Q19Basic Civil Engineering

A prismatic compass is one of the most vital, portable surveying instruments extensively used in traverse surveying to measure the magnetic bearing of survey lines with respect to the magnetic meridian. It is uniquely designed to allow the surveyor to sight the target object and read the measurement simultaneously. The prismatic compass operates entirely on the Whole Circle Bearing (WCB) system, meaning its graduated ring reads continuously from 0° to 360° in a clockwise direction.

Construction and Main Components: The prismatic compass consists of several intricate mechanical components housed within a compact frame to ensure frictionless movement and accurate readings.

• 1. Cylindrical Metal Box: The entire mechanism is housed inside a cylindrical box, typically made of non-magnetic materials like brass or aluminum, having a diameter of 8 to 12 cm. This box protects the delicate internal components from dust and wind.
• 2. Pivot and Agate Jewel: Exactly at the center of the box is a sharp, hard steel pivot. Resting perfectly on top of this pivot is an agate jewel bearing. This setup provides an ultra-low friction support point, allowing the magnetic needle to swing freely and align itself with the earth's magnetic field.
• 3. Magnetic Needle & Graduated Ring: Unlike a surveyor's compass, the prismatic compass uses a broad, flat magnetic needle. An aluminum graduated ring is rigidly attached directly to the top of this magnetic needle. Because they are attached, the ring and the needle rotate together. The ring remains stationary relative to the magnetic meridian (North), while the box rotates around it when taking a sight.
• 4. Graduations: The ring is graduated from 0° to 360°. Crucially, the zero (0°) is marked at the South end of the needle, 90° at West, 180° at North, and 270° at East. Furthermore, the numbers on the ring are printed inverted (upside down). This ingenious design ensures that when the numbers are viewed through the reflecting prism, they appear upright to the observer's eye.
• 5. Sight Vanes (Object Vane and Eye Vane): Diametrically opposite to each other on the box perimeter are the object vane and the eye vane. The Object Vane is a metal frame carrying a fine vertical horsehair or wire used to bisect the ranging rod at the target station. The Eye Vane contains the reflecting prism.
• 6. The Prism: The defining feature of the instrument. It is a 45-degree reflecting prism located at the eye vane. It reflects the inverted graduations of the ring upward to the observer's eye, allowing them to look horizontally at the object through a slit in the prism casing while simultaneously reading the bearing below.
• 7. Lifting Pin and Lever: A lever connected to a lifting pin is provided near the object vane. When the object vane is folded down flat for storage, the pin automatically lifts the magnetic needle off the steel pivot. This prevents unnecessary wear and tear on the delicate jewel bearing when not in use.
• 8. Brake Pin (Spring Brake): A small button located on the outside of the box. Pressing it applies a dampening spring against the edge of the aluminum ring to quickly stop its oscillation and bring the needle to rest.

Working Principle: When the compass is leveled, the needle aligns North-South. The observer rotates the compass box until the object vane's horsehair perfectly bisects the target. The observer then looks through the prism slit. The reading on the graduated ring that directly intersects the hair line is the WCB of the line.