Levelling Recap

EQUIPMENT

• Automatic level

– Consist of a telescope that is fixed rigidly to

a spindle and is supported on three foot

screws

– The level is supported by attachment to a

tripod.

– Tripod

• level is supported by attachment to the

tripod

– Staff – metric rule

Telescopic

staff/stave

2.5

2.6

metres

decimetres

centimetres

Each centimetre block = 10mm.

These are the only part of the

readings that need estimating

What are the main applications of levelling?

Levelling

• The level is used is used for two main

purposes:

1. To establish the reduced level, which

is the height of a point relative to the

original starting datum, of existing

features and ground

2. To establish datums that can be used

for the vertical control of building work

Datum Terminology

• Ordnance Datum (O.D.)

– lowest point in the British Isles

– Newlyn in Cornwall = 0.000m

E.G. 34.239 is 34metres and 239mm

above the mean sea level at Newlyn

• Ordnance Bench Mark (O.B.M.)

– Marks made by ordnance surveyors on

permanent structures

– Usually cut into stone or brick in the shape

shown below

Ordnance

Level

OBM

Survey Reference System

SLA introduced the Co-ordinated Cadastre system.

The new system defines property boundaries by co-

ordinates, rather than bearings and distances.

SLA has established the land survey control network as the

infrastructure to support survey work conducted by all

Registered Surveyors. This allows all land lots in

Singapore to be geographically linked to each other and to

the whole of Singapore.

Survey Reference System

We maintain 2 types of survey control:

•Horizontal Control Network

•Vertical Control Network

Horizontal Control Network

There are two categories of horizontal control: Primary and

Secondary control network. As a whole, they are known as the

Integrated Survey Network (ISN).

Survey Reference System

Vertical Control Network

The vertical control network consists of physical points on the

ground known as Precise Levelling Benchmarks (PLBM). There

are about 400 PLBMs located at 1 km intervals along major roads.

PLBM information is used to determine heights of locations in

Singapore.

Survey Reference System

Positioning with Global Navigation Satellite System

(GNSS)

SLA maintains a network of continuously operating satellite

reference stations on the ground, known as Singapore Satellite

Positioning Reference Network (SiReNT) . SiReNT is a

nation-wide infrastructure established and maintained by SLA

that supports real-time and post-processed high precision

satellite survey and positioning applications. With SiReNT, the

establishment of new control points is seamless and easy.

• Temporary Benchmark (T.B.M.)

• A TBM is a level that is transferred

from the OBM to a location that is

more practical for the work that is

required i.e. nearer.

• TBMs should be located:

– For excavations, the TBM should be

located away from the excavation line, but

not be too far away

– Not located on a wall that is going to be

demolished during the works

– Not located on a pavement by the site if

deliveries are to be made

• Examples of how a TBM can be fixed

include:

– A mark on a wall

– A bolt cast into a concrete slab

– An existing feature such as a step

TBM

Level Reading Definitions

• There are only three types of reading that can be taken.

– Backsight readings

– Foresight readings

– Intermediate sight readings

Change Point

• When you can no longer see all the points that need to be picked up – you move the instrument

• To continue the link – the staff remains in place

• Staff then has a FS reading taken and a BS reading taken on it’s single position

Booking Sheet

BS IS FS H.o.I. R.LEVEL REMARKS

2.000 50.000 OBM 1.500

2.000

First reading to book is the Back Sight

Last reading to book

is the Fore Sight

If any detail is being picked up

An Intermediate Sight is used

Must always have A known start point

Booking Sheet

BS IS FS H.o.I. R.LEVEL REMARKS

2.000 50.000 OBM 1.500

1.500 2.000

At a change point – the staff doesn’t move So two readings are booked on the same line (same staff = same line)

O.B.M.

50.000m

A B C D E F

1st set up 2nd set up 3rd set up

point point point

BS IS IS FS

2.000 2.500 2.250 1.500

BS IS FS

3.000 2.500 2.750

BS FS

1.750 3.500

Booking Information

Have a go at booking this information

Example Booking

Buildings

Known datum

TBM required here

A

B

CD E

F

GHI

Booking Sheet BS IS FS H of

I R.L. Remarks

1.003 BS Reading on known datum

Level is at A 1.645 1.524 FS Reading at B when level at A

BS Reading at B when level at C 2.835 2.134 FS Reading at D when level at C

BS Reading at D when level at E 2.884 2.163 FS Reading at F when level at E

BS Reading at F when level at G 1.058 1.332 FS Reading at H when level at G

BS Reading at H when level at I 2.272 FS Reading onto known datum

when level at I

Calculating the Reduced Level

• RISE AND FALL METHOD

• HEIGHT OF COLLIMATION METHOD

• The Height of the Instrument is calculated every time the instrument is set up and used to calculate the reduced level.

b

A

if the RL at A is 50.000m and the staff reading at b is 1.200m, then the height of the instrument is 51.200m

Calculations • Once you have checked the accuracy of you

readings you can move on to do the calculations.

• There are only two formulae that you need:

Height of Instrument =

Reduced level + Backsight

Reduced Level =

Height of Instrument – Foresight or Intermediate Sight

Example

Back

Sight

Intermediate

Sight

Fore

Sight

H of I R.L. Remarks

1.003 25.444 BS Reading on

known datum

1.645 1.524 at B

2.835 2.134 at D

2.884 2.163 at F

1.058 1.332 at H

2.272 FS Reading

onto known

datum

Example

Back

Sight

Intermediate

Sight

Fore

Sight

H of I R.L. Remarks

1.003 26.447 25.444 BS Reading on

known datum

1.645 1.524 26.568 24.923 at B

2.835 2.134 24.434 at D

2.884 2.163 at F

1.058 1.332 at H

2.272 FS Reading

onto known

datum

B

C

OBM

BS

1.551

A1

IS

2.018

A2

IS

2.155

A4

FS

1.550

B1

BS

2.573

B2

IS

1.347

B3

IS

3.002

B5

FS

2.056

C1

BS

1.863

C2

IS

1.432

C3

FS

2.637

A

Level positions

Staff positions

Staff positions where

level is moved

A3

IS

1.096

B4

IS

2.675

D

E

D1

BS

2.425

D2

FS

1.288

E2

FS

2.039

E1

BS

2.068

F

F1

BS

1.811

OBM

FS

2.721

Book the information

Accuracy

Allowable Misclosure = ±5n mm

Must always close a survey onto a known point

where n = number of Instrument Positions (FS readings)

CHECK Sum BS – Sum FS = First RL – Last RL

If more than is allowable, the levelling should be repeated.

If less than, the misclosure is spread equally between the CP values

BS IS FS H.o.I. R.LEVEL REMARKS

0.824 39.220 OBM1

1.628

0.790

0.380

2.154 1.224

2.336

2.757

2.555 0.461

2.275

0.436

0.227

0.716

0.652

0.233 OBM2

BS IS FS H.o.I. R.LEVEL REMARKS

0.824 40.044 39.220 OBM1

1.628 38.416

0.790 39.254

0.380 39.664

2.154 1.224 40.974 38.820

2.336 38.638

2.757 38.217

2.555 0.461 43.068 40.513

2.275 40.793

0.436 42.632

0.227 42.841

0.716 42.352

0.652 42.416

0.233 42.835 OBM2

5.533 1.918 42.835

1.918 3.615 39.220 3.615