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1.14.5. Local attraction
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Today we're discussing local attraction. Can anyone tell me what this means in the context of surveying?
Is it when something nearby affects the compass reading?
Exactly! Local attraction happens when metallic objects near the compass disrupt its alignment with the magnetic meridian. What types of objects do you think could cause this?
Electric wires and iron pipelines could be examples, right?
Yes, those are perfect examples! It could also include objects like steel structures and even your keys or glasses. Let's use a mnemonic to remember: 'Many Elephants Can Increase Local Attraction'—the first letter of each word reminds us of the common culprits: Magnetic ore, Electric wires, Chains, Iron, etc.
That’s a great way to remember! But how do surveyors detect this problem?
Good question! They measure the fore bearing and back bearing of a line. If the difference is 180°, everything's fine. If not, that’s a red flag for local attraction.
So, if we see that difference, what should we do?
We adjust our readings and check if the error distributes appropriately across the affected stations. Understanding this helps improve accuracy in our surveys.
To recap, local attraction is the disruption of compass readings by nearby metallic objects. Key objects include electric wires, iron pipes, and personal metal items. Surveyors can detect this issue using fore bearing and back bearing measurements.
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Now that we understand what local attraction is, let’s talk about its impacts. Can anyone share why it’s so critical to avoid or correct for local attraction in surveys?
I think incorrect bearings can lead to wrong mapping, right?
Absolutely! Inaccurate bearings can cause errors in property boundaries, land usage planning, and even construction. Can you think of more consequences?
If the boundaries are wrong, it could lead to legal issues between property owners.
Exactly! And it could also affect projects that rely on precise measurements, like infrastructure development. Understanding local attraction isn’t just about being technical; it has real-world implications!
So, if we're working in an area with a lot of metallic objects, we should be extra careful?
Yes! Always identify those objects and consider their influence. Before we finish, what did we learn about correcting local attraction errors?
We can adjust by ensuring the fore bearing and back bearing differ by 180°!
Exactly! In summary, local attraction can lead to significant errors in surveying outcomes, impacting legal and developmental processes. Awareness and correction strategies are essential.
Overview
Short Summary
Local attraction refers to the interference caused by nearby metallic objects that affect the accuracy of magnetic compass readings.
Medium Summary
This section discusses the phenomenon of local attraction where magnetic needles may be distorted by nearby ferrous materials, leading to inaccuracies in survey measurements. It describes the types of objects that can cause local attraction and outlines methods to identify and correct these errors in surveying.
Detailed Summary
Local Attraction
Local attraction occurs when a magnetic needle, expected to align with the magnetic meridian, is disturbed by nearby metallic or magnetic objects. These objects can include electric wires, iron pipelines, and even personal items like keys and spectacles. This phenomenon can lead to erroneous compass readings, which can significantly impact surveying accuracy.
Causes of Local Attraction
- Types of Interfering Objects: Common culprits include magnetic ore, steel structures, iron poles, electric poles, and miscellaneous metallic items. The presence of these materials near the surveying area can attract the magnetic needle of the compass and deflect it from its true north.
Identifying Local Attraction
- Detecting Errors: When measuring bearings, surveyors take the fore bearing and back bearing of a line. If the difference between the two measurements is exactly 180°, the observations are considered unaffected by local attraction. However, if there’s a discrepancy, it indicates that local attraction is affecting the measurements.
Addressing Local Attraction
- Error Distribution: If local attraction is identified, the error is distributed across the affected stations. This allows corrections to be made, helping ensure all lines meet the 180° difference requirement. It is vital for survey accuracy to be aware of local attraction and take corrective actions when necessary.
Audio Book
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Create a free accountA properly balanced and freely suspended magnetic needle is expected to show the direction of magnetic meridian. However, many-times the local objects, like electric wires, magnetic ore, iron pipe line, and metallic buttons can attract magnetic needle towards themselves, and can give erroneous results.
Detailed Explanation
Local attraction refers to the inaccuracies caused when the magnetic needle of a compass is influenced by nearby magnetic objects. Ideally, a magnetic needle points to the magnetic meridian, which is the direction of the magnetic north. However, various local factors such as electric wires, iron pipelines, or other metallic objects can distort this reading. This results in a deviation in the compass reading, making it less reliable.
Examples & Analogies
Think of a compass as a person trying to find their way in a crowded place. If there are signs (the magnetic objects) that are misleading, the person might end up walking in the wrong direction. Just like it's important for the compass to be in a clear area, it's crucial for someone navigating to avoid distractions.
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Create a free accountThe materials/objects which can cause error due to local attraction could be magnetic rock or iron ore, steel structures, iron poles, rails, pipe-lines, electric poles and wires, key bunch, ring, knife, iron buttons, steel framed spectacles, and survey chain, arrows, hammer, clearing axe, etc.
Detailed Explanation
Various materials can cause local attraction errors, including magnetic rocks, iron ores, and everyday metallic items like keys, rings, and even tools used in surveying. When these items are close to the compass, they can interfere with the magnetic field and cause the compass needle to deviate from true north, resulting in inaccurate measurements.
Examples & Analogies
Imagine you are trying to get a reading from a thermometer while holding it near a hot stove. The heat from the stove can affect the reading, just as metal objects can influence the compass needle. To get an accurate reading, you need to keep the thermometer away from the heat source.
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Create a free accountSpecial care is required to be taken to avoid error due to local attraction. However, in many circumstances, it is neither possible to remove the metallic objects from the survey field, nor changing the station of magnetic compass observations. In such cases, the fore bearing and back bearing of the line is taken, and those stations are considered unaffected by local attraction if the difference of fore bearing and back bearing of the lines is exactly 180°.
Detailed Explanation
To manage errors caused by local attraction, surveyors often take both fore bearing (the direction from one point to another) and back bearing (the direction back to the original point) readings. If both readings differ by exactly 180 degrees, it indicates that the compass readings are not affected by local attraction. This method allows surveyors to identify and mitigate the impacts of magnetic interference.
Examples & Analogies
It's like checking both the beginning and end of a journey to ensure you’ve remained on the right path. If you leave a place and return using the same route, the two paths should match up perfectly. If they don’t, there might be something wrong with your navigation, similar to how local attraction would distort the compass readings.
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Create a free accountThe difference in value is the error. Using this approach, the stations affected or unaffected by local attraction can be determined. The error due to local attraction is distributed at the affected stations, such that now all the lines will satisfy the check, i.e., Difference in fore bearing and back bearing of a line = 180°.
Detailed Explanation
Once the direction differences are calculated using the fore and back bearings, surveyors can determine error values caused by local attraction. This method ensures that all bearings can align properly in further calculations and help in achieving more accurate results.
Examples & Analogies
Think of it like using a level to check if a shelf is straight. If two readings show that it’s off by a particular amount in both directions, you can adjust accordingly to make sure everything aligns perfectly. Similarly, understanding and adjusting for local attraction helps ensure accurate survey results.
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Key concepts
Core takeaways and short definitions to help you quickly recall the key ideas from this section.
- Local Attraction:
Disturbance of the magnetic needle due to nearby metallic objects.
- Fore Bearing:
The measurement of direction from magnetic north to the target point.
- Back Bearing:
The measurement of direction back to the magnetic north from the target point.
- Error Detection:
Assessing differences between fore bearing and back bearing to identify local attraction.
Examples
Step-by-step examples to apply the section's ideas and test your understanding.
An electric pole 50 meters away causes the compass needle to deviate 10 degrees from true north.
Using readings from both fore bearing and back bearing, a surveyor discovers a discrepancy of 5 degrees, indicating local attraction affecting the site's measurements.
Memory aids
Imagine a surveyor trying to measure the land, but as the compass spins, it points like a fan! The fence made of steel and the wires from the pole cause confusion, making the true path a stroll.
Remember 'E.G. Pizza' for Local Attractors: Electric wires, Ground pipes, and Metallic keys affect bearings!
Flash Cards
Glossary
Local Attraction
A phenomenon where nearby metallic or magnetic objects disturb the alignment of a magnetic needle, affecting compass readings.
Fore Bearing
The direction of the survey line measured from the magnetic north towards the point of interest.
Back Bearing
The direction of the survey line measured backward from the point of interest to the magnetic north.
Error Distribution
The process of reallocating errors in measurements to account for inaccuracies introduced by local attraction.