Jawaharlal Nehru Technological University Hyderabad B.Tech II Year MINE SURVEYING Previous Question Paper (March - 2022 - R16)

The fundamental principle of chain surveying is triangulation. In this method, the area to be surveyed is divided into a network of triangles, as a triangle is the only simple plane figure that can be completely defined by the lengths of its three sides. By measuring the sides of these triangles with a chain or tape, the relative positions of the stations can be plotted on a plan without the need for angular measurements.

The process involves establishing a main survey line (base line) through the center of the area, and then forming a framework of triangles based on this line. The accuracy of the survey depends on the well-conditioned nature of the triangles, where angles are ideally between 30° and 120° to ensure clear intersections on the plan.

Sources of errors in chain surveying are categorized into three types:

• Compensating Errors: These are errors that tend to occur in both directions (positive and negative) and thus partially cancel each other out. Examples include slight inaccuracies in reading the tape, inconsistent tension, or minor errors in alignment.
• Cumulative Errors: These errors always occur in the same direction and tend to accumulate, significantly affecting the accuracy of the final measurement. These are often systematic and can be corrected if the source is identified. Examples include:
  • Incorrect length of chain or tape: If the chain is too long or too short, the error is proportional to the length measured.
  • Temperature variation: Expansion or contraction of the tape due to temperature changes compared to the standard temperature.
  • Slope error: Failure to measure the horizontal distance on sloping ground.
  • Sag: If the tape is not supported, it hangs in a curve, making the measured distance greater than the actual horizontal distance.
  • Pull: Applying a pull different from the standard tension for which the tape was calibrated.
• Mistakes: These are blunders committed by the surveyor due to carelessness or lack of attention. Examples include miscounting the number of chain lengths, reading the tape incorrectly (e.g., reading 6 instead of 9), or recording the wrong data in the field book. These can be avoided by verifying measurements and maintaining rigorous field procedures.

A compass is a fundamental surveying instrument used to determine the direction of a line with respect to a magnetic meridian. Compasses are primarily classified based on their construction, mounting, and method of observation.

• Surveyor's Compass: This instrument is designed for use with a tripod. It consists of a magnetic needle balanced on a sharp pivot at the center of a graduated circular box. The graduation ring is attached to the compass box and rotates with it. The readings are taken in the Quadrantal Bearing (QB) system, ranging from 0° to 90° in each quadrant. In this type, the needle remains stationary while the box rotates, and the observer reads the bearing directly by looking through the sights. It is generally less precise than a prismatic compass.
• Prismatic Compass: This is the most widely used portable compass in surveying. It features a magnetic needle attached to a circular card that rotates independently of the compass box. The graduations are engraved on the card in the Whole Circle Bearing (WCB) system, starting from 0° at the South end to 360° clockwise. A prism is provided at the eye vane, which allows the observer to read the graduation on the card simultaneously while sighting the object. This design enables the surveyor to take bearings without removing their eye from the object, significantly increasing speed and accuracy.
• Trough Compass: This is a specialized, elongated compass box containing a long, thin magnetic needle. It is primarily used with a plane table to mark the magnetic meridian on the drawing sheet. It does not have sights for taking bearings of lines but is essential for orienting the plane table.
• Hand Compass (Pocket Compass): These are compact, lightweight versions of the surveyor's compass, often used for rough reconnaissance or preliminary surveys. They may be held in the hand or mounted on a light staff. While portable, they lack the precision of tripod-mounted instruments due to the instability of the observer's hand.

Key differences between the primary types (Surveyor's vs. Prismatic) include: