For taking offsets with an optical square on the right hand side of the chain line it is held:
🤝 Understanding the Handling of an Optical Square
The way an optical square is held is based on ergonomics and the line of sight required for surveying. The goal is to hold the instrument steady while keeping the dominant hand free for signaling and marking.
When taking an offset to the right of the chain line, the surveyor needs to look forward along the chain line and simultaneously look to the right to see the object being ranged. Holding the instrument in the left hand achieves this efficiently.
🔬 Detailed Analysis of the Options
B. by left hand upright
This is the correct procedure. Holding the optical square upright in the left hand keeps the right hand (which is typically the dominant hand) free. This allows the surveyor to give signals to the chainman, hold a plumb bob, or make marks on the ground without putting down the instrument. It provides a stable and clear line of sight for both the forward station and the offset point.
Incorrect Methods:
- C. by right hand upright: This is inefficient. It forces the surveyor to use their non-dominant (left) hand for signaling and marking, which is awkward for most people. It can also cause the surveyor's right arm to obstruct the line of sight.
- A. & D. upside down: Holding the instrument upside down would invert the internal mirrors or prisms relative to the line of sight. This would prevent the direct and reflected images from aligning correctly, making it impossible to set out a 90° angle.
📝 Practical Steps for a Right-Hand Offset
Here is the standard procedure a surveyor follows:
- The surveyor stands on the chain line at the desired point for the offset.
- They hold the optical square upright in their left hand directly over the point on the chain line.
- Looking through the eyepiece, they align the instrument so that the forward ranging rod is visible through the unsilvered portion of the horizon glass.
- Using their free right hand, they signal to the assistant (chainman) to move a ranging rod at the object's location.
- The surveyor observes the reflected image of the assistant's rod in the silvered portion of the horizon glass. The assistant is instructed to move left or right until the reflected image is in perfect alignment with the directly viewed forward rod.
- When the two images are coincident, the assistant's rod is exactly at a 90° angle to the chain line.
For an offset on the left-hand side, the procedure is reversed: the instrument is held in the right hand.