Classification and Principle of Leveling

Classification of leveling

1. Different leveling:

It is the operation of leveling to determine the elevations of points. Some distance a part or to establish bench marks.

2. Check leveling:

It is the operation of running levels for the purpose of checking the series of levels, which have been previously fixed. At the end of each day’s work, a line of level is run, returning to the starting point of that day with a view to check the work done on that day.

3. Profile leveling:

It is the operation in which the object is to determine the elevation of points at known distance apart along a given line, and thus to obtain the accurate out line of the surface of the ground. It is called the longitudinal leveling or sectioning.

4. Cross sectioning:

It is the operation of leveling to determine the surface undulation or outline of the ground transverse to the given line and on either side of it.

5. Reciprocal leveling:

It is then method of leveling in which the difference in elevation between two points, accurately determined by two sets of observation when it is not possible to set up the level midway between the two points.

6. Barometric leveling:

It is the method of leveling in which the altitudes of points are determined by means of a barometer, which measures atmospheric pressure.

7. Hypsometry:

It is the method of leveling in which the heights of mountains are found by observing the temperature at which water boils.

8. Trigonometric leveling:

It is then process of leveling in which the elevations of points are computed from the vertical, angles and horizontal distance measured in the field.

Steps in Leveling:

When the level is set up and correctly leveled, the lines of collimation will be horizontal. When the telescope is rotated about its vertical axis, it will revolve in a horizontal plane known as the plane of collimation. Therefore all staff readings taken with the telescope will be vertical measurements made downwards from this plane. There are two essentials steps in leveling.

1. To find the elevation or R.L. of the plane of collimation (H.I) of the level by taking a back sight on a bench mark.

2. To find the levitation of R.L. of any other point by taking a reading on the staff held at the point.

Height of Instrument (H. I.) = R.L. of the plane of collimation 
                                         = R.L. of B.M. + B.S.
                R. L. of point    = H.I.-F.S.
                                         = H. I. – I.S.

It is the necessary that after every back side. [However many intermediate sight may be], there must be a foresight. Leveling should always commence from a permanent common bench mark and end on a permanent bench mark.

Principle of Leveling:

1. Simple leveling:

It is the simplest operation in leveling when it is required to find the difference in elevation between two points both of which are visible from a single position of the level. Suppose A and b are two such point and level is set up at 0, approximately mid way between. A and B but not necessary on the line joining them, after finding the reading on point A and point B, let the respective reading on A and B be 2.340 and 3.315 difference between them is 3.315-2.340=0.795 m.

2. Differential leveling:

This method is used in order to find out the difference in elevation between two points.

1. If they too apart.

2. If the difference in elevation between them is too great.

In such cases it is necessary to set up the level in several positions and to work in a series of stages. The method of simple leveling is employed each of the successive stages. The process is also known as compound continues leveling.
 
Methods of Determination of the Reduced Level of Point from the Staff Reading

1. Collimation Method:

It consist of finding the elevation of the plane of collimation ( H.I.)  for every set up of the instrument, and then obtaining the reduced level of point with reference to the respective plane of collimation.

1. Elevation of plane of collimation for the first set of the level determined by adding back side to R.L. of B.M.

2. The R.L. of intermediate point and first change point are then obtained by starching the staff reading taken on respective point (IS & FS) from the elation of the plane collimation. [H.I.]

3. When the instrument is shifted to the second position a new plane collimation is set up. The elevation of this plane is obtained by adding B.S. taken on the C.P. From the second position of the level to the R.L.  C.P. The R.L. of successive point and second C.P. are found by subtract these staff reading from the elevation of second plane of collimation Arithmetical check

                           Sum of  B.S. – sum of F.S. = last R.L. – First R.L.

2. Rise and Fall Method:

It consists of determining the difference of elevation between consecutive points by comparing each point after the first that immediately preceding it. The difference between there staff reading indicates a rise fall according to the staff reading at the point. The R.L is then found adding the rise to, or subtracting the fall from the reduced level of preceding point.

Arithmetic check

Sum of B.S. – sum of F. S. = sum of rise – sum of fall = last R. L. – first R.L.
 
Booking the staff readings:

The following points may be kept in mind entering the readings in a level field book.

1. The reading should be entering in the respective columns and in order their observation.

2. The first page is always a back side and the last one is ways a foresight.

3.  It a page finished with an IS reading, the reading is entered in the IS and FS columns on that page and brought forward to the next page.

4. The FS and BS of any change point are entered in the same horizontal line.

5. The RL of the line of the collimation is entered in the same horizontal line.

6. Bench marks and change points should be clearly described in the remark column.

Specimen pages of level field book: Collimation system

Specimen pages of level field book: Rise & Fall System