Full Form of RL in Civil Engineering

What is a Reduced Level in Civil Engineering?

Reduced level in civil engineering is a term that refers to equating the elevation of survey points. These survey points are equated with the help of an assumed datum.

This is a vertical distance that is above or below the datum plane, and the most common datum is mean sea level.

In simpler terms, "reduced level" is a term to describe a comparison in height. In technical terms, it is the height between the point of interest and the adopted level datum.

How to Calculate the Reduced Level (RL)?

The reduced level (RL) can be measured using two methods:

1. The rise and fall method
2. The height of the instrument method

The Rise and Fall Method

• The rise and fall method is one wherein the difference between consecutive points is calculated by comparing each point after the first with the points preceding it.
• Basically, the difference in the reading indicates the rise or fall of any reading that is smaller or greater than the preceding point.

How to Calculate the RL of Each Point

• The reduced level of each point is calculated by adding or subtracting the rise to or from the previous point's reduced level.
• For instance, if B.S. (backsight) - F.S. (foresight) is positive, then there is a rise.
• If, however, the B.S.-F.S. is negative, then there is a fall.
• Therefore, the R.L. is equal to the R.L. of the previous point plus the rise or fall.
• To check Σ B S. – Σ F.S. = Σ Rise – Σ fall = (First R.L.) – (Last R.L.)

What are the Specifications of this Method?

The specifications of this method are as follows:

• Firstly, it is both slow and simple.
• Secondly, it is precise as it considers the immediate sight.
• It is a laborious method.
• A thorough examination of the reduction of the reduced levels of the immediate station is carried out.
• For arithmetic accuracy, three checks are conducted, and the difference between the sum of the backsight and the foresight should be equal to the sum of the rise and fall between the first and last R.L.
• It is used primarily for calculating earthwork and other levelling operations that need precision.

Height of Instrument Method

• The height of the instrument method includes finding the elevation of the plane of collimation which is also called H.I.
• This is done for every setting of the instrument, and obtaining the reduced levels of the points using references according to the respective plan of the collimation.
• Firstly, the H.I. is calculated by adding the reduced level (of the starting point) to the backsight. I.e. H.I. = R.L. + B.S.
• Secondly, the reduced level (R.L.) of the immediate point and the first change point are obtained by subtracting the readings from the H.I.
• Additionally, when the instrument is shifted, a new plane of collimation is set up. Here, the height is calculated by adding the backsight reading to the reduced level of the first change point.
• The reduced level (R.L.) of the successive points as well as the second change point are determined by subtracting the staff reading from the new H.I.
• Most importantly, this process is repeated until every reduced level is worked out. After that, the arithmetic check takes place which is Σ B.S. – Σ F.S. = Last R.L. - First R.L.

What are the Specifications of this Method?

• The height of the instrument method only involves a few calculations. Therefore, it is less time-consuming, less labor-intensive, and saves time when it comes to immediate reading.
• There is no check on the reduced levels of intermediate stations.
• Additionally, there are two checks done for arithmetic accuracy. In other words, the difference between the sum of the backsight and that of the foresight should be equal to the difference between the first R.L. and the last R.L.
• Lastly, it is widely used for longitudinal operations as well as cross-leveling operations.

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