Levelling

Introduction:

Levelling is a means by which surveyors can determine the elevation of points, using other known points as references. Levelling is perhaps the most basic of surveying operations and forms an important fundamental part of almost every surveying project.

Equipment:

Levelling is carried out by the use of: a Spirit Level, often called an Engineer’s Level, and a level rod. The level rod resembles a large fold-up ruler but is not accurately referred to as such.

Basics of Levelling:
In levelling, the surveyor looks back (BS) to a point of known elevation to determine the elevation of his or her instrument (EI). The surveyor then looks forward (FS) to a point of unknown elevation and determines the elevation of that point using the elevation of his or her instrument (EI) and the value on the level rod read through the level’s telescope.

Once the elevation of a point is determined, that point can be used for determining the elevations of other points. In this way, the surveyor may “leap-frog” forward, eventually determining the elevations of points that are impractical from the initial location, and developing greater accuracy by means of a “closed traverse”.

A Sample Levelling Traverse:

Sample Traverse Data:

Surveyors accumulate levelling data in a specific fashion. This format ensures that all the relevant data is preserved and none is lost. Calculations are performed as you go, making mistakes easier to detect. This is important in levelling since a turning point may not be marked to revisit in case of error.

Sample Correction Calculations:

Adjust on basis of shot lengths. S shot lengths = 168 m (exclude IS’s from this)