The following difficulties are commonly met with while levelling: 1. Levelling Across a Hill or a Hollow 2. Levelling Up-Hill or Down-Hill 3. Staff Very Near the Instrument 4. When the Staff-Station is too Low or too High 5. Levelling across a Pond or a Lake 6. Levelling across a River 7. Levelling across Solid Obstruction like a Wall.
Difficulty # 1. Levelling Across a Hill or a Hollow:
In levelling across a hill, the level should not be set up on the top of the hill, but it should be set up on side shown in fig. 7.18. This reduces the number of instrument-settings. Similarly in levelling across a hollow, much time will be saved, if the instrument is set upon one side of the hollow as in fig 7.19 instead of in the bottom of the depression.
Difficulty # 2. Levelling Up-Hill or Down-Hill:
While levelling up-hill, the fore sight will be near the foot of the staff and the back sight heart the downhill the reverse is the case.
The error due to non verticality of the staff is small when the staff reading is small, but it is serious when the line of sight strikes near the top and the reading is large, in which case the error can be avoided by keeping the staff vertical by using a plumb-bob., or by waving the staff and noting the smallest reading.
Also while levelling along slopes of a hill, it is very difficult to equalise back and foresight distances and the progress of the work is considerably slow due to large number of instrument-settings and decreased length of sight distances, the instrument may be set up towards one side of the line along which levels are to be observed and ‘zing-zag’ sights may be taken.
Difficulty # 3. Staff Very Near the Instrument:
When the staff is held very near the instrument, the graduations on the staff are not clearly visible. In such a case, a piece of white paper is moved up or down the staff until its edge is bisected by the line of collimation and the corresponding reading is taken. When the level is set up over the staff-station itself the staff reading may be taken by viewing through the object glass or by measuring the height of the centre of the object-glass with the staff.
Difficulty # 4. When the Staff-Station is too Low or too High:
When the staff station is too low i.e., the line of collimation passes above the staff, a peg is driven and staff is held over the peg (fig. 7.21). The staff reading (be) is taken and the height of the top of the peg (ab) above the ground is measured with a tape.
The required staff reading equals (ab + bc).
When the staff-station is too high i.e., above the line of sight as in the case of a the-beam, roof-girder, the stringcourse etc. the staff is held inverted on the point, and the reading is then taken. This reading being negative is entered in the level-book with a negative sign.
Therefore, if the inverted staff reading is back sight it is negative and if the same is fore sight, then it is positive.
Difficulty # 5. Levelling across a Pond or a Lake:
This kind of difficulty can be got over by driving two pegs A and B flush with the water surface on opposite sides of a pond or a lake as shown in fig. 7.22.
Evidently the top of the pegs are at the same level. The R.L. of the peg A is determined by taking a fore sight on it. This is also the R.L. of the peg B. The instrument is then shifted and set up on the other side and a back sight is taken on peg B, the two pegs A and B together, being considered as a single change point.
Difficulty # 6. Levelling across a River:
The above method is not suitable in the case of fast flowing water of a river. In such a case, the method of reciprocal levelling employed to determine the true level between two point on opposite bank and the levelling is continued.
Difficulty # 7. Levelling across Solid Obstruction like a Wall:
The telescope is directed towards the wall where the line of sight cuts it (see fig 7.23).Measure height from this point ‘A’ to the top of the wall ‘B’ accurately with a steel tape. Find the R.L. of the top of the wall by adding the distance AB to the Height of Instrument.
Shift the instrument to the other side of the wall and mark a point ‘D’ where the line of sight strikes it. Measure CD as before. The height of instrument in the second position is then equal to the R.L of the top of the wall minus CD. Then proceed as usual.