What is Level Surveying?

Level surveying is a fundamental branch of surveying that deals with determining the height difference between points on the earth’s surface. It is also known as leveling or differential leveling. The primary goal is to establish elevations relative to a known reference point or datum, typically a benchmark. Level surveys are essential for various engineering and construction projects, mapping, and monitoring changes in the earth’s crust.

Principles of Level Surveying

Level surveying is based on the principle of establishing a horizontal line of sight using a leveling instrument. The instrument, typically a level or a digital level, produces a horizontal plane. By taking readings on graduated staffs held vertically at different points, the height difference between those points can be calculated. The fundamental principles are:

  • **Horizontal Line of Sight:** Leveling instruments generate a horizontal line of sight. This line is perpendicular to the direction of gravity at each point.
  • **Level Surface:** A level surface is a curved surface that is perpendicular to the direction of gravity at all points. The Earth’s surface is approximately a level surface.
  • **Height Difference:** The difference in elevation between two points is the vertical distance between the level surfaces passing through those points.

Instruments Used in Level Surveying

Several instruments are employed in level surveying, each with its own level of accuracy and application:

1. Level

The level is the primary instrument used to establish a horizontal line of sight. There are different types of levels:

  • **Dumpy Level:** A traditional type of level with the telescope rigidly fixed to the leveling base.
  • **Tilting Level:** A more modern level that allows the telescope to tilt slightly in the vertical plane for finer adjustments.
  • **Automatic Level:** These levels use a compensator mechanism to automatically level the line of sight, making them faster and easier to use.

2. Leveling Staff

Also known as a leveling rod, this graduated staff is held vertically at points where height measurements are needed. Staffs come in various materials (wood, aluminum, fiberglass) and graduations (meters, feet).

3. Tripod

A stable platform to support the level, ensuring it remains steady during measurements.

4. Staff Bubble (Optional)

A small bubble attached to the staff to ensure it is held perfectly plumb (vertical).

Procedure for Level Surveying

The basic steps involved in conducting a level survey are as follows:

1. Setting Up

  • **Choose Benchmark:** Start with a point of known elevation, a benchmark.
  • **Set Up Level:** Mount the level on the tripod over the benchmark, ensuring it is stable and leveled.

2. Taking Readings

  • **Backsight (BS):** Hold the staff vertically on the benchmark. The reading taken on the staff through the level’s telescope is the backsight.
  • **Foresight (FS):** Move the staff to the point whose elevation is to be determined. The reading taken is the foresight.

3. Calculating Height Difference

  • **Height Difference (HD) = BS – FS**

4. Moving Forward

  • To continue the survey, the staff at the foresight position becomes the new backsight. The level may need to be moved to a new position to maintain visibility.

5. Closing the Level Loop

  • For accuracy, the survey should end back at the original benchmark or another known point. This closes the level loop. Any discrepancy is the error in the survey.

Types of Leveling

There are several methods of leveling, each suited to different purposes:

1. Simple Leveling

Used to determine the elevation of a point relative to a single benchmark. It involves one backsight and one foresight.

2. Differential Leveling

Employed when the elevation difference between points over a longer distance needs to be determined. It involves multiple setups of the level and a series of backsights and foresights.

3. Profile Leveling

Used to determine the elevations of points along a line, such as the centerline of a road or pipeline. It creates a profile of the ground surface.

4. Cross-Section Leveling

Used to determine the elevations of points perpendicular to a baseline, often used in conjunction with profile leveling to create topographic maps.

5. Trigonometric Leveling

Utilizes the principles of trigonometry to determine elevation differences over longer distances or when obstacles prevent direct leveling. It involves measuring vertical angles and distances.

Applications of Level Surveying

Level surveying is crucial in various fields and applications:

1. Construction

  • Setting out foundations, floors, and other structural elements at the correct elevations.
  • Grading and leveling sites for buildings, roads, and other infrastructure.
  • Installing drainage systems with the proper slopes.

2. Engineering

  • Designing roads, railways, canals, and pipelines with appropriate gradients.
  • Constructing bridges and tunnels at accurate elevations.
  • Monitoring the movement of structures over time.

3. Mapping

  • Creating topographic maps that show the elevation contours of the earth’s surface.
  • Generating digital elevation models (DEMs) for various applications, including geographic information systems (GIS).

4. Monitoring

  • Observing changes in the earth’s crust due to subsidence, uplift, or tectonic activity.
  • Monitoring the movement of dams, embankments, and other structures.

Errors in Level Surveying and Precautions

Like all surveying techniques, level surveying is subject to errors. Some common errors and precautions to minimize them include:


  • **Instrumental Errors:** Errors due to imperfections in the level instrument itself, such as collimation error (line of sight not being perfectly horizontal). Regular calibration is essential.
  • **Natural Errors:** Environmental factors like temperature variations, wind, and atmospheric refraction can affect readings.
  • **Human Errors:** Mistakes in reading the staff, recording data, or setting up the instrument. Careful procedures and double-checking can reduce these errors.


  • **Calibrate Instruments:** Regularly calibrate levels and other equipment to ensure accuracy.
  • **Level the Instrument Carefully:** Take time to properly level the instrument using the leveling screws and bubble vials.
  • **Keep Staff Vertical:** Use a staff bubble or plumb bob to ensure the staff is held perfectly vertical.
  • **Short Sight Distances:** Keep sight distances short to minimize errors due to atmospheric refraction and curvature of the Earth.
  • **Balance Backsight and Foresight Distances:** Where possible, keep the distances from the level to the backsight and foresight staff positions roughly equal to compensate for any remaining instrumental errors.
  • **Check for Movement:** Ensure the level and staff are stable during readings. Avoid setups on unstable surfaces.

Advancements in Level Surveying

With technological progress, level surveying has seen advancements that increase efficiency and accuracy:

1. Digital Levels

Digital levels use electronic image processing to automatically read bar-coded staffs, eliminating the need for manual readings and reducing human error.

2. Laser Levels

Laser levels project a visible laser beam that serves as a horizontal or vertical reference, simplifying tasks such as grading and alignment.

3. GNSS/GPS Leveling

Global Navigation Satellite Systems (GNSS), such as GPS, can be used for precise leveling. However, specialized techniques and equipment are required for high-accuracy geodetic work.


Level surveying is a fundamental surveying technique essential for various applications, from construction and engineering to mapping and monitoring. Understanding the principles, instruments, procedures, and potential errors associated with level surveying is crucial for obtaining accurate elevation data. As technology continues to advance, we can expect further improvements in the accuracy, efficiency, and automation of level surveying techniques.

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