Which of the following instruments can be used for oblique offsets, at angles other than 45 degrees?
Correct Answer:
C. Adjustable cross-staff
📝 Detailed Explanation: Instruments for Setting Out Offsets
In chain surveying, offsets are lateral measurements taken from a main survey line (chain line) to locate details like corners of buildings, trees, or boundary markers. They can be perpendicular (90°) or oblique (any other angle). The choice of instrument depends on the type of offset required.
Understanding the Instruments
Let's analyze the function of each instrument listed:
- (a) Cross-staff: The simple or open cross-staff is one of the most basic instruments. It consists of two pairs of vertical slits at right angles to each other. Its sole purpose is to set out perpendicular offsets (90°).
- (b) Optical Square: This is a more precise instrument that uses the principle of reflection from mirrors or prisms to set out perpendicular offsets (90°). It is more accurate than a simple cross-staff but serves the same primary function.
- (d) French Cross-staff: This is a more advanced version of the simple cross-staff, typically having an octagonal shape. It has slits that allow a surveyor to set out angles of both 90° and 45°. While it can set out an oblique angle, it is limited to 45°.
- (c) Adjustable Cross-staff: This is the most versatile of the group. It consists of two cylinders, one on top of the other. The upper cylinder can be rotated relative to the lower one, and it is equipped with a graduated circle and a vernier scale. This design allows the surveyor to set and measure any desired angle with reasonable accuracy, making it the ideal tool for laying out oblique offsets at any angle, not just 45°.
Instrument Comparison
This table summarizes the capabilities of each instrument.
| Instrument | Primary Use / Angles it can set | Suitable for General Oblique Offsets? |
|---|---|---|
| Cross-staff | 90° only | No |
| Optical Square | 90° only (High Precision) | No |
| French Cross-staff | 90° and 45° | Only for 45° |
| Adjustable Cross-staff | Any angle (e.g., 30°, 60°, 75°, etc.) | Yes |
Therefore, for setting out an oblique offset at any angle other than the fixed 45°, the Adjustable cross-staff is the correct and necessary instrument.