Levelling Staff
A levelling staff is a crucial instrument in surveying, consisting of a straight, rectangular rod typically made of wood or modern composite materials. It is precisely graduated into metric or imperial units, with meters or feet as the primary divisions, further subdivided into smaller increments for accurate measurements.
The staff is designed to be held vertically on the point being measured, with its base representing the zero reading. When viewed through a levelling instrument, the line of sight intersects the staff at a specific point. This intersection provides a reading that represents the height of the instrument’s line of collimation above the ground point where the staff is positioned.
Levelling staff come in various designs to suit different surveying needs and conditions. Their accuracy and ease of use make them indispensable tools in a wide range of applications, from construction and civil engineering to topographic mapping and geological studies.
The functionality of a levelling staff relies on its clear, contrasting graduations that can be easily read from a distance through a telescopic sight. This allows surveyors to determine elevation differences between points with high precision, forming the basis for creating accurate topographic maps and ensuring proper grading in construction projects. Levelling staves come in various designs to suit different surveying needs and conditions. They can be broadly categorized into two main classes:
I).Self-reading staff
II).Target staff.
Types of Levelling Staffs
i.Self-reading Staff
A self-reading staff is one on which the observer can directly read measurements through the telescope without any intermediate steps. This type of staff allows for straightforward and efficient observation, eliminating the need for additional tools or personnel to record readings. Self-reading staffs are commonly used in levelling and surveying tasks due to their simplicity and ease of use. They come in three primary types, as outlined below
1.Solid staff
Solid staffs are typically 3 meters in length and are constructed as a single, continuous piece. Due to the absence of hinges or sockets, they offer greater accuracy in reading measurements, making them ideal for precise levelling tasks. However, their rigid structure also makes them less convenient to transport and handle in the field. As a result, solid staffs are predominantly used for specialized levelling work where precision is a priority, despite their practical limitations in terms of portability.
2.Folding or hinged staff
A folding or hinged staff is typically constructed from well-seasoned timber, ensuring durability and precision. It measures 4 meters in total length, composed of two 2-meter sections connected by a hinge. The staff has a width of 75 mm and a thickness of 18 mm, providing sturdiness while remaining lightweight for practical use. To protect against wear and tear, the foot of the staff is fitted with a brass cap. In some cases, a plummet is also included to help the staffman check and maintain the verticality of the staff during use.
Each meter of the staff is subdivided into decimeters, with each decimeter further divided into 20 segments, each 5 mm wide. Decimeter markings from 1 to 9 are displayed in black, while meter numerals are shown in red. Traditionally, the graduations are marked in an inverted style so that they appear upright when viewed through a telescope. However, modern levelling staffs feature upright graduations for easier reading.
When not in use, the staff can be folded for convenience, offering several practical configurations:
- One 2-meter section can fold onto the other, making it compact and easier to carry.
- The two sections can be detached, allowing the use of a single 2-meter piece in flat or plain areas.
- When the sections are locked together, the staff functions as a single, rigid rod for accurate levelling tasks.
3. Telescopic or Sopwith-Type Staff
The telescopic, or Sopwith-type, staff consists of three interlocking sections. The top section is a solid piece measuring 1.25 meters, while the central and lower sections, measuring 1.25 meters and 1.5 meters respectively, are hollow. These sections fit into each other in a telescopic manner, allowing the top section to slide into the central portion. When fully extended, the staff reaches a total length of 4 meters. Brass spring catches secure the upper two sections in place when extended.
The smallest division on this levelling staff is 5 mm. Meter numerals, located on the left side, are marked in red, while decimeter numerals (1 to 9) are displayed on the right side and marked in black. The numeral 10 is replaced by the letter “M” to indicate the end of each meter length. Graduations are marked upright, although they appear inverted when viewed through a telescope.
When using the telescopic staff, it’s crucial to ensure that all three sections are fully extended to utilize the complete 4-meter length accurately.
While 4 meters is a common size, telescopic staffs are available in longer versions, typically ranging from 5 to 7 meters or more, depending on the specific requirements of the surveying task. The length is determined by the number of sliding sections and the extension mechanism. Larger telescopic staffs are often used in projects requiring higher precision or when working in areas with significant elevation changes.
ii. Target Staff
The target staff consists of two sections: an upper rod measuring 6 feet and a lower rod measuring 7 feet. The upper rod slides into the lower one, making the staff adjustable in length. A movable target is attached to the staff, which can be adjusted up or down along the rod.
The staff is graduated in feet, with subdivisions in tenths and hundredths for precise readings. To take a reading, the level man instructs the staff man to raise or lower the target until it is perfectly bisected by the level’s line of sight. Once the target is correctly aligned, the staff man clamps it in place and records the reading. This process allows for accurate measurement in levelling tasks, with the target aiding in clear, precise readings.
