TYPES OF DIRECT LEVELLING

The following are the different types of direct levelling:

(i) Simple levelling

(ii) Differential levelling

(iii) Fly levelling

(iv) Profile levelling

(v) Cross sectioning

(vi) Reciprocal levelling.

Simple Levelling

It is the method used for finding difference between the levels of two nearby points.

Differential Levelling

If the distance between two points A and B is large, it may not be possible to take the readings on A and B from a single setting.In such situation differential levelling is used. In differential levelling the instrument is set at more than one position, each shifting facilitated by a change point.

 

Fly Levelling

If the work site is away from the benchmark, surveyor starts the work with a back sight on the benchmark by setting instrument at a convenient point. Then he proceeds towards the site by taking foresights and backsights on a number of change points till he establishes a temporary benchmark in the site. Rest of the levelling work is carried out in the site. At the end of the work again levelling is carried out by taking a set of convenient change points till the bench work is reached. This type of levelling in which only backsight and foresights are taken, is called fly levelling, the purpose being to connect a benchmark with a temporary benchmark or vice versa. Thus the difference between fly levelling and differential levelling is only in the purpose of levelling.

Profile Levelling

This type of levelling is known as longitudinal sectioning. In highway, railway, canal or sewage line projects profile of the ground along selected routes are required. In such cases, along the route, at regular interval readings are taken and RL of various points are found. Then the section of the route is drawn to get the profile. For drawing profile of the route, vertical scale is usually larger compared to scale for horizontal distances. It gives clear picture of the profile of the route.

 

Cross-Sectioning

In many engineering projects, not only longitudinal profile but also the profile of cross-sections at regular intervals are required. These profiles help in calculating the earthworks involved in the projects. shows the scheme of such work in which longitudinal profile is found by taking readings at 20 m interval along chain lines AB, BC and readings are taken at an interval of 3 m on either side. The distances on the cross-sections are treated as left or right of the lines as they are found while facing the

forward station of survey. The cross-sectional length depends upon the nature of the project.

 

Reciprocal Levelling

In levelling, it is better to keep distance of backsight and foresight equal. By doing so the following errors are eliminated:

(i) Error due to non-parallelism of line of collimation and axis of bubble tube.

(ii) Errors due to curvature and refraction.

But in levelling across obstacles like river and ravine, it is not possible to maintain equal distances for fore sight and backsight. In such situations reciprocal levelling as described below is used: