REDUCTION OF LEVELS There are two systems of working out the reduced levels of points,

THE COLLIMATION SYSTEM: It consists of finding the elevation of theplane of collimation (H.I.) for every set-up of the instrument and thanobtaining the reduced levels of points with reference to the respectiveplane of collimation.

THE RISE AND FALL SYSTEM: It consist of determining the difference ofthe levels between consecutive points by comparing each point afterthe first with that immediately preceding it and than the reduced levelof each point is found out by adding the rise to, or subtracting the fallfrom the reduced level of the preceding point.

COMPARISION The collimation system is less tedious more rapid and involves less

calculations. However, mistakes made in reduction of levels of the intermediate

points remain undetected, while in the rise and fall system, there isa complete check on each of the intermediate reductions.

The first system is generally used in profile leveling and in settingout levels for constructional work, while the second one ispreferred for differential leveling, and other important work.

BOOKING THE STAFF READINGS The following points may be borne in mind in entering the

staff readings in a level field book, The readings should be entered in the respective columns

and in the order of their observation. The first entry on the page is always a backsight and the

last one always a foresight. In carrying forward the readings from one page to the

next, if the last entry happens to be an intermediate sight,it is entered in both, I.S. and F.S. columns, and in the B.S.and I.S. columns as a first entry on the next page. Theentries in the remaining columns against it should also berepeated on the next page.

Contd . . . The fore and back sights of the change point should be

written in the same horizontal line. The R.L. of the plane of collimation should be written in

the same horizontal line opposite the backsight. Bench marks, change points, and other important points

should be briefly but accurately described in the Remarkscolumn, and their sketches drawn on the left-hand side ofthe page.

CLASSIFICATION OF LEVELING Differential Leveling :-

It is the operation of leveling to determine the elevations of pointssome distance apart or to establish bench marks. It is also known as"Taking Flying Levels".

Check Leveling :- It is the operation of running levels for the purpose of checking a

series of levels, which have been previously fixed. Profile Leveling :-

It is the operation in which the object is to determine the elevations ofpoints at known distances apart along a given line, and thus to obtainthe accurate outline of the surface of the ground. It is also called thelongitudinal leveling or sectioning.

Cross Sectioning :- It is the operation of leveling to determine the surface undulations or

the outline of the ground transverse to the given line and on eitherside of it.

Reciprocal Leveling :- It is the method of leveling in which the difference in elevation

between two points is accurately determined by two sets ofobservations when it is not possible to set up the level midwaybetween the two points.

Barometric Leveling :- It is the method of leveling in which the altitudes of points are

determined by means of a barometer, which measures atmosphericpressure.

Hypsometry :- It is the method of leveling in which the heights of mountains are

found by observing the temperature at which water boils. Trigonometrical Leveling :-

It is the process of leveling in which the elevations of points arecomputed from the vertical angles and horizontal distances measuredin the field.

Contd . . .

DIFFERENTIAL LEVELING This method is used in order to find the difference in elevation between

two points when, they are too far apart the difference in elevation them is too great there are obstacles intervening

This method is also known to be fly leveling or compound or continuousleveling.

PROFILE LEVELING In this method, also called the longitudinal leveling or sectioning The object of this leveling operations is to determine the undulations of

the ground surface along a predetermined line, which is usually the centreline of a road, railway, canal or pipe line.

The line of section may be a single straight line, or may consist of a seriesof straight lines changing direction, or connected by curves.

The leveling operations always start from a bench mark and end on abench mark.

If the permanent bench mark is not near the line of section, flying levelsmay be run from the permanent bench mark to establish a bench marknear the line of section.

The following points should be borne in mind while running a profile. The chainage of the staff points is continuous from the beginning to

the end section line. The back-sight and fore-sight distances should be approximately equal. Readings on bench marks and change (or turning) points should be

taken to the nearest thousandth of a meter and those on intermediatepoints to the nearest hundredth of a meter.

The bubble must traverse, i.e. must be in the centre of its run when thebacksights and foresights are taken.

The positions of the features, such as a nalla, river, road, cart-track,foot-path etc. crossed by the section line should be completely locatedby noting the bearings of their centre lines, and their widths, or bymeans of offsets.

Contd . . .

PLOTTING THE PROFILE

CROSS SECTIONING Cross-sections are the sections run at right angles to the centre line

and on either side of it for the purpose of determining the lateraloutline of the ground surface.

For this purpose they are taken at each 30 m station on the centreline.

The length of the cross-sections depends upon the nature of thework, e.g. for a road it may be 30 to 60 m on each side of the centreline, and in the case of a railway, 200 to 300 m, or more on eitherside of the centre line.

PLOTTING THE CROSS-SECTIONS

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