A cross-staff is a simple surveying instrument that is primarily used to set out lines perpendicular to a main survey line. This means it helps in establishing right angles on the field.

Understanding the Use of a Cross-Staff in Surveying

The cross-staff is a basic tool used in chain surveying. Its main purpose is to establish perpendicular offsets from a survey line to locate details or boundaries. This is crucial for creating accurate maps or plans of an area.

How a Cross-Staff Works

There are different types of cross-staffs, but the basic principle involves sighting along the main survey line and then sighting at a 90-degree angle to that line. The instrument typically has sights or slits arranged at right angles to each other.

• Open Cross-Staff: This is the simplest type, consisting of four metal arms or sights fixed at right angles on top of a staff or pole. The surveyor looks through one pair of sights aligned with the survey line and uses the other pair to sight along the line perpendicular to it.
• French Cross-Staff: This type is more versatile as it allows setting out not only right angles but also angles of 45 degrees and 135 degrees. It usually has eight octagonal faces with sights.
• Adjustable Cross-Staff: This allows setting out any angle, not just fixed ones, by using a circular plate graduated in degrees.

Regardless of the type, the most common and fundamental use of any cross-staff is to accurately establish a 90-degree angle from an existing line.

Analyzing the Options

Let's look at why the other options are not the primary use of a cross-staff:

• Measuring Distance: Distances in chain surveying are typically measured using a chain or tape, not a cross-staff. The cross-staff only helps in establishing direction, specifically a right angle.
• Measuring Acute Angle: While a French cross-staff can measure/set out 45-degree angles, the primary and fundamental use of the basic cross-staff is the right angle. More advanced instruments like a theodolite are used for measuring arbitrary acute or obtuse angles accurately.
• Levelling: Levelling is the process of determining the relative heights or elevations of points on the ground. This is done using instruments like a dumpy level, auto level, or digital level, along with a levelling staff. A cross-staff has no function related to determining vertical heights.

Therefore, the primary and intended use of a cross-staff is for setting out lines perpendicular to a survey line, which means setting out right angles (90 degrees).

Conclusion

Based on the function and design of the instrument, the cross-staff is used for establishing a right angle relative to a survey line. This is a fundamental operation in basic surveying techniques like chain surveying.

Revision Table: Cross-Staff Key Facts

Additional Information: Basic Surveying Tools

Surveying involves various instruments to measure distances, angles, and elevations. Understanding the specific function of each tool is essential:

• Chains and Tapes: Used for linear measurements (distances). Chains are older and less accurate than tapes.
• Prismatic Compass: Used for measuring magnetic bearings (angles relative to Magnetic North).
• Plane Table: Used for simultaneous field surveying and plotting. Allows plotting details directly on paper in the field.
• Optical Square/Prism Square: Like a cross-staff, used for setting out perpendiculars, often considered more accurate than a simple cross-staff.
• Levelling Instruments: Used to determine differences in elevation between points.
• Theodolite: A precise instrument for measuring horizontal and vertical angles.
• Total Station: An electronic instrument that combines a theodolite with an electronic distance measurement (EDM) device, used for measuring angles, distances, and coordinates.

Each tool plays a specific role in different types of surveying work. The cross-staff remains a simple yet effective tool for establishing right angles, particularly useful in basic chain surveying where detailed angular measurements are not the primary focus.