1.15. Measurement of Levels

Levelling is a branch of surveying which deals with measurements in vertical planes. It is the operation performed to determine and establish the elevations of points on the surface or beneath the Earth. It is also done to determine differences in elevation between points to draw the contours, find out the slope, and control the grades during construction. Levelling is of prime importance to the engineers for both design as well as execution of a project on the ground. The success of many engineering projects would depend upon accurate determination of elevations.

Levelling is a crucial aspect of surveying that focuses on measuring differences in height. When engineers design structures like buildings or roads, knowing how much higher or lower one point is compared to another helps them create accurate plans. It allows for the proper grading of land and ensures that each point is built at the right elevation, which is essential for the structural integrity and functionality of the project.

Imagine building a playground on a hill. If you don't know how steep the hill is, you might end up with a slide that is too high or a swing that tips over. Just like in construction, knowing the heights of different parts of the playground helps ensure everything is safe and fun for the kids.

The pre-requisite to compute the elevations of ground points is that the elevation of at least one point should be known with respect to which the elevations of other points can be computed. The point whose elevation is known to start the levelling work is known as the Bench Mark (BM).

A Bench Mark is a reference point with a known elevation. It's essential for any levelling process because it serves as the starting point for determining the heights of other locations. By knowing one fixed elevation, surveyors can calculate the relative elevations of various other points in the area, ensuring accuracy in their measurements.

Think of a surveyor starting at sea level when measuring the height of a mountain. The sea level is the bench mark, allowing the surveyor to determine how high the mountain is in relation to that base point.

The instrument used to measure the elevations is known as Level, which is used along with a Levelling staff or Levelling rod. The object is of levelling, therefore, is to; (i) determine the elevations of given points with respect to a datum, and (ii) establish the points of required height above or below the datum line.

A leveling instrument helps determine the height of points by creating a straight line of sight. The leveling staff, or rod, is used to take measurements against this line. When the level is set correctly, surveyors can read the staff to see how far above or below the reference height each point is, ensuring accurate construction and planning.

Imagine you're trying to hang pictures on a wall. You have a level tool to ensure the pictures are straight. The level indicates a straight line, while the staff is like the wall where you take your measurements to ensure everything hangs evenly.

The levelling is used for many applications, such as; (i) to determine or to set the plinth level of a building, (ii) to decide or set the road, railway, canal or sewage line alignment, (iii) to determine or to set various levels of dams, towers, etc., and (iv) to determine the capacity of a reservoir.

Levelling has a wide variety of applications in civil engineering and construction. It helps engineers decide how high to build foundations for buildings, set the proper angle for roads, and ensure that water systems are constructed at the right heights for drainage and effectiveness. Each application relies on precise measurements to ensure that everything functions as intended.

When building a dam, engineers need to know how deep to dig and how high to build the walls. If they miscalculate, the dam could overflow or collapse. Levelling ensures everything is built perfectly to control water levels effectively.

It is necessary to understand few technical terms often used in levelling. These are explained below (Refer to Figure 1.18). (a) Level surface: It is a surface perpendicular to the direction of gravity at all points. The surface of a still lake is a good example of level surface.

Understanding the technical terms used in levelling helps professionals accurately communicate and perform their work. For instance, a 'level surface' is crucial because all measurements in levelling are based on the premise that this surface is flat and even. Knowing what a level surface is allows surveyors to align their tools and measurements accordingly.

Consider a large, still pond. The water's surface is perfectly flat, just like a level surface in surveying. If you were to build a dock over the pond, you would want it to be straight and aligned with that level surface so that boats can float evenly without tipping over.

Various types of leveling instruments are: (a) Dumpy level (b) Tilting level (c) Engineers level (d) Automatic level (e) Digital level (f) Laser level.

There are different types of levelling tools suited for various tasks in surveying. Each type, such as the Dumpy level or Auto level, has specific features that make it ideal for certain situations, like ease of use, accuracy, or the ability to automatically adjust to level. Understanding these instruments allows surveyors to select the best tool for their specific requirements.

Think of a toolbox filled with different tools. Just like a carpenter chooses a specific saw or hammer for a job, a surveyor selects a particular leveling instrument based on the needs of the project, like setting up a foundation or determining a road's grade.

--