QUESTION: Describe the methods of inverted staff reading & reciprocal levelling

1       Describe the methods of inverted staff reading & reciprocal levelling

 

                       In levelling there are other levelling method which is inverted staff reading and reciprocal levelling. Inverted staff it may be necessary to determine the reduced levels of points such as the soffit of a bridge, underpass or canopy. Generally, these points will be above the line of collimation. To obtain the reduced levels of such points, the staff is held upside down in an inverted position with its base on the elevated points. When booking an inverted it is entered in the levelling table with a minus sign, the calculation proceeding in the normal way, taking this sign into account. An inverted staff position must not be used as a change point since there is often difficulty in keeping the staff vertical and in keeping its base in the same position for more than one reading. An inverted staff position must be used as a change point because it is often difficult to keep the staff vertical and to keep iys base in the same position for more than one reading.

                           True differences in height are obtained by ensuring that back sight and fort sight lengths are equal when levelling. This eliminates the effect of any collimation error that may be present in the level used and also eliminates the effects of curvature and refraction. There are certain cases, however, when it may be not be possible to take readings with equal sight lengths as, for instance, when a line of levels has to be taken over a wide gap such as river. In these cases, the technique of reciprocal levelling can be adopted.

                     Figure down shows two point A and B on opposite sides of a wide river. The line of collimation has been assumed to be elevated above the horizontal plane. This may not be the case but does not affect the calculations. To obtain the true difference in level between A and B a level is placed at I1 about 5m from A, and a staff is held vertically at A and B . Staff readings are taken at A (a1) and B (b1). The level is next taken to position I2 where readings a2 and b2 are recorded. Since the observations are taken over the same sighting distances with the same level, the effects of the collimation error will be the same for both cases. When repciprocal levelling with one level, the two sets of observations must follow each other as soon as possible so that refraction effects are the same and are therefore eliminated. Where this not possible, two levels have to be used simultaneously. It must be realised  that the levels should have the same collimation error or the true height different will not be obtained.

                  

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