Auto level staff reading is one of the most essential practices in land surveying and construction for determining the difference in elevation between points on the ground. The auto level (also known as a Dumpy Level or Automatic Level) is an optical instrument used along with a leveling staff to measure relative heights and prepare contour maps, road profiles, and leveling plans. Understanding how to take accurate staff readings is critical for civil engineers, surveyors, and site supervisors to ensure precise leveling and gradient calculation in construction projects.
What is Auto Level Staff Reading?
Auto level staff reading refers to the process of observing and recording readings on a graduated staff placed at various points on the ground using an auto level instrument. The main objective is to determine the difference in height (or elevation) between two or more points. The instrument maintains a horizontal line of sight, and the reading observed on the staff helps calculate Reduced Levels (RLs) of the points. The auto level gives accurate and stable readings because it automatically levels itself using an internal compensator.
Components Used in Auto Leveling
- Auto Level Instrument: Optical device with a telescope and internal leveling mechanism.
- Tripod Stand: Supports the auto level and keeps it stable.
- Leveling Staff: A graduated rod (usually in meters) used to take readings.
- Levelling Bubble: Ensures the instrument is horizontally aligned.
- Plumb Bob or Markers: Used to position the instrument correctly on the station point.
Types of Staff Readings in Auto Leveling
When using an auto level, three types of staff readings are commonly taken:
1. Backsight (BS)
Backsight is the first reading taken on a known point of elevation (called a Benchmark or BM). It establishes the Height of Instrument (HI). This reading is always taken on a point whose RL is already known.
2. Intermediate Sight (IS)
Intermediate Sight is a reading taken between the backsight and foresight on points whose RLs are to be determined. These readings help in mapping surface elevations.
3. Foresight (FS)
Foresight is the last reading taken on a point before shifting the instrument to a new position. It helps in transferring the level from one setup to another.
Procedure for Taking Auto Level Staff Readings
- Set up the Instrument: Fix the auto level on the tripod and level it using foot screws until the bubble is centered.
- Take the Backsight (BS): Place the staff vertically on the benchmark (BM). Note the reading visible through the telescope crosshairs.
- Calculate the Height of Instrument (HI): Add the BS reading to the known RL of the benchmark.
- Take Intermediate Sights (IS): Place the staff at required points on the ground and record the readings.
- Take Foresight (FS): Finally, take the FS on the last point before moving the instrument.
- Shift the Instrument: Move to a new station, take a new BS on a known point, and repeat the process.
Formula for Calculations in Auto Leveling
Two main methods are used to calculate Reduced Levels (RLs) from staff readings:
1. Height of Instrument (HI) Method
Formula:
- Height of Instrument (HI) = RL of Benchmark + BS
- RL of any point = HI – Staff Reading (IS or FS)
Example:
| Station | Staff Reading (m) | Type | RL (m) | HI (m) |
|———-|——————–|——|——–|——–|
| A (BM) | 1.250 | BS | 100.000 | 101.250 |
| B | 1.550 | IS | 101.250 – 1.550 = 99.700 | 101.250 |
| C | 2.000 | FS | 101.250 – 2.000 = 99.250 | — |
Explanation: RL of Benchmark = 100.000 m, so HI = 101.250 m. Using HI, RLs of other points are calculated.
2. Rise and Fall Method
Formula:
- Rise = Previous Reading – Next Reading (if positive)
- Fall = Next Reading – Previous Reading (if positive)
- RL = Previous RL + Rise – Fall
Example:
| Station | Staff Reading (m) | Rise (m) | Fall (m) | RL (m) |
|———-|——————|———–|———-|——–|
| BM | 1.250 | — | — | 100.000 |
| 1 | 1.550 | — | 0.300 | 99.700 |
| 2 | 2.000 | — | 0.450 | 99.250 |
This method is used to check accuracy as the total rise minus total fall should equal the difference between the first and last RLs.
Reading Line of Sight in Auto Level
The line of sight in an auto level instrument must be perfectly horizontal. The telescope contains three crosshairs—one central and two horizontal stadia hairs. The staff reading is always taken at the center hairline, while the stadia lines are used for tacheometric distance measurement if required.
Precautions During Auto Level Staff Reading
- Always ensure the staff is held vertically; use a circular bubble on the staff if available.
- Focus the telescope properly to avoid parallax error.
- Keep the distance between instrument and staff moderate (not more than 100 meters).
- Avoid taking readings during strong wind or vibration, which can disturb alignment.
- Take multiple readings if the line of sight is long to ensure accuracy.
- Always close the leveling loop by returning to a known benchmark for error checking.
Types of Leveling Staff Used
1. Telescopic Staff
Made of aluminum or fiberglass, consisting of multiple sliding sections for easy transport. Graduations are usually in centimeters and meters.
2. Folding Staff
Made of wooden or metallic sections joined by hinges. It’s less accurate than telescopic staff but commonly used in short-distance leveling.
3. Rigid Staff
Used for precise work such as in laboratories or benchmark leveling where high accuracy is required.
Example of Auto Level Reading Table
| Station | B.S (m) | I.S (m) | F.S (m) | HI (m) | RL (m) | Remark |
|---|---|---|---|---|---|---|
| BM | 1.245 | — | — | 101.245 | 100.000 | Benchmark |
| 1 | — | 1.785 | — | 101.245 | 99.460 | Ground Point |
| 2 | — | 2.105 | — | 101.245 | 99.140 | Ground Point |
| 3 | — | — | 2.350 | — | 98.895 | Last Point |
Applications of Auto Level Staff Reading
- Road and highway construction: To ensure proper slope and drainage.
- Foundation work: To check the level differences before excavation or concreting.
- Canal and irrigation works: To determine gradients and water flow levels.
- Topographic surveys: To prepare contour maps.
- Building projects: For plinth, floor, and roof level checks.
Advantages of Auto Level Over Dumpy Level
- Automatic leveling through internal compensator saves time.
- High accuracy due to stable horizontal line of sight.
- Ease of use, even for long leveling lines.
- Faster setup and reading process.
- Durable and reliable for both field and construction site conditions.
Conclusion
Auto level staff reading is a fundamental process in surveying that ensures accurate elevation measurements. By using methods like Height of Instrument (HI) and Rise and Fall, engineers can calculate Reduced Levels (RLs) precisely for any construction or land development project. Following IS 7322:1974 and standard field procedures guarantees accuracy, reduces human error, and ensures that every structure built stands on a properly leveled foundation.
