Rise and Fall Method in Surveying
Surveying is required to accurately determine the size, shape, and elevation of the Earth's surface.The rise and fall method is a levelling technique used in surveying to determine the difference in elevation between two points. We must compute the difference in the staff's elevation at the two points using this method. We know the RL of ground for one of the two stations where the staff readings are taken, but we need to locate the RL of the other station.
The rise and Fall method
It is a method for calculating the difference in elevation between two consecutive points by comparing each point after the one immediately preceding it. The difference in staff readings indicates a rise and fall. Following that, RL is calculated by adding the rise to or subtracting the fall from the previous point's reduced level.Method to calculate R.L(Reduced Level)
fall (F) = foresight (F.S.) – backsight (B.S.)
fall (F) = intermediate sight (I.S.) – backsight (B.S.)
rise (R) = backsight (B.S.) – foresight (F.S.)
reduced level (R.L.) = precede reduced level + rise (R)
reduced level (R.L.) = precede reduce level – fall (F)
fall (F) = intermediate sight (I.S.) – backsight (B.S.)
rise (R) = backsight (B.S.) – foresight (F.S.)
reduced level (R.L.) = precede reduced level + rise (R)
reduced level (R.L.) = precede reduce level – fall (F)
We have two methods for determining the reduced level of a point in the field from staff readings:
- Height of Instrument or Collimation method
- Rise and fall method.
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| Rise and fall method |
What exactly is the rise and fall method?
Instead of calculating the instrument's height, we calculate the difference between the staff readings at the two stations. The two stations where staff readings are taken are set up in such a way that we know the RL of one station's ground and want to find the RL of the other.
- A level is placed in such a way that the maximum number of stations is visible. Now, the staff is stationed at a known RL station, and the reading is observed, say R1. The staff is then transferred to a station where the RL is to be determined, and the reading is taken, which says R2.
- Now, if R1 > R2, the former station's ground is lower than the latter station's ground. As a result, the ground surface is rising.
- Similarly, if R2 is greater than R1, the ground at the latter station is deeper / lower than the ground at the former station. As a result, the ground surface has sunk.
What is Back Sight?
Backsight is a first reading of a staff (levelling rod) that remains unchanged when the levelling apparatus is moved to another or new point after it has been set up and levelled on the first point. Simply put, it is the backward reading of the previous station point. It is measured against a benchmark or known reduced level.What is Intersight?
Inter-sight refers to all readings taken between backsight and foresight.What is Fore Sight?
Foresight is a reading of the staff or levelling rod taken in the forward direction of the levelling process or the staff reading to be determined, and it is the final reading of the entire surveying process. It is regarded as negative and subtracted from the Instrument's Height to determine the RL of the point.
What is a Benchmark?
A benchmark is a fixed point of known elevation through which the reduced level of another point is determined.It is arbitrary or permanent reading through which survey work is completed and the reduced level is calculated. The surveyor calculates the elevation of the points using an identified or permanent benchmark for GPS surveys.
Rise and Fall Method Formula
Determine the rise or fall using:B.S-I.S or I.S-F.S
Where;B.S= back sight
F.S= foresight
I.S= intermediate sight
If,
B.S-F.S=+ve (Rise) & B.S-F.S=-ve (Fall)
Determining new R.L
If,
B.S-F.S=+ve (Rise) & B.S-F.S=-ve (Fall)
Determining new R.L
New R.L= Old R.L-fall = Old R.L+rise
Arithmetic checking
Arithmetic checking
New R.L= Old R.L-fall = Old R.L+rise
Advantages of the Rise and Fall Method
- The advantage of the Rise and fall method is that it gives more accuracy in the reading and it includes many steps.
- This method is quite complicated and is not easy to use.
- It is used where the change point is more and the reduced level takes more time.
- Imagination or visualization is required for the nature of the ground.
- A complete check is done for all the readings.
Applications of the Rise and Fall Method:
Construction Projects:
Topographic Mapping:
Topographic maps depict natural and man-made land features such as contours and elevations. The rise and fall method is useful for producing accurate topographic maps. Surveyors can collect elevation data at various points throughout the surveyed area, allowing them to create contour lines that represent elevation changes. This data is extremely useful for land planning, environmental studies, and infrastructure development.Read More
Drainage and Water Management:
Water management is essential in both urban and rural areas. By providing elevation data required for proper grading and slope calculations, the rise and fall method aids in the design of effective drainage systems. This method is used by surveyors to determine the best location for drainage channels, pipes, and retention ponds in order to ensure efficient water flow and prevent flooding.Road and Highway Design:
The rise and fall method is used in road and highway design to determine appropriate grades, curves, and alignments. The method is used by surveyors to measure elevation changes along a proposed route, ensuring a safe and efficient road design that minimizes steep slopes and maximizes visibility. Accurate elevation data obtained via the rise and fall method contributes to the smooth and well-designed operation of transportation networks.Example of Rise and Fall method
- The following staff readings were obtained during a levelling work, with the instrument shifted after the fourth, seventh, and tenth.
- Readings: 2.305, 0.940, 0.865, 1.325, 2.905, 1.185, 1.205, 2.015, 1.365, 0.985, and 1.785.
- If the RL of the second turning point is 200.00, find the reduced levels of the remaining points.
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| Example of rise and fall method |
Arithmetic Check :
∑B.S –∑F.S = ∑Rise – ∑Fall = Last R.L – First R.L
= 7.225 – 4.315 =4.19 – 1.28 = 201.545 – 198.635
= 2.91=2.91=2.91
Hence proved