A prism square is used to:
🎯 Core Concept: Setting Out Right Angles
In chain surveying, where the primary measurements are linear, establishing accurate right angles (90°) is crucial for creating offsets from the main survey line. Several hand-held instruments are used for this purpose, with the prism square being a modern and highly efficient option.
🔬 Principle of Operation
The prism square operates on the optical principle of double reflection. It contains a solid prism of glass. The key feature is that the reflecting surfaces are precisely cut at a fixed angle of 45° to each other. According to the law of reflection, this fixed internal angle ensures that a ray of light is deviated by exactly 90°. This makes the instrument inherently stable and accurate without needing any adjustment.
📊 Comparison of Right-Angle Instruments
Here’s how the prism square compares to other similar instruments:
| Instrument | Principle | Key Feature | Advantage | Disadvantage |
|---|---|---|---|---|
| Prism Square | Double Reflection (Prism) | Fixed internal prism (45° angle). | Highly accurate, no adjustment needed, robust. | More expensive than a cross-staff. |
| Optical Square | Double Reflection (Mirrors) | Two mirrors fixed at 45° to each other. | More convenient and accurate than a cross-staff. | Mirrors can get knocked out of alignment, requiring adjustment. |
| Cross-Staff | Line of Sight | Two pairs of vertical slits providing lines of sight at 90°. | Simple, inexpensive, and easy to use. | Less accurate, prone to sighting errors. |