lecture 1 precise levelling

PRECISE LEVELLING

INTRODUCTION AND INSTRUMENTATION

SUG213 : ENGINEERING SURVEYING II

Lecture By:Zuraihan MohamadDept. of Surveying Sciences & GeomaticsFSPUUiTM Arau

Aim Of The Topic

At the end of this lecture, student should be able to :

Understand what is precise levelling and the needs.

Understand the methods and instrumentation of precise levelling

Lecture content

Concept of levelling

Historical background of Precise Levelling in Malaysia

National Geodetic Vertical Datum (NGVD)

Malaysia Tidal Network

Types of Levelling

Precise Levelling

Equipment of precise levelling Optical precise levelling Motorized l precise levelling Digital precise levelling

QUESTION… Before we begin

What is P.L?

Why do we need P.L?

Are there any differences between P.L n O.L??

Say that that there are indeed differences, they are in terms of what???

Concept of Levelling

The concept of levelling is based on the horizontal line of the instrument (level) with the level line of the point on the earth.

If the heights of A and B can be measured, the height differences can be calculated using the formula :

If the RLA is known, RLB can be calculated as:

h = a - b

RL B = RLA + h

Memory Test… Which is what???

a

Level

b

Level line through level instrument

Level Line through B

Level Line through A

A

B

Staff Staff

???

??????

???

Concept of Levelling

The name given to the method of determining differences in heights between points

A method of height transfer from one point to another

Being used in Geodetic and Engineering Surveys

Using special instrument called : level

Historical Background

Several methods of height measurement are being used in surveying and construction works;

Trigonometric heighting - Theodolite

Barometric heighting - Barometer (pressure)

Hydrostatic Levelling - Water tube

Tacheometry - Staff

GPS - Receiver

Direct measurement - Tape

Levelling - Level

1st vertical datum was established in 1912 based on Mean Sea Level (MSL) produced by British Admiralty.

At Port Swettenham (Port Kelang)

1 year tidal observations

Also known as Land Survey Datum (LSD)

But no records and evidence available

Mean Sea Level

Average level taken up by the sea Coincide with the Geoid Change regularly due to tide Best observation period is 18.6 years

Use of Tide Observation Data

Determine precise vertical datum Information for research in geodesy, geodynamic and scientific studies Tide & flood prediction Port activities and navigation Marine boundaries, hydrography and aquaculture Delivery of fixed record of sea level To obtain tidal harmonic constant To study tidal characteristics For tidal prediction.

National Geodetic Vertical Datum (NGVD)

JUPEM initiated the establishment of NGVD

12 tidal stations were established in 1981

Objectives:

To observe tide levels continuously

18.6 years complete cycle of moon regression

To obtain tidal harmonic constants

To study tidal characteristics For tidal prediction

Malaysia Tidal Network

1995 (established n in operation) - 21 tidal stations

12 are installed in Peninsular and 9 in east Malaysia

Each station is connected by precise levelling networks

Tidal Station

Types of Levelling

LEVELLING

Precise levelling Ordinary levelling

Optical level Motorized level

Optical Level Digital level

Differentiated by the set of instruments, observation methods and accuracy

Digital level

Precise Levelling

Also known as the highest order of levelling works

Readings observed and recorded to decimals of a millimeter

Used for :

Basic levelling framework of a country

Transfer height to bench marks

Precision engineering structure Irrigation Scheme, Dam, Tunnels

Precision dimensional surveys

Equipments of Precise Levelling

Level (Precise type)

Invar or bar-coded staff

Survey Tripods

Change plate (staff support)

Staff bubble

Handles and steadying rods (bipod)

Thermometer

Umbrella

OPTICAL PRECISE LEVEL

1. Optical Precise Level

Precise type With parallel plate micrometer

Manufacturer quote: “Std dev less than 1 mm per double run of levels over a km” can be considered as precise

Glass diaphragms (eye piece) – vertical line, levelling line and two stadia lines (upper and lower)

Example

1. Parallel Plate Micrometer

To be used with precise level

To provide precise reading (decimal of mm)

Components

Micrometer readings

2. Compensator

Available for automatic level only To compensate error on collimation line

Components

3. Precise Staff

Two precise/Invar staffs

Invar Strip – stable material (low sensitivity to heat)

Small expansion coefficient

Graduations 0.02ft / 0.01 m on invar strip

With handle / staff holder

To be calibrated every 3 months

Components

MOTORIZED PRECISE LEVEL

2. Motorized Precise Level

Were used in the DSMM (Late 80s) - No longer used

Modification of conventional method

Three vehicles – 1 for the level and observer, 2 for the staffs

Invar staffs are fitted to the vehicles

Advantage:

Faster and convenient

Disadvantage:

High cost

Not suitable in busy roads

DIGITAL PRECISE LEVEL

3. Digital Precise Level

Use digital level (automatic level) and bar-coded staffs

Use infrared detector to scan the bar-coded staff

Scanned staff image is compared to actual staff pattern stored in the instrument

Provide staff reading and horizontal distance

Advantages:

Levels are recorded automatically

Reduce human error (reading and booking)

Reduce observation time

Include processing software

Example

Leica DNA 03 Digital Precise Level

Elements:

1 On/ off button

2 Base plate

3 Foot screws

4 Horizontal circle

5 Lever to unlatch battery

6 Battery compartment

7 Button to unlatch card compartment

cover

8 Card compartment cover

9 Display

10 Circular level

11 Hand grip with aiming sight

12 Ocular

13 Keyboard

14 Objective

15 Battery GEB111 (optional)

16 PCMCIA or CF-card with

adapter (optional)

17 Battery GEB121 (optional)

18 Battery adapter GAD39; 6

single cells (optional)

19 Light duct for circular level

20 Plug stopper for crosshair

adjustment knob

21 RS232 serial interface

with external power supply

22 Measuring button

23 Focusing drive

24 Endless horizontal drive

(bi-directional)

Leica DNA 03 Digital Precise Level

Leica DNA 03 Digital Precise Level

Characteristics:

Limited target distance <30m

Minimum ground clearance of >0.5m required to minimized refractionary influences of ground proximity.

Double observance (BFFB, aBFFB) to increase the reliability of measurement and to reduce possible errors caused by staff sinking.

Applying alternating observations procedures (aBFFB = BFFB FBBF) to eliminate horizontal tilt ( residual error of the automatic compensator)

Use an umbrella in strong sunlight.

ADDITIONAL EQUIPMENT IN PL

Change PlatesBipods

1. Base/Change Plate

Staffs are to be supported on turning points

Made from mild steel

Round head and collars

To reduce error during turning (for soft ground)

To support precise staff

For long observation period

Stable (verticality)

2. Handles/Bipods