Prism Square Function in Chain Surveying

A prism square is an optical instrument commonly used in chain surveying. Its primary purpose is to set out perpendicular lines, also known as offsets, from a main survey line to features on the ground. This capability is crucial for accurately plotting details of an area being surveyed.

Understanding the Prism Square

• A prism square typically consists of two right-angled prisms, or a combination of mirrors, arranged within a small, circular or square metal casing.
• It operates on the principle of optical squares, utilizing reflection to establish a precise 90-degree angle.
• When a surveyor looks through the instrument, they can simultaneously see an object on the main survey line (through direct vision) and an object that is at a right angle to that line (through reflection from the prisms).

Chain Surveying and Offsets

Chain surveying is a method of surveying where only linear measurements are taken. It's suitable for small, relatively flat areas. To accurately map the features (like buildings, trees, fences, etc.) in the field, it's essential to measure their perpendicular distances (offsets) from the main survey lines. This is where the prism square becomes indispensable.

Analyzing the Options for Prism Square Function

Let's evaluate each given option to determine the correct function of a prism square in a chain survey.

• To set a line at right angles to a survey line: This is the exact and most important function of a prism square. By using its optical properties, the instrument allows the surveyor to accurately establish a line that is precisely 90 degrees to the existing survey line. This is fundamental for taking offsets to details on either side of the chain line, which are then used to plot the survey.
• To mark survey stations in between survey lines: Survey stations are typically marked using pegs, ranging rods, or arrows. A prism square is not used for marking points; it's an instrument for establishing angular relationships.
• To check the alignment of a survey line: The alignment of a survey line, known as ranging, is usually checked by direct observation, using ranging rods, or through instruments like a theodolite for longer lines. A prism square is designed for setting perpendiculars, not for checking the straightness or alignment of a line itself.
• To get an enlarged view of the station marks: Instruments like telescopes or magnifying glasses provide enlarged views. A prism square works by reflecting light to establish right angles; it does not magnify objects.

Conclusion on Prism Square Utility

The primary utility of a prism square in chain surveying is to facilitate the setting out of perpendicular offsets. This ensures that the details of the ground features can be accurately measured and then plotted relative to the main survey lines, forming a precise map.