land surveying chapter 2 leveling p1

Land Surveying / Engineering Surveying I

Chapter 4 : Levelling (Part 1)

Levelling General term applied to any of the various processes by

which elevations of points or differences in elevation are determined.

A measurement process whereby the difference in height between two or more points that are some distance from each other can be determined.

Introduction

Levelling

Introduction

Levelling Levelling is done when we need to :

• Establish new vertical control (Bench Mark or Temporary Bench Mark).

• Determine the heights of discrete points. • Provide spot heights or contours on a plan. • Provide data for road cross-sections or volumes of earthworks. • Provide a level or inclined plane in the setting out of construction

works

Introduction

Levelling

Introduction

Levelling Levelling results are used to

(i) Design highways, railroads, canals, sewers, water supply systems and other facilities having grade lines that best conform to existing topography.

(ii) Lay out construction projects according to planned elevations.(iii) Calculate volumes of earthwork and other materials.(iv) Investigate drainage characteristic of an area.(v) Develop maps showing general ground configurations.(vi) Study earth subsidence crusted motion.

Introduction

3.1 Terminology and Definitions3.2 Levelling Equipment3.3 Theory of Levelling3.4 Forms of Levelling3.5 Levelling Procedure3.6 Errors in Levelling

Contents

Vertical at A

A Horizontal line at A

Level line through A

Ground surface

Mean sea level

3.1 Terminology and Definitions

Vertical line • A line that follows the direction of gravity as indicated by

a plumb line.


Horizontal plane• A plane perpendicular to the direction of gravity.• In plane surveying, a plane perpendicular to the plumb

line.


Horizontal line• A line in a horizontal plane.


Level surface • A curve surface that at every point is perpendicular to

the local plumb line (the direction in which gravity acts.)• Level surfaces are approximately spheroidal in shape.• A body of still water is the best example.


Level line• A line in a level surface (curved line).


Mean Sea Level (MSL)• The elevation of sea differs from station to station

depending on local influences of the tide.• A reference surface to which the heights of all points in a

survey or on a site are referred. • A surface which defines a datum height.


Vertical datum• Any level surface to which elevations are referred, e.g.

MSL.

Elevation• The vertical distance from a vertical datum, usually MSL,

to a point or object.• If the elevation of point A is 244.95 m, A is 244.59 m

above its adopted datum.


Bench mark (BM)• A relatively permanent object,

natural or artificial, having amarked point whose elevationabove or below an adopteddatum is known or assumed.

• Common examples are metaldisks set in concrete, referencemarks chiselled on large rock, etc.



Bench mark (BM)

Temporary Bench mark (TBM)• A point placed to provide

a temporary reference point. • Common examples are

peg, nail, spike, etc.


Levelling• An operation of determining the difference in elevation

between points on the earth’s surface.• A reference surface or datum is established and an

elevation assigned to it.• Differences in the determined elevations are subtracted

from or added to this assigned value and result in the elevations of any point.


Vertical control• A series of bench marks or other points of known

elevation established throughout an area, also termed basic control or level control.


Equipment frequently usedin levelling is as follows :• Level.• Tripods.• Levelling staff or rod.• Rod level.

3.2 Levelling Equipment

Level Level consists of some important

parts, namely telescope,focusing knobs, base, etc.


Tripods It supports the level base and

keeps it stable during theobservations.

It consists of a tripod head towhich the level’s base isattached, 3 wooded or metallegs which are hinged at thehead, and metal pointed shoeson each leg to press or anchorinto the ground to achieve afirm setup.


Levelling staff (levelling rod) The length of the instrument is

3, 4, or 5m. Different colours must be used

to show the graduation marksin alternate meters, the mostcommon colours being blackand red on a white background.


Levelling staff (levelling rod) Used for measuring distances

vertically above or belowpoints on which it is heldrelative to a line ofcollimation as definedby the level.

Since the staff is used tomeasure a vertical distance,it must be held vertically.


Levelling staff (levelling rod)


Rod level It ensures fast and correct

rod plumbing. Its L shape is designed to

fit the rear and side facesof a levelling staff, whilethe bull’s-eye bubble iscentred to plumb the rodin both directions.


3.3 Theory of Levelling

Height of Instrument (HI)• The height of instrument in levelling is the elevation or

the reduced level (RL) of the line of sight or line of collimation.

Reduced level (R.L.)• The R.L. of a point is the elevation of the point referred to

any datum which may be either assumed or which may be referred to the MSL.

• The height of a point above the datum.


Back sight (BS)• Back sight (BS) is used to denote the staff reading kept

over a point whose elevation is already known.• The terms back have nothing to do with direction.• Always the first reading from a new instrument station.

Fore sight (FS)• Fore sight (FS) is the staff reading kept over a point whose

elevation is required to be known.• The terms fore have nothing to do with direction.• Always the last reading from the current instrument

station.


Intermediate point• All points in between the BS and FS whose elevation is

required to be determined are termed intermediate points.

• This reading is called intermediate sight (IS).• Any sighting that is not a back sight or foresight.

Change point (CP) or turning point (TP)• A point which the level staff is held for the fore sight and

back sight.


Rise and fall• The difference in elevation at any point which is

determined with respect to its preceding point.

Arithmetical check• A check made in order to check that the readings are

properly booked or entered in their correct respective column.

• Can also be done for correct calculation of R.L..


Inverted staff reading• When R.L. of point above

HI is required, the staff issimply held upside downon the point and thereading is book with anegative sign.

• The reading is calledinverted staff reading.


Inverted staff reading• Levelling operation with inverted staff reading



Rise and Fall method• The rise or fall in elevation at any point is determined

with respect to its preceding point.

Rise or Fall = BS – FS = BS – IS1

= IS1 – FS = IS1 – IS2

Arithmetical check :Σ BS – Σ FS = Σ Rise – Σ Fall

= Last R.L. – First R.L.

Height of the Plane of Collimation method (HPC method)• The levels of the various points are computed by

deducting the staff readings at those points form the R.L. of the line of sight in that section.

HI = R.L. of BM + BSR.L. of B = HI – FS

Arithmetical check :Σ BS – Σ FS = Σ Rise – Σ Fall

= Last R.L. – First R.L.


Observation ProcedureFigure below illustrates a typical levelling situation, where the reduced levels of several points B, C, D, E, F and G are to be determined relative to a point A which is the bench mark. The levelling is to be closed on a second bench mark H.

The instrument has to be setup up twice in this particular case, although in a practical levelling exercise there could be many more set-up points.


Observation ProcedureEvery time the instrument is set up, the FIRST sight taken from

that position is called BACK sight (BS). Likewise the LAST sight taken, prior to moving the instrument, is called a FORE sight (FS). Thus, in set-up number 1, point A is a backsight, while point E is a foresight. Any other sight observed between backsight and foresight is an INTERMEDIATE sight (IS). Points B, C and D are therefore intermediate sights.


Observation ProcedureAt set-up number 2, the sight taken to point E is a BS; point F is

an IS; point G is an IS; finally point H is a FS. It should be noted that sight G is taken to the underside of a beam, which is higher than the instrument. In such a case, the staff is held upside down against the point while the reading is taken. Such a sight is called an inverted staff reading. Point E, where a foresight followed by a backsight is taken, is called a change point.


Observation ProcedureBoth readings are entered on the same line of the field book.

Each point is given a separate line in the field book and its reading is entered on that line in its respective column, either BS, IS or FS.

At each point of thesurvey, the staffholder holdsthe staff on the mark andensures that it is heldvertically (guided by therod level), facing towardsthe instrument.


Observation ProcedureThe observer directs the telescopetowards the staff and using thefocusing screw, brings the staffclearly into focus.

Parallax should already have been eliminated in setting up the instrument in which case there should be no apparent movement of the cross-hairs when the head is moved up and down.

The observer then reads the figures on the staff and enters the reading on the appropriate line and column in the field book. The reading is taken once more and checked against the field book entry.


Rise and Fall Method


BS IS FS Rise Fall Reduced Level Remarks0.850 0.650 Bench Mark A

1.500 0.65 0 B

1.050 0.45 0.450 Bench Mark C

HPC Method


BS IS FS HPC Reduced Level Remarks

2.400 207.900 205.500 Peg A

1.800 206.100 Peg B

Simple Levelling• The elevations of various points are determined from 1

set of the instrument as in case of levelling a building site or for establishing some bench marks nearly.

• The level is not shifted and is to be used when the area is small and for taking the levels of an existing structure.

Compound Levelling (Differential Levelling)• To be carried out for a great distance and it is not possible

to do the levelling from 1 set up of the instrument, the level will have to be shifted.

3.4 Forms of Levelling

Longitudinal or Profile Level• A type of compound levelling in which the undulations of

the ground along the center line of a proposed road or canal or for any other type of road survey are required.

• Its objective is to give the elevations at known distance apart and then obtain the profile along a given line.


Reciprocal levelling• In levelling, it is essential and desirable that the level

should be set up mid-way between the BS and FS, so that error due to the line of collimation not being perfectly horizontal is eliminated.

• Therefore, this levelling is applied if it is not possible to keep the level exactly mid-way due to river, deep gutter or a ravine intervening.


Reciprocal levellingExtra notes :• Accurate determination of the difference in level

between two points by setting up the instrument near each of the points in succession and taking observations of staff readings on both.

• It is useful when it is not feasible to set up the level midway between the points.


Reciprocal levelling


Reciprocal levelling• This procedure is used for either differential or

trigonometric leveling when along sight across a wide river, ravine, or similar obstacle must be made.

• This long sight will be affected by curvature and refraction and by any small error in aligning the line of sight with the bubble axis.


Reciprocal levelling• The alignment error can be minimized by balancing the

long sight and computing the curvature.• The atmospheric conditions will vary so much over an

open expanse that the refraction correction will be quite erratic.

• Reciprocal leveling is desired to minimize the effect of the atmosphere as well as the line of sight and curvature corrections.


Reciprocal levelling


Flying levelling• A reconnoissance level over the course of a projected

road, canal, etc.• A method when a bench mark of known elevation is

available, a series of levels may be run along a general direction to reach some other point whose level can be determined and fixed as a temporary B.M., if necessary.

• It is a very approximate form of levelling in which no distance are measured and distances as long as possible are taken.


Flying levelling• It is also employed for the determination of approximate

levels of different points.• In such cases, the distances as long as possible between

the level and staff are measured.• It is done where rapidity but low precision is required. It

is normally used for reconnaissance surveys or for approximate checking of the level.


Precise levelling• A very accurate method of differential levelling for

establishing bench mark or where high precision is required. Refine instruments and procedures enhance the accuracy of the work. It is a specialized form of levelling and is usually conducted by government agencies like Survey of India.

• It is carried out for establishing some permanent bench marks or some important works where greatest precision is required.


Cross levelling• When it is required to know the nature of the ground on

either side of the centre line of the proposed route, levels are taken at right angles to the general direction of the proposed alignment.

• Cross section are laid at right angles to proposed direction of the road and at suitable distances and levelling carried along this cross section.

• Generally, the profile levelling and cross-section levelling are done simultaneously, to help in estimation of the earthwork involved in a route.


1) Setting up a level2) Duties of a rodperson (staffman)3) Booking the readings

(Rise & Fall method or HPC method)4) Arithmetical check

3.5 Levelling Procedure

Gross errors Wrong staff readings.

• Misplacing the decimal point, reading the wrong meter value and reading the staff wrong way up.

Using the wrong cross-hair• The observer reads against one of the stadia lines. • This error is common in poor visibility.

3.6 Errors in Levelling

Gross errors Wrong booking

• The reading is noted with the figure interchanged, e.g. 3.020 m instead of 3.002 m.

Omission or wrong entry• A staff reading can easily be written in the wrong column or

even omitted entirely.


Constant errors Non-verticality of the level staff.

• If the staff leans forward or backward, it is not easily be detected by the staff man.

• The error can be eliminated by fitting the staff with a circular spirit level or rod level.

Collimation error in the instrument• If the line of sight is not perfectly horizontal when the bubble

of the spirit level is central, there will be a constant error in the staff reading.

• The error can be entirely eliminated by making BS and FS equal in length.


Constant errorsEliminating the collimation error :


Random errors Effect of wind and temperature

• The stability of the instrument may be affected, causing the height of collimation to change slightly.

Soft and hard ground• When the instruments set on soft ground it is likely to sink

slightly as the observer moves around it. Change points

• At any CP, the level staff must be held on exactly the same spot for both FS and BS.

Human deficiencies• Errors arise in estimating the millimeter readings, particularly

when visibility is bad or sights are long.


If the levelling is carried out between two known stations, e.g., from 1 BM to another BM), the difference between the calculated and known values of the R.L. of the final BM is called misclosure.

where n is number of instrument positions.

3.7 Misclosure & Adjustment

If the actual misclosure is found greater than the allowable misclosure, the levelling should be repeated.

If the misclosure is within the allowable misclosure, then it is distributed throughtout the reduced levels.

The usual method correction is to apply an equal, but cumulative, amount of the misclosure to each instrument position.

3.7 Misclosure & Adjustment

land surveying chapter 2 leveling p1

Documents