The primary difference between a target staff and a self-reading staff lies in who takes the reading and the specific mechanism employed for measuring elevations in surveying.
A self-reading staff is directly observed and read by the instrument operator (levelman) through the telescope of the leveling instrument. In contrast, a target staff features a movable target that is adjusted by the staff man, who then takes the reading directly from the staff at the target's position, often with enhanced precision.
Understanding Self-Reading Staffs
A self-reading staff, also known as a direct-reading staff or leveling rod, is the more common type used in routine leveling operations.
- Mechanism: It consists of a graduated rod, typically marked in meters, decimeters, and centimeters. The instrument operator, looking through the leveling instrument's telescope, directly reads the graduation on the staff that aligns with the horizontal crosshair.
- Usage: They are widely used for general leveling, profiling, and contouring where quick and efficient readings are required.
- Precision: The precision of readings depends on the clarity of the staff markings, the magnification of the telescope, and the observer's ability to interpolate readings between the smallest graduations.
Understanding Target Staffs
A target staff is designed for situations demanding higher precision, as it reduces the potential for reading errors by the instrument operator over long distances or in conditions with poor visibility.
- Mechanism: Unlike a self-reading staff, a target staff is equipped with a movable target. This target is a distinct part of the staff that the staff man slides up or down the staff's main body.
- Operation: The instrument operator guides the staff man to adjust this movable target until its center precisely aligns with the horizontal crosshair in the leveling instrument's telescope. Once aligned, the staff man then reads the elevation directly from the position of the target on the staff.
- Design Features: The movable target is often circular or oval, painted in contrasting colors (such as red and white in alternate quadrants) to enhance visibility. Crucially, it is typically fitted with a vernier at its center. This vernier allows the staff man to take readings to a much finer precision than the main graduations alone, often to millimeters or even half-millimeters.
- Usage: Target staffs are particularly useful for precise leveling work, establishing benchmarks, or when operating over very long sight distances where direct reading by the instrument man would be difficult or less accurate.
Key Differences at a Glance
The table below summarizes the core distinctions between these two types of leveling staffs:
| Feature | Self-Reading Staff | Target Staff |
|---|---|---|
| Reading Performed By | Instrument operator (levelman) through the telescope | Staff man, by adjusting and reading a movable target on the staff |
| Movable Target | Absent | Present (adjustable along the staff) |
| Vernier | Generally absent | Often fitted with a vernier for enhanced precision |
| Design | Simple graduated rod | Graduated rod with an attached, adjustable circular or oval target, often colored red and white in alternate quadrants |
| Precision | Moderate, dependent on observer's skill and instrument optics | High, due to direct reading by staff man and the use of a vernier |
| Common Application | General leveling, construction site leveling | High-precision leveling, establishing control points, long-distance leveling |
| Communication Needed | Minimal (instrument man reads directly) | Essential (instrument man guides staff man for target adjustment) |
For more information on leveling techniques and surveying equipment, you can explore resources on land surveying and leveling.
