**Cross-staff is used for setting out right angles**

The cross-staff is a simple instrument that has been used for centuries in surveying and engineering. It is primarily used for setting out right angles, which is an essential step in many surveying and construction tasks.

**What is a cross-staff?**

The cross-staff consists of a long staff or rod with a sliding arm attached to it at a right angle. The sliding arm can be moved along the staff to measure or set out right angles accurately.

**How does it work?**

To set out a right angle using a cross-staff, the following steps are typically followed:

1. Choose a starting point and mark it on the ground or surface where the right angle needs to be set out.
2. Hold the cross-staff at the starting point with the sliding arm pointing towards the desired direction of the right angle.
3. Extend the sliding arm until it aligns with a reference point in the distance.
4. Walk along the sliding arm while keeping the staff vertical and aligned with the reference point.
5. Stop when the sliding arm intersects with the reference point, forming a right angle.
6. Mark the point where the sliding arm intersects with the reference point.
7. Repeat the process from the starting point in a different direction to complete the right angle.

**Why is setting out right angles important?**

Setting out right angles accurately is crucial in various surveying and construction tasks, such as:

1. Building foundations: Right angles are necessary to ensure the corners of a building or structure are square and aligned correctly.
2. Road construction: Right angles are used to set out road intersections, curves, and other geometric features.
3. Land surveying: Right angles are essential for dividing land into plots, measuring boundaries, and creating accurate maps.
4. Pipe and utility installation: Right angles are used to lay pipes, cables, and other utilities in a grid pattern or specific alignment.

**Conclusion**

The cross-staff is a valuable tool for surveyors and engineers as it allows for the accurate setting out of right angles. This instrument is essential in various construction and surveying tasks, ensuring that structures, roads, and utilities are correctly aligned and positioned.