1.15.6.c - Fly levelling
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Introduction to Fly Levelling
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Today, we will explore fly levelling, a surveying technique essential for connecting survey sites to benchmarks. Can anyone tell me why benchmarks are important in levelling?
I think benchmarks are crucial because they provide a reference point for measurements.
Exactly! Benchmarks guide us in determining elevations accurately. Now, when might you use fly levelling instead of other methods?
We might use fly levelling when the survey site is too far from the nearest benchmark.
Great observation! Fly levelling allows us to extend our reach to distant areas without losing accuracy.
Setting Up for Fly Levelling
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To begin fly levelling, we set up our level instrument near the BM. What do you think the first step would involve?
We need to take a back sight reading on the BM.
Correct! After taking the back sight reading, what comes next?
Then we start moving towards the survey site and take fore sights and back sights on change points.
Exactly! The process of taking readings continues until we establish a temporary benchmark at the site.
Closing the Levelling Operation
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After establishing the temporary BM, how do we finalize our levelling measurements?
We take a last reading back on the temporary BM to check for closing errors.
Correct! This closing reading helps us to identify any discrepancies that occurred during the levelling process.
How do we calculate those closing errors?
We compare the sum of the back sight and fore sight readings to verify consistency in our measurements.
Importance of Fly Levelling
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Why do you think fly levelling is a preferred method in certain surveying scenarios?
It helps connect distant areas back to a known elevation reference without complicating things.
That's right! Fly levelling balances the need for accuracy with the practical challenges of surveying over long distances.
It also allows us to handle elevation changes in complex terrains.
Exactly! The effectiveness of fly levelling lies in how it adapts to practical surveying environments.
Introduction & Overview
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Quick Overview
Standard
Fly levelling is utilized when a survey site is distant from a benchmark. The level instrument is setup near the benchmark to capture initial readings, followed by fore sights and back sights on change points, establishing a temporary benchmark at the survey site for calculations.
Detailed
Fly Levelling
Fly levelling is a critical technique used in surveying, particularly when a survey site exists far from a predefined benchmark (BM). In this method, surveyors start by setting a level instrument near the BM to obtain a back sight reading on the benchmark. From there, they progressively move toward the survey site, taking fore sight and back sight readings at various change points. This process continues until a temporary benchmark is established at the actual site of the survey.
Once a temporary benchmark is established, differential levelling methods are applied to carry out further observations and calculations. Finally, a closing reading is made back on the temporary BM to identify and quantify any closing errors that may have occurred during the levelling operation. The significance of fly levelling lies in its ability to effectively connect distant survey locations back to known benchmark heights, ensuring accurate elevation determinations and facilitating smooth project execution.
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Definition of Fly Levelling
Chapter 1 of 3
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Chapter Content
If the survey site is away from the bench mark, the level instrument is set up near the BM and a back sight reading is taken on the BM. The survey work proceeds towards the site by taking fore sights and back sights on a number of change points till a temporary BM is established at the survey site.
Detailed Explanation
Fly levelling is a method used when the survey area is distant from a known benchmark (BM). In this approach, the level instrument is initially placed close to the BM to take a reference reading. From this point, the surveyor makes their way toward the target area, taking measurements (fore sights) at various intermediate points (known as change points). This establishes a new temporary benchmark closer to the survey site, enabling further levelling works to be carried out in that area.
Examples & Analogies
Imagine you're setting up a camera to capture a distant mountain peak, but your tripod can't reach the trailhead nearby. First, you stabilize your camera close to the nearby hill (the BM), take a shot, and then gradually move toward the mountain while capturing photos along the way to ensure you've accurately documented the path to the peak.
Carrying out Differential Levelling
Chapter 2 of 3
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Chapter Content
Now the levelling work is carried out at the site as per differential levelling method. At the end of the work, last reading is taken again on temporary BM. It will help determining the closing error in levelling work.
Detailed Explanation
After establishing a temporary benchmark within the survey site, the surveyor employs the differential levelling method to take continuous elevation measurements from that point. This includes measuring back to the original BM and using the gathered data to calculate the height differences accurately. At the conclusion of the levelling process, the last measurement is again compared against the temporary BM to check for any discrepancies, known as closing errors, to ensure the accuracy of the entire levelling operation.
Examples & Analogies
Think of it like keeping a detailed diary of your travels. You start from home (the BM), take notes of every new location you visit (the change points), and at the end of your journey, you compare your final note with your first to see if anything seems off, ensuring your traveled path is accurately recorded.
Purpose of Fly Levelling
Chapter 3 of 3
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Chapter Content
The purpose of fly levelling is to connect the survey site to a BM away from the site, and then carry out the levelling work using differential levelling approach.
Detailed Explanation
Fly levelling is essential for establishing a reliable connection between a survey site and a benchmark that might be far away. It allows for a systematic method of obtaining precise elevation data that is vital for construction, road alignment, and other project needs. It effectively bridges the gap, ensuring that measurements taken in a new area can be directly related to the known height reference, thus guaranteeing accuracy in subsequent construction or landscaping projects.
Examples & Analogies
Consider it like needing to build a bridge between two islands. First, you establish a solid foundation on one island (the known BM) and then measure the height of the bridge to the second island at various points (the survey site). This method allows you to ensure that your bridge is level and structurally sound by referring back to both locations in a consistent way.
Key Concepts
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Fly Levelling: A method to connect distant survey sites to known benchmarks.
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Benchmark: A reference point with a known elevation.
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Back Sight and Fore Sight: Measurements taken during levelling to establish elevation differences.
Examples & Applications
When surveying for a new construction site that is far from any established benchmark, fly levelling is employed to ensure elevations are accurately measured and recorded.
In a mountainous region, fly levelling allows engineers to connect distant sites without needing to visit the benchmark multiple times.
Memory Aids
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Rhymes
Fly towards the site, don’t lose the height, take a sight, and measure right!
Stories
Once upon a time, a surveyor named Sam had to measure heights all day long. Far from his benchmarks, he knew he needed to fly—fly levelling that is—so he could connect every height high. He set his instrument, took his sights, and soared through the fields, ensuring every level was right.
Memory Tools
BFS: Back Sight, Fore Sight - to connect and compute. Remember BFS to not miss a loop!
Acronyms
FLY
Fly Levelling Yields accurate heights.
Flash Cards
Glossary
- Benchmark (BM)
A point of known elevation used as a reference for measuring other elevations.
- Back Sight
A reading taken on a level staff held at a point of known elevation.
- Fore Sight
A reading taken on a level staff held at a point where the elevation needs to be determined.
- Temporary Benchmark
A reference elevation established at a survey site for measurements.
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