Principles of Levelling

Levelling is an integral part of surveying. In this brief article, Mr. Rajnish Shukla, Survey Manager, FGS Engineers & Innovators, describes the principles of levelling.

Levelling is the process of determining the position of points or objects on the earth’s surface in terms of their height or elevation relative to a datum or reference. Commonly, the mean sea level (MSL) is used as the datum. Levelling is done using a levelling instrument or level, a levelling staff or rod, a chain or tape for measuring horizontal distances and a field book for logging. The levelling instrument consists of a telescope with its eyepiece and a bubble tube.

How to determine the level?

When the instrument is accurately levelled, the axis of the bubble tube and the line of sight will be horizontal, and the bubble must be central. When the telescope is rotated, the line of sight shall remain in a horizontal plane. This process is called instrument levelling. The levelling instrument is used to sight a staff kept on a known or unknown point or elevation. The primary reading thus taken for a known height forms the datum or reference. Based on the datum, the height or elevation of the line of sight can be read with the staff readings.

Assume that h1= 1.225, and h2= 0.885

Reduced level of the instrument Height is, RL = (A+h1), 100 + 1.225 =101.225

Reduced level of the point B = (Instrument Height-h2), 101.225-0.885=100.34

A levelling instrument can help us establish a horizontal line, which may not be a level line. Nevertheless, for short distances the difference between level line and horizontal line can be negligible. To eliminate instrument errors, the points measured have to be equidistant from the levelling instrument. If the point is lower, the staff reading will be higher; if the point is higher, the staff reading will be lower.

Terms used in Levelling

Station: Station is the target point for which the elevation has to be determined. It may also be the point to be marked at a given elevation. From the station, the staff is held while making observations with the levelling instrument.

Height of the Instrument: With regard to the datum, this is the reduced elevation of the line of sight. Height of the instrument and height of the line of site above the ground are hence different.

Height of the instrument= elevation of benchmark (BM) + backsight

Suppose that back side staff reading is 1.225m and Benchmark is 100.00m; then

Height of instrument (HI) = 100.00+1.225= 101.225 m.

Balancing of Sight: Balancing of sight is carried out to avoid instrumental and other errors. Balancing of sight is done by comparing the distances of the points where backsight (BS) and foresight (FS) are taken. The distances thus measured from the instrument station must be approximately the same.

Differential Levelling: Differential levelling or compound levelling or continuous levelling is done when the points whose heights are to be determined, are far apart, or if there is a huge difference in elevation between them. For differential levelling, readings are taken at many positions and at each position, simple levelling is done.

Change Point (CP): Change point (CP) is a point that indicates the shifting of level. At this station, both BS and FS readings are taken. Care must be given in fixing change points because stable and well-defined objects must be chosen for getting accurate results.

Let's take a look at some key terms related to differential levelling.

Backsight (BS) Reading: BS reading is taken at a point of known elevation; for instance, on a benchmark (BM) or change point (CP). BS reading is also known as plus sight. It is the first staff reading taken after the level is set up.

Foresight (FS) Reading: FS reading or minus sight is the elevation which has to be determined through the process of levelling. The foresight reading is the last reading taken before the instrument is shifted. Towards the change point also, the foresight reading is taken.

Elevation of station = height of instrument (HI) – foresight (FS) or intermediate sight (IS)

Intermediate Sight (IS) Readings: These are the staff readings or elevations to be measured at other points before the foresight is taken. i.e. Intermediate sight readings are all the readings between the backsight (BS) and foresight (FS).

Format for collection of levelling data from the field:

Precautions to be taken in Levelling:

To get accurate and precise readings in levelling, utmost care has to be taken. Since this is a foundational process, any error in levelling could possibly have a domino effect on the later processes. A surveyor has to bear in mind the following points before getting on with levelling on the field.

• For accurate reading, the staff should be kept vertically. As an alternative method, the staff can be waved and the lowest reading is take. Both methods are found to be equally good.
• In order to obtain an accurate line of sight, the bubble in the levelling instrument has to be at the centre and the foot screws must be used carefully.
• Mark the readings in the increasing direction so as to avoid errors. The readings have to be taken on the portion of the staff between two vertical crosshairs.
• Verify for congruence of foresight and backsight distances.
• Make sure not to disturb the tripod of the instrument, once it is suitably placed.
• Careful in fixing turning or changing point, for it has to be a fixed or stable point.
• Ensure the elimination of errors such as parallax, incorrect reading, etc.
• The instrument is weather-sensitive and hence, avoid working in very hot conditions.