CE 371 Surveying

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LEVELLING PROCEDURES

Dr. Ragab Khalil

Department of Landscape Architecture Faculty of Environmental Design

King AbdulAziz University Room LIE15

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Overview

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• What is leveling

• Leveling Applications

• Leveling Theory

• Leveling Equipments

• Leveling procedures

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What is leveling

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• A measurement process whereby the difference in height between two or more points can be determined

• A measurement process whereby the elevation of points can be determined

Elevation is a vertical distance above or below a reference Datum.

A (10.00 m)

C (-3.00 m) B (4.00 m) Height difference

Elevation

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When do we level?

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Typical examples include :

• To establish new vertical control (BM or TBM)

• To determine the heights of discrete points

• Design highways, railroads, canals, sewers, and water supply systems.

• To provide spot heights or Develop contour

maps showing general ground configuration

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When do we level?

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Typical examples include :

• To provide data for road cross-sections or volumes of earthworks

• To provide a level or inclined plane in the setting out of construction work

• Investigate drainage characteristics of an area

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Definitions

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• Datum



A reference surface to which the heights of all points in a survey or on a site are referred



May be arbitrary or a national height datum



The surface which defines the national height datum is (approximately) Mean Sea Level (MSL)

In Surveying, the reference Datum is the mean sea level (MSL).

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Leveling terms

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Wolf/Ghilani, Elementary Surveying: An Introduction to Geomatics, 10e, c2002, Prentice Hall

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More definitions

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• Level surface

 A curved surface that at every point is perpendicular to the plumb line.

 It is almost ellipsoidal in shape.

 Mean sea level is a level surface.

 All points on a level surface have equal elevations.

 Level surfaces are not parallel due to ellipsoidal earth shape.

• Horizontal surface

 A horizontal surface will be tangent to a level surface

 A plane normal to the direction of gravity

 Over short distances (<100 m) the horizontal surface and the level surface will coincide

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More definitions

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• Vertical Line :

A line that follows the direction of gravity as indicated by a plumb line.

• Level Line:

A line in a level surface, therefore, a curved line.

• Horizontal Line:

A line in a horizontal plane. In plane surveying, a line perpendicular to the vertical line.

• Elevation:

The vertical distance from a datum to a point.

• Bench Mark (BM):

A relatively permanent object, natural or artificial, having a marked point whose elevation is known or

assumed.

• Vertical Control :

A series of bench marks or other points of known elevation established throughout precise leveling.

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Definitions (cont.)

limit of practical

coincidence (~100 m) direction of gravity

level surface

horizontal surface

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Curvature and refraction

• Earth Curvature

 Earth curvature causes the horizontal plane at a point to depart from the level surface as one goes away from the point.

 C=0.0785 k²

 (C: curvature effect in m & k: distance in km)

• Atmospheric Refraction

 Atmospheric Refraction causes a light ray passing through earth's atmosphere to bend or refract toward the earth's surface.

 R= .011k²

 (R: refraction effect in m & k: distance in km)

• Curvature and Refraction Effect

 Notice that curvature effect increases rod reading, while refraction effect decreases it

 CR=0.0675 k²

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Curvature and refraction

• (R+C)² =R² + k²

• R² + 2RC +C² = R² + k²

• C(2R+C) = k²

• C=k²/ (2R+C)  k²/ 2R

• C = (r+h) = k²/ (2x 6370)

• C = 0.0785 k²(m)

• ^{r = 0.14 C}

• h = 0.0675 k²(m)

level surface

horizontal surface

o R

K A

E

B r

h Line of Sight

R

C

R = Mean Radius of Earth

= 6370 km

C = Effect of Curvature r = Effect of refraction h = Effect of Curvature

& refraction

k = length of Sight ( km)

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Ex.

A level instrument at point A is used to sight level rods at B &

C. AB=385m, AC=210 m, rod readings are: r

_B

=3.874 m, &

r

_C

=1.865 m. Compute the combined curvature and refraction correction and the corrected values of r.

Solution:

Combined correction (for point B) = CR_B = 0.0675(385/1000)2 = 0.010 m Corrected rod reading at B = r_B - CR_B = 3.874 - 0.010 = 3.864 m

Combined correction (for point C) = CR_C = 0.0675(210/1000)2 = 0.003 m Corrected rod reading at C = r_C - CR_C = 1.865 - 0.003 = 1.862 m.

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Leveling methods

• Taping:

• Barometric Leveling

• Differential Leveling

• Trigonometric Leveling

• Photogrammetry

• Satellite and Inertial Systems

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Leveling Theory

To perform the leveling we need a horizontal plane, we get it by using the level

We need also a vertical scale to measure the vertical distance from the earth surface to that horizontal plane. This scale is known as Staff

Level Staff

Plane of Collimation

Elevation Reading

HI = Elev (A) + BS

_A

A (10 m)

B C

MSL (0.0 m)

BS IS FS

HI

Elev (B) = HI - FS

_B

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11.25 2.10

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Equipment

• Level

• Tripod

• Staff

• Change plate

• Staff bubble

• 50 m tape measure (sometimes)

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The level

• Level parts

• Definitions

• Level types

• Level setup

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The level

Vertical axis

Bubble

Base

Leveling screw

object lens eyepiece

A surveying optical telescope

cross hairs

base plate

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More definitions

• Horizontal line



The line that path through the centres of the object lens and the eyepiece.

• Line of sight



The line that path through the centre of the object lens and intersection of the cross hairs.

• Line of collimation



The line of sight when it coincides with the

horizontal line.

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Collimation error

• Occurs when the line of sight (as defined by the cross-hairs) is not horizontal

• Leads to an incorrect staff reading

horizontal line

error

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Types of level

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Tilted Level

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Automatic Level

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compensator

Compensating Prism in Automatic Level

Automatic compensator orients line of sight in a horizontal plane

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Precise Level

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Digital Level

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Laser Level

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Level setup

1. Adjust the Bubble

2. Focusing

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Tripod

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ةماقلا

1.7 1.8

1.9 II

The staff

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Reading an E-type levelling staff

Read value at the

horizontal cross hair

The value is ?

1.900

1.910

1.920

1.930

1.932

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1.100

1.110

1.120

1.130 1.133

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More definitions

• Reduced Level (RL) {Elevation}



The height of a point above the datum

• Benchmark (BM)



A stable reference point of known RL



Usually used as the starting and finishing point when levelling

• Temporary Bench Mark (TBM)



A point placed (e.g. peg, nail, spike) to provide a

temporary reference point

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More definitions

• Backsight (BS)



Always the first reading from a new instrument station

• Foresight (FS)



Always the last reading from the current instrument station

• Intermediate sight (IS)



Any sighting that is not a backsight or foresight

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More definitions

• Change point (CP) {Turning Point (TP)}



Location of the staff when the level is moved



Change points should be...

 Stable

 Well defined

 Recoverable

 e.g. sharp rock, nail, change plate, etc...

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Rules for levelling

• Always commence and finish a level run on a Benchmark (BM or TBM)

• Keep foresight and backsight distances as equal as possible

• Keep lines of sight short (normally < 50m)

• Never read below 0.5m on a staff (refraction)

• Use stable, well defined change points

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Levelling procedures

• Three stages:



Observation procedures



Booking procedures



Reduction procedures

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Observation procedures

• The level instrument is set up and leveled approximately halfway between the point

• A backsight (BS) is taken on a rod held on point 1.

• A foresight (FS) is taken on a rod held on point 2.

• In many situations, one instrument setup is not enough, therefore, several intermediate points or Turning points must be established.

Each turning point is sighted twice, first as a FS from the setup before, then as a BS from the setup after

1

2 3

4

1.235 3.148

2.352 1.869 0.725

Elev. 15 m

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Booking procedures

point BS IS FS HI Elev. Adj.

Elev.

1 (BM) 1.235 15.000

2 (TP) 2.352 3.148

3 1.869

4 0.725

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Reduction procedures

point BS IS FS HI Elev. Adj.

Elev.

1 (BM) 1.235 15.000

2 (TP) 2.352 3.148

3 1.869

4 0.725

16.235

13.087 15.439

13.570 14.714

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Summery

• What is leveling

• Leveling Applications

• Leveling Theory

• Leveling Equipments

• Leveling procedures