Lecture 1 precise levelling


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Lecture 1 precise levelling

  2. 2. 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
  3. 3. 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
  4. 4. 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???
  5. 5. 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 : h = a - b If the RLA is known, RLB can be calculated as: RL B = RLA + h
  6. 6. Memory Test… Which is what??? Staff Staff Level ??? a b Level line through level instrument Level Line through B B Level Line through A A ??? ??? ???
  7. 7. 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
  8. 8. 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
  9. 9.  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
  10. 10.  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.
  11. 11. National Geodetic VerticalDatum (NGVD) JUPEM initiated the establishment of NGVD 12 tidal stations were established in 1981Objectives: 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
  12. 12. 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
  13. 13. Tidal Station
  14. 14. Types of Levelling LEVELLING Precise levelling Ordinary levelling Optical level Motorized Optical Level Digital level level Digital levelDifferentiated by the set of instruments, observation methods and accuracy
  15. 15. 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
  16. 16. 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
  18. 18. 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)
  19. 19. Example
  20. 20. Components1. Parallel Plate Micrometer To be used with precise level To provide precise reading (decimal of mm) Micrometer readings
  21. 21. Components2. Compensator Available for automatic level only To compensate error on collimation line
  22. 22. Components3. 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
  24. 24. 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
  26. 26. 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
  27. 27. ExampleLeica DNA 03 Digital Precise Level
  28. 28. Leica DNA 03 Digital Precise Level Elements:1 On/ off button 16 PCMCIA or CF-card with2 Base plate adapter (optional)3 Foot screws 17 Battery GEB121 (optional)4 Horizontal circle 18 Battery adapter GAD39; 65 Lever to unlatch battery single cells (optional)6 Battery compartment 19 Light duct for circular level7 Button to unlatch card compartment 20 Plug stopper for crosshair cover adjustment knob8 Card compartment cover 21 RS232 serial interface9 Display with external power supply10 Circular level 22 Measuring button11 Hand grip with aiming sight 23 Focusing drive12 Ocular 24 Endless horizontal drive13 Keyboard (bi-directional)14 Objective15 Battery GEB111 (optional)
  29. 29. 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.
  30. 30. Change PlatesBipodsADDITIONAL EQUIPMENT IN PL
  31. 31. 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)
  32. 32. 2. Handles/Bipods  To support precise staff  For long observation period  Stable (verticality)