Leveling is a part of surveying, in which relative heights of different objects on or below the earth surface are measured. The undulations of ground can be measured using leveling. But it is not such an easy job.
Different ground conditions, lot of natural formations, structural parts etc. are interfered while leveling a ground. They provide difficulties to the surveyor and staff men. In such cases, there are some methods to resolve the difficulties and to continue the whole work.
Different Situations Those Makes Leveling Difficult
Some situations making leveling difficult are:
- When the staff is close to the instrument
- Leveling across a large lake
- Leveling across a river
- Leveling a cross a solid wall
- Bench mark lies above line of collimation
- Leveling along steep slope
- Leveling across depression or rising ground
Sometimes it is necessary to place the staff very near to the instrument for leveling. In that case, the instrument loses its focus and becomes blur. It is difficult to note down the exact staff reading.
This problem arises whenever the staff is as close as below the focusing limit. This can be solved by using white paper. Holding staff vertical and a white paper is slowly moved up and down in front of it.
The paper edge is observed through telescope and stopped at a point where the line of collimation meets edge of white paper. The staff reading at that point is noted.
Leveling Across a Large Lake
If a large pond or lake is there across the leveling line, then it is difficult to perform the leveling across that pond. In that case, some assumptions are to be made to continue the work.
Generally, in ponds or lakes water is in still position so, we can consider it as a level surface. Two pegs are fixed at the two banks of pond as the top surface of peg is visible or coincide with water surface.
Now, the staff is placed at first peg and using level, RL is measured by taking foresight (FS). Then we assumed that the RL of second peg point is as same as first peg point. Finally, the level is shifted to the other bank side and back sight (BS) reading is taken from second peg point.
Leveling Across a River
In a river, water always flows so, we cannot consider it as leveled surface like in pond or lake. The water levels are different at two banks of a river. For this problem to resolve, reciprocal method of leveling is used.
Two pegs are fixed at two banks of river and RL of 1st peg positioned is taken in usual manner. RL of 2nd peg position is measured by obtaining the true difference level between two pegs using reciprocal leveling.
Leveling Across a Solid Wall
When a solid wall is interfered in leveling, it is difficult to measure RL of the points in that wall position. In such a case, two pegs are driven on both sides of the wall as the staff just touching the wall. Now find out the height of the instrument is measured by focusing any bench mark or Champion point.
Then, read the staff reading on one side of the wall note down it. Then using staff measure the height of the wall. Now we have all the data from which RL of every point can be calculated. Similarly, do this process on the other side of the wall.
Benchmark lies Above Line of Collimation
When the bench mark lies above the line of collimation such as on the bottom of bridge girder, lower surface of culvert etc., in such cases, fore sight reading of point A is taken first using the level positioned at O.
Now the staff is held at benchmark (under the girder) in inverted position from the girder. Now the back-sight reading is noted from the inverted staff and it is described with negative sign with showing “staff held inverted” in remarks column.
Leveling Along a Steep Slope
When it is required to perform leveling along steep slope areas like hilly regions it is difficult to obtain same values in back sight and fore sight readings. This problem can be resolved by positioning level set up in zigzag paths.
Benchmark lies Above Line of Collimation
When the bench mark lies above the line of collimation such as on the bottom of bridge girder, lower surface of culvert etc., in such cases, fore sight reading of point A is taken first using the level positioned at O.
Now the staff is held at benchmark (under the girder) in inverted position from the girder. Now the back-sight reading is noted from the inverted staff and it is described with negative sign with showing “staff held inverted” in remarks column.
Leveling Along a Steep Slope
When it is required to perform leveling along steep slope areas like hilly regions it is difficult to obtain same values in back sight and fore sight readings. This problem can be resolved by positioning level set up in zigzag paths.
The instrument position in that zigzag path may be decided by keeping back sight and fore sight reading equal. Finally, RL of every point is calculated. This can be done for both positive and negative steep slopes.
Leveling Across Depression or Rising Ground
When a high-rise ground or depressed ground is reached while leveling, then the instrument should not be kept at peak positions like peak rise or lowest of ground. In case of rising ground, the instrument is fixed in a position such that the line of collimation should pass through the apex point of that high-rise ground.
Similarly, in case of depressed ground, the instrument is positioned at one side of depressed portion and staff is positioned at other side of depression.
