Understanding BS (Back Sight Reading) and FS (Fore Sight) in Surveying

The planning infrastructure of the building project, delineating property boundaries or the accuracy in the measurements of the project has the ability to make or break the construction project. In this regard, precision is necessary. This precision can be achieved by applying various tools and techniques, in which, Backsight (BS) and Foresight (FS) are the fundamental concepts in building construction.

Backsight (BS) is the process of observing the point of elevation, which was previously established. The backsight acts as the reference for subsequent measurement. At this point, all kinds of measurements in the construction process have been started.

Foresight (FS) is the process of sighting a point in which elevation needs to be determined. The reading of foresight (FS) helps to allow the surveyors to calculate the new elevation’s point while it is combined with the known elevation of BS.

Different processes of using BS and FS in surveying

There are different processes that have been involved in using BS and FS in surveying which include:

The establishment of backsight (BS)

The surveyor is involved in locating and sighting the benchmark along with a known elevation. This process helps to establish the backsight (BS).

Taking the foresight (FS)

After establishing the BS, the points are sighted by the surveyor whose elevation is required to determine. This sight is known as the foresight (FS).

Recording the measurements

The measurements from the levelling instrument have been recorded for both the FS and BS readings. The horizontal distance, the slope distance and the vertical angle have been included in these types of readings.

Calculating elevations

The record measurements have been used along with the known elevations of BS for calculating the elevations. The FS point’s calculations have been done by using trigonometric principles or levelling formulas.

Verifying and adjustment process

Once the calculation is done, the accuracy of the measurements has been verified by the surveyors by redundant checks or by comparing the points of the elevations with other nearby points. In this regard, necessary adjustments have been made for ensuring precisions.

BS and FS’s applications in surveying

BS and FS have been used in various applications within the surveying process which include:

Engineering

BS and FS help to create more accurate topographic maps and profiles that have been used in infrastructure projects.

Construction

BS and FS help to ensure the precise grading process as well as the levelling of the building sites in the construction units.

Land Surveying

The property boundaries have been also established with the help of BS and FS. In addition, BS and FS help to conduct cadastral surveys.

Geomatics

BS and FS are also involved in mapping land features and helping to monitor the changes over time.

Importance of using BS and FS

The measurements of BS and FS are important in the surveying process which helps to ensure the vertical measurement accuracy. More precise elevation profiles and contour maps can be created by the surveyor while they have established the reference point (BS) as well as determine the subsequent point’s elevations (FS). These precise elevation profiles are important for several construction and engineering projects.

Levelling in Surveying

Levelling is the process in which the height of one level compared to another has been determined. Levelling in surveying has been used for establishing the elevation of points at a given elevation in relation to a datum or for developing the elevation of points in relation to a datum. The levelling in surveying is important to measure buildings, landscapes or other types of building assets.

Levelling instruments in the surveying process

The instruments that have been used for the levelling process are:

A. Dumpy Level
B. Tilting Level
C. Automatic Level
D. Digital Level
E. Water Level

Learn about different types of Levelling in Surveying

Geometric Levelling

This levelling method is done on the basis of the use of an automatic level of optical level in which, a levelling rod is used for measuring the height differences between two points.

Trigonometric Levelling

This method of levelling has been conducted on the basis of the use of angles and distances that have been measured along with a theodolite. This levelling process involves the measurement of the vertical and horizontal angles as well as the distance between points of interest. This helps to determine the differences in heights by using the trigonometric formulas.

GPS Levelling

In this type of levelling method, satellites have been used for determining the elevations of different points. This levelling method helps to employ altimeters or barometers for measuring the air pressures at different locations. After that, the height difference has been calculated on the basis of the pressure variations.

To learn more, watch the following video tutorial.

Video Source: Billing Engineer Civil

Barometric Levelling

The atmospheric pressure has been used in this levelling method which helps to estimate the elevation of different points.

Barometers or altimeters have been employed for measuring the air pressures at different locations. After that, the height differences have been calculated on the basis of the pressure variations.

Hydrostatic Levelling

This type of levelling has been used for communicating vessels which helps to determine the differences in heights.