In civil engineering and surveying, the term inter sight (often written as intermediate sight) plays a crucial role in leveling operations and determining elevation differences across a site. Understanding what inter sight means and how it is used is fundamental for engineers, surveyors, and construction professionals to ensure accurate leveling, proper foundation design, and smooth execution of infrastructure projects.
This article provides a complete explanation of inter sight, its purpose, how it differs from back sight and fore sight, and its practical application in leveling and field surveying.
What Is Inter Sight in Surveying?
Inter sight (Intermediate Sight) refers to the staff reading taken on a point between the back sight and the fore sight during leveling operations. It is neither the first nor the last reading at a particular instrument setup but represents points whose elevations are required for detailed profiling or contouring.
In simpler terms, an inter sight is taken on intermediate points of the ground to determine their elevation relative to the known benchmark or datum. These points help create an accurate representation of the ground surface, which is vital for design and construction planning.
Example:
If a level instrument is set up and staff readings are taken as follows:
- Back Sight (BS): 1.250 m (on a benchmark)
- Intermediate Sight (IS): 2.030 m (on ground points)
- Fore Sight (FS): 2.890 m (on change point)
Then the inter sight (2.030 m) represents a ground point between the benchmark and the change point, helping to measure its elevation.
Purpose of Inter Sight
The primary objective of taking inter sight readings is to obtain elevations of multiple intermediate points without changing the position of the leveling instrument. It helps in:
- Determining elevations at multiple locations on uneven terrain.
- Drawing contour maps and longitudinal sections of roads, canals, or pipelines.
- Calculating ground profiles for excavation and embankment works.
- Ensuring uniform grading and slope control in large projects.
Difference Between Back Sight, Fore Sight, and Inter Sight
To understand inter sight clearly, it’s important to differentiate it from the other two key readings in leveling: back sight and fore sight.
| Term | Definition | Purpose | Position in Sequence |
|---|---|---|---|
| Back Sight (BS) | First staff reading taken on a known elevation point (benchmark or change point). | Used to determine height of instrument (H.I.). | Always first reading after setting up the instrument. |
| Intermediate Sight (IS) | Staff readings taken on intermediate points between BS and FS. | Used to find elevation of different ground points. | Taken between BS and FS. |
| Fore Sight (FS) | Last staff reading taken on a point before the instrument is shifted. | Used to calculate reduced level of the last point. | Always the last reading before moving the instrument. |
Example of Reading Sequence:
BS → IS → IS → IS → FS
Procedure for Taking Inter Sight Readings
Inter sight readings are obtained during the leveling process using a dumpy level, auto level, or digital level. The steps are as follows:
1. Instrument Setup
The level is set up and properly leveled using the foot screws to ensure the line of sight is horizontal.
2. Take Back Sight (BS)
The first reading is taken on a benchmark or a known point of elevation.
3. Take Intermediate Sight (IS)
Without moving the instrument, the staff is held at several intermediate points along the survey line, and readings are recorded. These readings represent the inter sight values.
4. Take Fore Sight (FS)
The last reading is taken on a point where the instrument will be moved next (change point).
5. Repeat Process
After shifting the instrument, the process is repeated, ensuring continuous leveling along the line of sight.
Computation of Reduced Level Using Inter Sight
To find the Reduced Level (R.L.) of any point using inter sight, the Height of Instrument (H.I.) method or Rise and Fall method is used.
By Height of Instrument Method:
Formula:
Height of Instrument (H.I.) = R.L. of Benchmark + Back Sight
Then,
R.L. of any intermediate point = H.I. – Inter Sight
Example Calculation:
If:
- R.L. of Benchmark = 100.000 m
- Back Sight (BS) = 1.250 m
- Inter Sight (IS) = 2.030 m
Then,
H.I. = 100.000 + 1.250 = 101.250 m
R.L. of IS point = 101.250 – 2.030 = 99.220 m
Thus, the intermediate point has an elevation of 99.220 m above the datum.
Applications of Inter Sight in Construction
Inter sight readings are extensively used in construction and infrastructure projects for:
- Road and railway surveys: To determine ground levels along alignment.
- Drainage and canal design: For maintaining uniform gradient and flow direction.
- Building layout: To check site elevation and cut/fill requirements.
- Earthwork computation: To prepare longitudinal and cross-sections.
- Contour mapping: To represent surface elevation variations accurately.
Advantages of Using Inter Sight Readings
- Reduces the number of instrument setups.
- Saves time and effort during large-scale leveling operations.
- Provides detailed elevation data for accurate ground profiling.
- Improves precision in design and construction planning.
- Ensures efficient drainage, slope control, and grading.
Common Mistakes to Avoid
While taking inter sight readings, some common errors can affect accuracy:
- Improper leveling of the instrument: Leads to incorrect readings.
- Misidentification of points: Confusing IS with BS or FS affects calculation.
- Parallax error: Focusing incorrectly on the staff markings.
- Poorly maintained staff: Dirty or faded markings can result in wrong readings.
- Incorrect recording sequence: Always maintain a proper field book order.
Example of Leveling Field Book with Inter Sight Readings
| Station | Back Sight (m) | Intermediate Sight (m) | Fore Sight (m) | Height of Instrument (m) | Reduced Level (m) | Remarks |
|---|---|---|---|---|---|---|
| BM1 | 1.250 | – | – | 101.250 | 100.000 | Benchmark |
| A | – | 2.030 | – | 101.250 | 99.220 | Intermediate Point |
| B | – | 2.640 | – | 101.250 | 98.610 | Intermediate Point |
| C | – | – | 3.150 | 101.250 | 98.100 | Change Point |
Difference Between Intermediate Sight and Change Point
A change point (CP) is the point where the level instrument is shifted for the next setup, while inter sight refers to readings taken on intermediate points.
| Parameter | Inter Sight (IS) | Change Point (CP) |
|---|---|---|
| Function | Determine elevation of intermediate points | Reference point when instrument is shifted |
| Position | Between BS and FS | Acts as FS for one setup and BS for the next |
| Frequency | Multiple in one setup | One per instrument shift |
FAQs About Inter Sight
1. What is inter sight in surveying?
Inter sight is a leveling staff reading taken on intermediate ground points between the back sight and fore sight to determine their elevations.
2. Is inter sight used for determining height of instrument?
No, inter sight readings are used to determine ground elevations. Only the back sight is used to calculate the height of instrument.
3. How many inter sight readings can be taken in one setup?
Any number of inter sight readings can be taken before a fore sight, depending on the number of required elevation points.
4. What is the accuracy requirement for inter sight?
Accuracy depends on the leveling instrument used, but typically ±3 mm per 1 km of leveling is acceptable for standard construction projects.
5. Can inter sight be used in contour mapping?
Yes, multiple inter sight readings across a site are essential for preparing accurate contour maps.
Conclusion
The concept of inter sight is fundamental in the field of surveying and leveling. It helps engineers and surveyors determine ground elevations efficiently and accurately without frequently moving the instrument. By using inter sight readings correctly, one can create precise elevation maps, profiles, and contour plans essential for designing and executing construction projects.
