifying Lens:
   • A magnifying lens might be present to assist in reading fine details on maps or graduated scales.
10. Graduated Circles:
    • The compass housing often includes graduated circles or scales that help in reading angles with precision.

Surveyor compasses are essential tools for land surveyors and are used in various applications, including topographic mapping, boundary surveys, construction layout, and geodetic control surveys. They are valued for their accuracy, stability, and the ability to provide reliable angular measurements in the field.

b) prismatic Compass

A prismatic compass is a type of compass used for measuring magnetic bearings of distant objects. It incorporates a prism and a sighting mechanism, allowing the user to simultaneously read the compass direction and view a distant object through the sighting mechanism. Prismatic compasses are commonly used in various field activities such as navigation, surveying, and reconnaissance. Here are some key features and components of a prismatic compass:

1. Prism:
   • The prismatic compass gets its name from the prism, which is a glass or optical component that reflects the image of the compass card (with direction markings) into the line of sight. This allows the user to read the compass direction while looking at a distant object.
2. Compass Card:
   • The compass card is a circular disc with markings indicating the cardinal directions (north, east, south, west) and degrees. It is housed in a transparent or semi-transparent container.
3. Sighting Mechanism:
   • Prismatic compasses are equipped with a sighting mechanism, typically consisting of a peep sight or notch that allows the user to align the compass with a distant target accurately.
4. Hinged Prism and Reading Lens:
   • The prism is often hinged, allowing the user to swing it away when taking a reading and close it for protection during transport. Some prismatic compasses also have a reading lens that magnifies the compass card for easier reading.
5. Bubble Level:
   • A bubble level is usually integrated into the compass housing to ensure that the instrument is held horizontally, aiding in accurate readings.
6. Adjustment Screws:
   • Prismatic compasses may have adjustment screws or mechanisms that allow the user to calibrate or adjust the instrument for accuracy.
7. Tripod Mount:
   • Some prismatic compasses have a threaded base that allows them to be mounted on a tripod for stability, particularly when taking precise readings over an extended period.
8. Base Plate:
   • The compass is often mounted on a base plate, which may have a ruler or scales for measuring distances on maps.

Prismatic compasses are particularly useful in situations where accurate readings of bearings to distant objects are required, such as in land navigation, mapping, and reconnaissance activities. They offer a compact and portable solution for obtaining directional information in the field.

Difference between surveyor compass & prismatic Compass

Both the surveyor compass and the prismatic compass are tools used for navigation and measuring angles, but they serve different purposes and have distinct features. Here are the key differences between a surveyor compass and a prismatic compass:

1. Purpose:
   • Surveyor Compass: It is primarily designed for land surveying and geodetic work. Surveyor compasses are used to measure horizontal angles accurately, and they often come with additional features such as clinometers for measuring slopes and alidades for sighting.
   • Prismatic Compass: This type of compass is designed for measuring magnetic bearings to distant objects. It is commonly used in field activities such as navigation, reconnaissance, and mapping.
2. Sighting Mechanism:
   • Surveyor Compass: It typically has a more sophisticated sighting mechanism, such as an alidade, to accurately aim at distant survey points. This is crucial for precise angle measurements in surveying.
   • Prismatic Compass: It includes a prism and a simpler sighting mechanism (like a peep sight or notch) that allows the user to simultaneously read the compass direction and view a distant object through the prism.
3. Additional Features:
   • Surveyor Compass: It may include features like a clinometer for measuring slopes and a bubble level for ensuring the instrument is held horizontally.
   • Prismatic Compass: While it may include a bubble level, it is primarily focused on providing a quick and accurate reading of magnetic bearings.
4. Use in Surveying:
   • Surveyor Compass: Ideal for precise angle measurements in land surveying, topographic mapping, and construction layout.
   • Prismatic Compass: Suited for quick and accurate readings of magnetic bearings, making it useful in field navigation, reconnaissance, and mapping where precise angles may not be as critical.
5. Mounting Options:
   • Surveyor Compass: Some surveyor compasses are designed to be mounted on tripods for added stability during prolonged surveying tasks.
   • Prismatic Compass: While some may have a tripod mount, they are often more portable and may be used handheld in the field.
6. Versatility:
   • Surveyor Compass: More versatile in terms of applications related to land surveying and precise angle measurements.
   • Prismatic Compass: Geared towards quick magnetic bearing readings in the field but may lack some of the features needed for detailed land surveying tasks.

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In summary, the surveyor compass is tailored for accurate angle measurements in surveying, while the prismatic compass is designed for quick and convenient measurements of magnetic bearings in various field activities. The choice between the two depends on the specific needs and requirements of the task at hand.

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