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In levelling, ___________ is also called minus sight.
Inverted intermediate sight
fore sight
back sight
intermediate sight
fore sight
In levelling, the Fore Sight (FS) is termed a 'minus sight' because its reading is subtracted from the Height of Instrument (HI) to determine the Reduced Level (RL) of the forward station. It is the final reading taken on a leveling staff held at a point whose elevation is to be determined.
In levelling, the Fore Sight (FS) is termed a 'minus sight' because its reading is subtracted from the Height of Instrument (HI) to determine the Reduced Level (RL) of the forward station. It is the final reading taken on a leveling staff held at a point whose elevation is to be determined.
RL=HIтИТFS тАФ Elevation calculation at a forward station
HI=BM+BS тАФ Height of instrument calculation
The principle relies on the relation: RL=HIтИТSight┬аReading. For a Fore Sight, the staff reading is subtracted from the established Height of Instrument to calculate the elevation of the new point. It marks the termination of the current instrument station set-up.
Back Sight (BS) is known as the 'plus sight' as it is added to the known RL to find the HI.
Intermediate Sights (IS) are also 'minus sights' as they are subtracted from the HI to find the RL of intermediate points.
Fore Sight (FS) is essentially the final reading taken before the instrument is shifted to a new location.
In any setup, the last reading is always a Fore Sight.
Allows for continuous measurement across varying topography
Provides a systematic check for leveling errors
Cumulative errors can occur if staff readings are taken incorrectly
Requires stable turning points to maintain survey accuracy
Profile leveling for roads and railways
Establishing benchmarks for construction projects
| Feature | Back Sight | Fore Sight |
|---|---|---|
Sight Type | Added to RL | Subtracted from HI |
The term 'minus sight' specifically refers to the mathematical operation applied to the HI in the Rise and Fall or Collimation method.
Option A is incorrect as inverted sights are used for ceilings/underside of bridges, Option C is a plus sight, and Option D is an intermediate reading.
B is correct тАФ Fore sight is termed a minus sight because it is subtracted from the height of instrument to obtain the reduced level of the point.
Remember the mnemonic: BS is a Plus Sight (B+), and FS/IS are Minus Sights. This is essential for verifying your arithmetic in the leveling table during exams.
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