Reciprocal Levelling

This is a leveling operation in which the height difference between two points is precisely determined by two sets of correlation observations.

This method is very useful when equipment is unable to set between two points due to obstruction such as a valley, river, etc., and if the sights are longer than normally allowed.

Staff reading errors, earth curvature, and incomplete adjustment of equipment are key to such long scenes.

Special methods such as leveling each other should be used to reduce these errors. In this method, the instrument is said to be located at one point A, mounted on one side of the valley, and readings are taken on the staff held in position A (Fig. 6.43 (a)) and on the staff at B. On the other side of the valley.

Also, watch this for a better understanding of reciprocal levelling :

Let these readings be a and b respectively. Proximal reading a is flawless, but b breading has an error e due to curvature, refraction, and friction.

Also Read: Auto level

The instrument is then moved closer to B on the other side of the valley and the reading is taken on the staff holding in B and A. Let these readings be c and d (Figure 6.43 (b)).

Proximate reading c is flawless, but reading d for the reasons discussed above will have an error. Let h be the true difference in height between A and B.

In case 1 (Fig. 6.43 (a)), h = (b – e) – a
In case 2 (Fig. 6.43 (b)), h = c – (d – e)

2 h = (b – a) + (c – d)

Or    h = 1/2 [(b – a) + (c – d)]

And e = 1/2 [(b – a) – (c – d)]

In the above derivations, it is assumed that the effect of refraction is the same when making observations from both stations.

However, if only one point is used, the time to move the instrument to the opposite bank is delayed, at which point the value of refraction may change.

Also Read: Methods of traversing

Therefore, to ensure better results, some surveyors recommend using two steps, one at each bank, and scenes were taken at once.

Although this gives good results, each stage may have a different collimation error. Therefore the equipment must be interchangeable and the entire procedure must be repeated. The average of the four values ​​is the possible difference in the level between the two points.