lecture 1 precise levelling
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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