What is meant by back sight (BS), fore sight (FS), and intermediate sight (IS)?

Short Answer:

In leveling, Back Sight (BS), Fore Sight (FS), and Intermediate Sight (IS) are terms used to take height measurements with a leveling instrument.

• Back Sight (BS) is the first reading taken on a known benchmark or reference point.
• Fore Sight (FS) is the last reading taken at the final survey point.
• Intermediate Sight (IS) is any reading taken between the back sight and fore sight.
  These readings help in calculating height differences and determining elevations in surveying.

Detailed Explanation:

In surveying, leveling is used to measure height differences between points. To do this, readings are taken using a leveling instrument and a leveling staff. The three main types of readings in this process are Back Sight (BS), Fore Sight (FS), and Intermediate Sight (IS).

1. Back Sight (BS):

• This is the first reading taken after setting up the leveling instrument.
• It is taken on a benchmark (BM) or any known elevation point.
• It helps in determining the Height of Instrument (HI), which is the reference for further measurements.
• Example: If a surveyor places the staff on a benchmark with a known RL (Reduced Level), the reading observed through the instrument is the BS.

2. Fore Sight (FS):

• This is the last reading taken before moving the instrument to another position.
• It is taken on a new point where the next benchmark or reduced level needs to be determined.
• FS helps in calculating the final elevation of the surveyed area.
• Example: If a surveyor is leveling a road, the FS is the reading taken at the last point before shifting the instrument forward.

3. Intermediate Sight (IS):

• These are any additional readings taken between the BS and FS.
• They help in determining elevations of multiple points without moving the instrument.
• IS readings are useful in detailed surveys, where several points need to be measured in one setup.
• Example: If a surveyor measures different points along a construction site, the readings taken between BS and FS are IS readings.

Importance of BS, FS, and IS in Leveling:

• Ensures accurate height measurements for buildings, roads, and land grading.
• Helps in calculating reduced levels (RL) by using height differences.
• Reduces errors by providing multiple reference points in a single setup.

Conclusion:

Back Sight (BS), Fore Sight (FS), and Intermediate Sight (IS) are essential for leveling in surveying. BS is the first reading taken at a known point, IS readings are taken in between, and FS is the last reading before moving the instrument. These readings ensure accurate elevation measurements for construction and land development.