The term “Inter Sight” may sound abstract at first, but in the context of civil engineering, surveying, and construction design, it plays a crucial role in ensuring accuracy, alignment, and precision in project execution. Understanding Inter Sight is essential for surveyors, engineers, and architects who work with levels, gradients, and sight lines in land development and infrastructure projects. This article explains in detail what Inter Sight is, how it is used in surveying, its significance, applications, advantages, and methods of observation.
What is Inter Sight?
Inter Sight (often referred to as Intermediate Sight) is a reading taken on a leveling staff between the Back Sight (BS) and Fore Sight (FS) during leveling operations in surveying. It is a staff reading observed on points whose elevations need to be determined but are not used as turning points.
In simpler terms, Inter Sight is the staff reading on intermediate points between the initial and final stations of a leveling instrument setup. It helps surveyors measure and record intermediate ground elevations without moving the leveling instrument.
Formula
If the Height of Instrument (H.I.) is known, the Reduced Level (R.L.) of an intermediate point can be calculated as:
R.L. = H.I. – Inter Sight (I.S.)
Understanding the Components Related to Inter Sight
To clearly understand Inter Sight, one must know the key terms associated with leveling:
1. Back Sight (BS)
A Back Sight is the first staff reading taken on a point of known elevation (like a benchmark) after setting up the leveling instrument. It helps in calculating the Height of Instrument (H.I.).
2. Fore Sight (FS)
A Fore Sight is the last staff reading taken before shifting the leveling instrument to a new position. It helps determine the elevation of the next station or turning point.
3. Intermediate Sight (IS or I.S.)
The Intermediate Sight (Inter Sight) readings are taken between BS and FS to measure the elevation of intermediate points without moving the instrument.
4. Height of Instrument (H.I.)
It is the elevation of the line of sight of the leveling instrument above the benchmark or datum level.
H.I. = R.L. of Benchmark + Back Sight
Example of Inter Sight Calculation
Let’s consider a simple example from a leveling survey:
| Observation | Staff Reading (m) | Type of Sight | Height of Instrument (H.I.) | Reduced Level (R.L.) |
|---|---|---|---|---|
| Point A | 1.200 | Back Sight | — | 100.000 (Benchmark) |
| 101.200 | ||||
| Point B | 1.500 | Inter Sight | — | 101.200 – 1.500 = 99.700 |
| Point C | 1.800 | Inter Sight | — | 101.200 – 1.800 = 99.400 |
| Point D | 2.000 | Fore Sight | — | 101.200 – 2.000 = 99.200 |
In this example:
- Point A is the benchmark (BM).
- Points B and C are Inter Sight points.
- Point D is the Fore Sight point.
The Inter Sight readings helped determine the elevations of intermediate points without changing the instrument’s position.
Purpose of Inter Sight in Surveying
The main purposes of using Inter Sight readings include:
- Determining elevations of intermediate points between major stations.
- Creating a ground profile for road, canal, or pipeline design.
- Reducing the number of instrument setups, saving time and effort.
- Ensuring continuous leveling accuracy along a section or alignment.
Procedure of Taking Inter Sight Readings
- Set up the Leveling Instrument on a stable ground surface.
- Take a Back Sight (BS) reading on the benchmark or known point.
- Calculate the Height of Instrument (H.I.).
- Take Inter Sight (I.S.) readings on intermediate points along the line or alignment.
- Take a Fore Sight (FS) reading on the last point before moving the instrument.
- Use the H.I. method or Rise and Fall method to calculate the Reduced Levels of all points.
Methods of Leveling Using Inter Sight
1. Height of Instrument (H.I.) Method
In this method:
- Calculate the H.I. using:
H.I. = R.L. of Benchmark + Back Sight - Then, for each intermediate point:
R.L. = H.I. – Inter Sight
2. Rise and Fall Method
Here, the difference between consecutive staff readings determines whether the ground is rising or falling.
It is slightly more accurate but requires detailed calculation.
Applications of Inter Sight in Civil Engineering
Inter Sight readings are vital in a variety of construction and infrastructure projects:
1. Road Construction
Used to prepare longitudinal and cross-sectional profiles to design road gradients, slopes, and drainage.
2. Railway and Canal Projects
Helps measure the elevation of intermediate points along alignment for smooth gradient design and water flow.
3. Building Site Leveling
Determines cut and fill quantities before foundation construction.
4. Irrigation Projects
Ensures uniform slope in irrigation channels and pipelines by determining intermediate ground levels.
5. Earthwork and Topographical Surveys
Creates contour maps and elevation models using multiple Inter Sight readings.
Advantages of Using Inter Sight
- Reduces the need to move the instrument frequently.
- Improves efficiency and accuracy in determining multiple ground levels.
- Helps prepare elevation profiles for design work.
- Simple and quick to perform in the field.
- Provides continuous data along an alignment.
Limitations of Inter Sight
- Line of sight obstruction (trees, buildings, or uneven ground) can affect accuracy.
- Human error in reading or recording can lead to elevation discrepancies.
- Instrument stability must be maintained to prevent false readings.
- Not suitable for large elevation differences in one setup.
Difference Between Back Sight, Fore Sight, and Inter Sight
| Aspect | Back Sight (BS) | Fore Sight (FS) | Inter Sight (IS) |
|---|---|---|---|
| Definition | First reading from known point | Last reading before instrument shift | Reading on intermediate points |
| Purpose | To find H.I. | To determine next station R.L. | To find R.L. of in-between points |
| Sequence | Taken first | Taken last | Taken between BS and FS |
| Effect on H.I. | Adds to R.L. | Subtracts from H.I. | Subtracts from H.I. |
| Example | On benchmark | On turning point | On intermediate point |
Example in Practical Fieldwork
During a road survey:
- The level is set near a starting point with a benchmark R.L. of 100.00 m.
- BS on BM = 1.50 m, so H.I. = 101.50 m.
- IS on midpoints = 1.80 m and 2.10 m, giving R.L. = 99.70 m and 99.40 m.
- FS = 2.30 m gives next station R.L. = 99.20 m.
These Inter Sight readings help plot an accurate longitudinal profile, critical for road gradient and drainage design.
Common Mistakes in Taking Inter Sight Readings
- Not properly leveling the instrument before observation.
- Misrecording readings or interchanging IS and FS.
- Ignoring parallax error during staff observation.
- Incorrect staff positioning on uneven ground.
- Failing to recheck instrument stability during long surveys.
Tips for Accurate Inter Sight Observation
- Always ensure the line of sight is horizontal before taking readings.
- Use a tripod on firm ground to avoid movement.
- Recheck readings for consistency before moving the instrument.
- Maintain clear field notes with correct symbols and columns.
- Use automatic levels or laser levels for higher precision.
FAQs About Inter Sight
What is the meaning of Inter Sight in surveying?
Inter Sight (Intermediate Sight) is a staff reading taken on intermediate points between back sight and fore sight, used to find their elevations.
Is Inter Sight used to find Height of Instrument?
No, Inter Sight is used to calculate Reduced Levels (R.L.), not to find H.I.
Can multiple Inter Sights be taken from one setup?
Yes, you can take as many Inter Sight readings as required, provided the instrument remains stable.
What is the symbol for Inter Sight in field books?
Inter Sight is generally denoted by I.S. in leveling field books.
What happens if Inter Sight is misrecorded?
Incorrect Inter Sight readings can lead to errors in elevation and design profiles.
Conclusion
Inter Sight is a fundamental concept in leveling and surveying, enabling engineers to determine elevations of multiple intermediate points efficiently from a single instrument setup. It simplifies fieldwork, saves time, and provides accurate ground profiles essential for road, canal, and building construction projects. Mastering the use of Inter Sight not only improves survey accuracy but also ensures that every construction project begins on a precisely leveled foundation.
