Keywords: Levelling, difficulties in levelling, errors in levelling
The last two topics in leveling are the difficulties and precautions to be taken for an error-free levelling process. Let’s jump into the first section, difficulties in levelling.
Some of the difficulties that are commonly encountered in levelling are:
When the staff station is more than 3metres above the line of sight
Solution: Measure the distance (d) from line of sight to the staff placed at the station accurately with a tape.
RL of the station = HI+d+3.00
When the staff station is much below the line of sight
Solution: Raise the staff till the horizontal cross hair exactly with 3.00 mark
Measure the distance from the station to the bottom of the staff (d)
RL of the station = HI-3.0-d
If the staff is too near the instrument
Solution: When the staff is placed too near the level, the graduations will not be distinctly visible. Such situation has to be avoided. If unavoidable, then adopt any of the following two methods:
- i. By moving the edge of a white sheet of paper, locate the point corresponding to the line of sight on the staff. Then read the staff using the naked eye.
- ii. Turn the telescope through 180°; then take the reading through the objective. Move a pencil point on the staff till it coincides with the line of sight. Now, take the reading corresponding to the point by naked eye.
Levelling across a hill
Solution: The instrument should be set on one side of the hill rather than on the top. Such a set up saves time.
Levelling across a hollow
Solution: Set up the instrument sufficiently low to enable the levels of the hollow to be obtained. Also setting up the instrument to one side of the hollow, saves time.
Levelling on steep slope
Solution: To equalize BS and FS distances, take the instrument along a zigzag route. To keep the staff vertical, use a plumb bob or a circular level.
Levelling across a pond
Solution: The surface of still water is a level surface. The RL of the water surface could be determined from one side and the leveling can be continued from the other side of the pond. Alternatively, reciprocal leveling could be done.
Levelling across a high wall
Solution: The RL of the top of the wall is determined and the leveling continued from the other side of the wall.
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