Local Attraction in Compass Survey – Meaning, Causes, Detection & Correction
🔹 Introduction
In surveying, Compass Surveying is one of the most widely used methods to measure directions (bearings) using a Magnetic Compass.
However, sometimes the magnetic needle does not point towards the true magnetic north due to nearby magnetic influences.
This error is known as Local Attraction.
🧭 What is Local Attraction? (Definition)
Local Attraction is the deviation of the magnetic needle from its normal (magnetic north) position due to the presence of magnetic substances or electrical sources nearby.
👉 In simple words:
When the compass needle is pulled away from the correct direction by nearby metal objects or magnetic fields, it is called Local Attraction.
⚙️ Causes of Local Attraction
Local attraction occurs due to several reasons, such as:
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🧲 Iron objects near the instrument (vehicles, nails, rods, etc.)
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⚡ Electric or power lines carrying current
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⛓️ Metallic chains or measuring tapes used during survey
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⌚ Metallic accessories like wristwatches, belts, or phones
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⛰️ Natural magnetic rocks or minerals in the area
📍 Effect of Local Attraction
When local attraction occurs, the compass needle gives incorrect readings, which causes:
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Wrong bearing measurements
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Errors in plotting and mapping
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Incorrect direction and angle calculations
Hence, it’s essential to detect and correct local attraction before finalizing survey data.
🔍 Detection of Local Attraction
Local attraction can be detected by comparing Fore Bearing (FB) and Back Bearing (BB) readings:
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Take the Fore Bearing from station A to B.
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Take the Back Bearing from station B to A.
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If the difference between them is not exactly 180°,
then Local Attraction exists at one of the stations.
🧮 Example:
If Fore Bearing (A→B) = 60°
then Back Bearing (B→A) should be 240°.
If not, Local Attraction is present.
🧮 Correction of Local Attraction
There are two main ways to correct local attraction:
1. By Comparing Fore and Back Bearings
If the difference between FB and BB is not 180°, identify and adjust the readings of the affected stations.
2. By Using a Station Free from Local Attraction
Select one station where the difference between FB and BB is exactly 180°.
Use that station as a reference to correct other bearings.
📘 Example Table
| Station | Fore Bearing | Back Bearing | Difference | Result |
|---|---|---|---|---|
| A–B | 60° | 240° | 180° | No Local Attraction |
| B–C | 110° | 285° | 175° | Local Attraction Present |
Here, the second line shows that either station B or C is affected by local attraction.
🧾 Conclusion
Local Attraction is a common error in compass surveying caused by magnetic influences near the instrument.
It leads to incorrect bearings and must be checked carefully during every survey.
To avoid this:
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Keep metallic objects away from the compass
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Compare fore and back bearings
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Apply corrections if needed
Magnetic Dip – Definition, Meaning, Causes & Examples
🔹 Introduction
The Earth behaves like a giant magnet, and because of this, a magnetic needle (like in a compass) does not remain perfectly horizontal.
Instead, it tilts slightly upward or downward depending on your location on Earth.
This tilting of the needle is known as Magnetic Dip or Angle of Dip.
🧭 What is Magnetic Dip? (Definition)
👉 In simple words:
The angle made by the magnetic needle with the horizontal plane when it points toward the Earth’s magnetic field is called the Magnetic Dip or Angle of Dip.
It shows how much the Earth’s magnetic field is inclined (tilted) at a particular place.
⚙️ Cause of Magnetic Dip
The Earth’s magnetic field has two components:
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Horizontal Component (H) – Keeps the needle aligned with the magnetic north-south direction.
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Vertical Component (V) – Pulls the needle upward or downward.
Because of the vertical component, the magnetic needle does not stay level — it tilts, creating the Dip Angle.
🌍 Magnetic Dip at Different Places on Earth
| Location | Dip Angle | Description |
|---|---|---|
| At the Magnetic Equator | 0° | Needle remains perfectly horizontal |
| At the Magnetic Poles | 90° | Needle points vertically downward |
| Between Equator & Poles | Between 0° and 90° | Needle is inclined downward |
📐 Instrument Used – Dip Circle
The Dip Circle is a special instrument used to measure the Angle of Dip.
It has a magnetic needle mounted so that it can rotate in a vertical plane and measure the inclination.
📘 Example
If the Magnetic Dip at a location is 40°, it means the magnetic needle is inclined 40° below the horizontal plane at that place.
🧾 Conclusion
Magnetic Dip is the angle that the Earth’s magnetic field makes with the horizontal plane.
It varies with latitude:
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0° at the Equator
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90° at the Magnetic Poles
Understanding the Magnetic Dip is important in Surveying, Navigation, and Geomagnetism to ensure accurate compass readings and direction measurements.
Magnetic Declination – Definition, Causes, Types & Examples
🔹 Introduction
The Earth’s magnetic field does not perfectly match the Earth’s geographical north and south poles.
Because of this difference, the magnetic needle of a compass does not point exactly toward the true north, but slightly east or west of it.
This angular difference is called Magnetic Declination.
📘 Definition of Magnetic Declination
👉 In simple words:
The angle between the True North (Geographical North) and the Magnetic North (shown by a compass) at any place on the Earth is called Magnetic Declination.
This angle may be toward the east or toward the west, depending on your location.
🧭 True North vs Magnetic North
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True North → The direction along the Earth’s surface towards the geographic North Pole.
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Magnetic North → The direction indicated by the north end of a magnetic needle (compass).
Since the magnetic and geographic poles are not at the same place, a small angle forms between them — this is the Magnetic Declination.
⚙️ Types of Magnetic Declination
| Type | Description | Example |
|---|---|---|
| East Declination | When the Magnetic North lies east of True North | Compass needle deflects to the right |
| West Declination | When the Magnetic North lies west of True North | Compass needle deflects to the left |
👉 For example:
If the compass needle points 5° east of the true north, the magnetic declination is 5° East.
🌍 Causes of Magnetic Declination
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Uneven distribution of Earth’s magnetic field
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Irregular magnetic materials (rocks, ores) in the Earth’s crust
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Local magnetic influences like power lines, iron structures, etc.
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Natural movement of magnetic poles over time (they slowly shift every year)
🧮 Example of Magnetic Declination
Suppose at a particular location:
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The True North bearing = 0°
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The Compass (Magnetic) bearing = 4° East
Then,
Magnetic Declination = 4° East
If the compass points 3° West of True North,
then Declination = 3° West
📐 Importance of Magnetic Declination in Surveying
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Helps convert magnetic bearings (from compass) into true bearings (for maps).
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Required for accurate navigation and mapping.
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Used in civil engineering, defense, aviation, and marine navigation.
Without declination correction, all directions and bearings can become inaccurate.
📊 Variation of Magnetic Declination
Magnetic Declination changes with:
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Time (Yearly variation)
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Place (Latitude & Longitude)
So, surveyors must always check updated declination values before conducting any compass survey.
🧾 Conclusion
Magnetic Declination is the angle between True North and Magnetic North.
It may be east or west depending on your location.
It’s an important correction factor in surveying, navigation, and mapping, ensuring that all compass readings are accurate and reliable.
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