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A short class with Sarah and her students. The other ways are under More.
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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.
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.
Disturbance of the magnetic needle due to nearby metallic objects.
The measurement of direction from magnetic north to the target point.
The measurement of direction back to the magnetic north from the target point.
Assessing differences between fore bearing and back bearing to identify local attraction.
If metal's near, beware my dear, the needle will not steer clear!
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!
MICE for magnetic interference: Metallic items, Interfering wires, Chains present, Errors all round.
Term
What is Local Attraction?
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Term
How do you detect Local Attraction?
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A phenomenon where nearby metallic or magnetic objects disturb the alignment of a magnetic needle, affecting compass readings.
The direction of the survey line measured from the magnetic north towards the point of interest.
The direction of the survey line measured backward from the point of interest to the magnetic north.
The process of reallocating errors in measurements to account for inaccuracies introduced by local attraction.
Local attraction refers to the interference caused by nearby metallic objects that affect the accuracy of magnetic compass readings.
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.
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.