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E4014 Construction Surveying

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At right angles to these points a band of low water will lie along the meridian ... an object by timing the two way journey of pulse of sound energy and converting ... – PowerPoint PPT presentation

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Title: E4014 Construction Surveying


1
E4014 Construction Surveying
  • Hydrographic Surveys

2
Charts And Maps
  • Hydrographic Chart
  • an information medium and a tool for maritime
    traffic for the safety and ease of navigation
  • contains information on
  • least water depths
  • nature of ocean bottom
  • sea and swell conditions
  • surface navigational dangers
  • other navigational significant data

3
Charts And Maps
  • Hydrographic Chart (cont)
  • Also contains some non-coastal information
  • mariner is interested in the general shape of
    features that can be seen from the water
  • accurate height required only when judging at
    what distance a feature may be visible

4
Charts And Maps
  • Hydrographic Chart (cont)
  • only show topographical and cultural features
    when they will aid
  • a safe landfall upon an unknown coast and to
    safely enter a harbour
  • manoeuvring and docking within a harbour
  • the conduct of efficient harbour business

5
Charts And Maps
  • Hydrographic Chart (cont)
  • Three distinctive features
  • working space
  • chart is a navigational plotting and recording
    board
  • overlap
  • enables projection of a course from one chart to
    the next using connecting points of navigational
    importance
  • scale and shape

6
Charts And Maps
  • Hydrographic Chart (cont)
  • Pilot publications
  • contains information which cannot be included on
    the chart because of space legibility
  • Notice to Mariner publications
  • recently acquired information
  • alerts to navigational hazards

7
Charts And Maps
  • Topographic Maps
  • Depicts the physical form ( in two dimensions )
    of the surface of the earth and features
    resulting from human occupancy
  • information is
  • qualitative as to location and use
  • quantitative as to area and volume
  • format is rectangular with minimum overlap
  • use is for activities on land and geographical
    studies

8
Charts And Maps
  • Bathymetric Maps
  • Topographic maps of a nations submerged lands
  • depict the shape of the sea floor through the use
    of isobaths
  • do not show navigational information or shipping
    hazards

9
Charts And Maps
  • Bathymetric Maps (cont)
  • Primarily used as an aid to scientific studies
    associated with
  • marine mining
  • oil and gas exploration
  • coastal zone management
  • pollution control
  • boundary and other international jurisdictional
    matters
  • engineering and research purposes

10
Soundings
  • Sounding
  • Measurement of the vertical depth from the level
    surface of the water to the bed of the lake,
    river or sea
  • a series of soundings whether taken at random
    points or on a grid can be used to prepare a plan
    showing the topographic features of the land
    covered by the water

11
Soundings
  • Sounding (cont)
  • simplest case
  • sounding survey of a lake with no tidal or wave
    pattern
  • water would be assumed to be level and accepted
    as datum for the soundings i.e. RL 0.00m
  • soundings would be taken in a systematic way and
    the fix positions are plotted
  • depth of the soundings are plotted so that the
    decimal point represents the the position of the
    sounding on the chart e.g.
  • 6.5 would indicate a depth of 6.5m at that point
    on the chart
  • 6.5 indicates 6.5m above datum

12
Soundings - Sounding Datums
  • In coastal areas soundings are normally related
    to a tidal datum such as Mean Sea Level
  • Hydrographic charts must always have a lowest
    water as their datum
  • mariners reading a depth of 10.0 m expect a boat
    drawing 9.0m to be safe during all tide ranges

13
Soundings
  • Sounding Datums (cont)
  • Each sounding made on tidal water must be
    corrected for the height of the tide at the time
    the sounding was taken
  • a tide board is attached to a suitable structure
    and set vertically in the water in the vicinity
    of the survey area
  • preferable to be visible from the boat
  • tide board readings and the time are observed and
    recorded at constant intervals e.g.15min

14
Soundings
  • Sounding Datums (cont)
  • the zero mark on the tide board is determined by
    levelling from a BM
  • differences between AHD and Mean Sea Level are
    available and the soundings can be corrected
    accordingly

15
Soundings - Sounding Datums (cont)
  • conventional levelling found that
  • the 3.000 graduation on the gauge is exactly 19
    metres below the BM.
  • Therefore the zero mark on the tide board has an
    RL of -2.375m.
  • Conversely the datum (AHD) zero is at 2.375m on
    the tide board.

16
Soundings - Sounding Datums (cont)
  • During a sounding survey the tide board was
    observed every 15 minutes. The results of
    soundings taken during one 15 minute period are
    shown below. Reduce the soundings to AHD

17
Soundings - Sounding Datums (cont)
(2)
Interpolate with respect to time
18
Soundings - Sounding Datums (cont)
(2)
19
Tides
  • Caused by the combined gravitational effects of
    the sun and moon, with the moon having the major
    effect
  • influenced by
  • terrestrial gravity
  • earths rotation
  • land masses
  • weather systems

20
Tides - Semi-Diurnal Tide
  • Consider the earth to be stationary and covered
    with a layer of water. Assume that the position
    of the moon is fixed such that it has zero
    declination

21
Tides - Semi-Diurnal Tide
  • On the side of the earth nearest the moon there
    is a net force towards the moon
  • whilst on the opposite side there is a net force
    away from the moon.
  • The small net forces at the North and South poles
    will be towards the centre of the earth.

22
Tides - Semi-Diurnal Tide
  • At the two points on the equator, nearest and
    farthest from the moon there will be permanent
    high water.
  • At right angles to these points a band of low
    water will lie along the meridian on either side
    of the earth

23
Tides - Semi-Diurnal Tide
  • When the moon moves into perigee (closest point
    to the earth during the moons elliptical orbit)
    the high waters will be higher and the low waters
    will be lower.
  • The opposite effect occurs when the moon moves
    out to apogee (most distant point)
  • These minimum and maximum tides are known as
    perigean and apogean respectively

24
Tides - Semi-Diurnal Tide
  • When the earth rotates the four tides, two high
    and two low, move around the earth, in a 24 hour
    period.
  • This is known as a semi-diurnal tide
  • the poles have a permanent low tide

25
Tides - Diurnal Tide
  • The moon's declination causes inequalities in the
    tides that occur.
  • successive high tides at a point on the earth
    with a latitude equal to X (and Y) will NOT be
    equal.
  • The high tide at X will not be as high as that at
    Y

26
Tides - Diurnal Tide
  • At all points on the earth where the latitude on
    earth is greater than the moons co-declination,
    there is only one high tide and one low tide per
    day.
  • This is known as the diurnal tide
  • The form of tide varies between the extremes of
    diurnal and semi-diurnal, with those exhibiting
    both characteristics being known as 'mixed'

Moons co-declination
27
Tides - Spring Tide
  • The sun has a similar tide producing effect on
    the earth to that of the moon. However the forces
    involved are not as great.
  • At new and full moon the sun, moon and earth are
    nearly in a straight line. The tide raising
    forces act together to produce tides with a large
    range. These tides occur fortnightly and are
    called spring tides

28
Tides - Neap Tide
  • At the moon's first and last quarter the moon's
    tide raising force is to some extent counteracted
    by the sun's tide raising force producing tides
    with a small range.
  • These tides are called neap tides (neap being
    high water at its lowest value).

29
Soundings
  • Tide Levels
  • Mean Sea Level ( MSL )
  • average height of the sea in all states of the
    oscillation
  • equivalent to the level which would exist in the
    absence of all tidal forces
  • approximates the geoid

30
Soundings
  • Tide Levels (cont)
  • Mean Tide Level ( MTL )
  • average value of the heights of high and low
    water
  • Mean High Water Springs ( MHWS ) and Mean Low
    Water Springs ( MLWS )
  • average values derived from a sufficiently long
    series of high water springs and low water
    springs

31
Soundings
  • Tide Levels (cont)
  • Mean High Water Neaps ( MHWN ) and Mean Low Water
    Neaps ( MLWN )
  • average values derived from a sufficiently long
    series of high water neaps and low water neaps

32
Soundings
  • Tide Levels (cont)
  • Highest Astronomical Tide ( HAT ) and Lowest
    Astronomical Tide ( LAT )
  • highest and lowest levels that can be predicted
    to occur under average meteorlogical conditions
  • may not be reached each year
  • do not take storm surges into account and hence
    considerably higher and lower levels may still
    occur

33
Soundings
  • Tide Levels (cont)
  • Indian Spring Low Water ( ISLW )
  • level suggested by Sir Charles Darwin for Indian
    waters
  • mathematically derived from tide data

34
Tide Gauges - Tide Board
  • merely a vertical staff with a broader face than
    a levelling staff. This may be graduated every 5
    or 10 centimetres, according to the accuracy
    required. The tide board is often difficult to
    read due to the surface chop or wave action

35
Tide Gauges - Float Gauge
  • A float is attached to the bottom of a staff.
  • The staff is then held in a box which is fixed in
    a vertical position.
  • Only the bottom of the box is open to the sea.
  • The float rises and falls with the tide, the
    staff running up and down through guides on the
    inside of the box.
  • The staff can be read through a special
    inspection opening in the side of the box.

36
Tide Gauges - Automatic Tide Gauge
  • normally established permanently at an official
    tide station.
  • The tidal fluctuations are recorded on a chart
    attached to a drum which revolves with time
  • The gauge may need to be visited only once every
    seven days to change the paper chart and reset
    the drive mechanism, i.e. wind the clock

37
Tide Gauges - Water Pressure Tide Gauge
  • Operates on the changes of water pressure due to
    tide rising and falling
  • completely self contained instrument designed to
    measure and record tidal movements when mounted
    on an underwater offshore structure or on the sea
    bed
  • Due account must be made for barometric pressure
    reading and the necessary corrections applied

38
Establishing Tide Levels
  • If tide observations are made over a period of
    time, statistical values for various types of
    tide can be arrived at, such as mean sea level
    (MSL), MHWS, MLWN, etc.
  • The degree of variation that can occur at a point
    for observations taken over different time
    periods will of course depend upon the range of
    the tide at that point.

39
Establishing Tide Levels
  • As a guide, the following figures are
    appropriate
  • Figures based on one full day's observation may
    vary considerably from observations made on
    another day as meteorological conditions play a
    big part.
  • Figures based on one full lunar month's
    observations (291/2 days) may vary by up to 25cm
    from values taken from another month's
    observations on the East coast of Queensland.
    This figure is the semi-annual and annual
    variation of MSL.
  • Figures based on one full year's observations may
    vary from values taken from another year's
    observations.

40
Establishing Tide Levels
  • Thus it can be seen that to obtain the figure for
    tidal variation at a point observations should be
    taken over at least one lunar month.
  • More accurate results are obtained when
    observations are taken over a full year.
  • The most accurate results cannot be obtained
    until continuous observations have been made over
    at least 19 years.

41
Establishing Tide Levels
  • Ships are concerned with the least amount of
    water that may be below them.
  • the datum for depths at sea, and navigation
    charts is normally a low water datum.
  • Two datums which have been used in the past for
    navigation charts are
  • LWST and ISLW,
  • both of which are to be progressively replaced by
    LAT.

42
Soundings
  • Sounding Equipment
  • Sounding Rod
  • 5m long, plate or shoe on end to prevent sinkage
    into soft bottom
  • commonly used in creeks, rivers, shallow dams or
    lakes
  • Lead Line
  • lead weight attached to the lines end to take it
    to the bottom

43
Soundings
  • Sounding Equipment (cont)
  • Sonar Equipment
  • Sound Navigation Ranging
  • measure the range of an object by timing the two
    way journey of pulse of sound energy and
    converting the result to units of distance

44
Soundings - Echo Sounder
  • sounding device utilising a fixed beam with a
    vertical axis
  • shape and width of the beam varies

45
Soundings - Echo Sounder
  • several components
  • Recorder
  • paper record or trace
  • digital display
  • punch tape
  • cassette recorder
  • digital readout to a computer

6
1
7
2
5
3
4
46
Soundings - Echo Sounder
  • Recorder
  • paper record or trace
  • digital display
  • punch tape
  • cassette recorder
  • digital readout to a computer

47
Soundings - Echo Sounder
  • Recorder
  • when a horizontal fix occurs a fix button is
    pressed and the moment is recorded on the trace
  • fix is numbered so that it can be correlated with
    position fixes made
  • time is recorded at the first and last fix and
    for every tenth fix - enables the tide gauge
    readings to be correlated with the depth readings

48
Soundings - Echo Sounder
  • Transducer
  • transmitting transducer vibrations generates
    pressure waves which are projected into the water
    medium and detected by a receiving transducer
  • may be
  • hull mounted
  • mounted in a shoe, or
  • towed in a fish enabling the sounder to operate
    at any depth
  • Receiving Amplifier
  • amplifies the weak return signal

49
Soundings - Echo Sounder
  • The shape of the beam is assumed to be a cone
  • the frequency of the sound signal determines the
    beam width and the reflective and penetrative
    capabilities of the pulse
  • the wider the beam width the wider the lane
    widths and a lesser number of runs are required

50
Soundings - Echo Sounder
  • the width of the beam varies with the water depth
  • possible to miss a prominent high point if the
    line spacing does not allow for any overlap
  • If a wide beam is used on a sloping seabed then
    an incorrect depth which (will be that of the
    first returning signal) will be recorded for the
    depth immediately under the boat

51
Soundings - Echo Sounder
  • Thus for precise work it is essential that a
    narrow beam is used.
  • disadvantage is an increase in the number of
    lines necessary to cover the same area in order
    to achieve a saturated examination.

52
Soundings - Echo Sounder
  • A dual frequency echo sounder using a narrow beam
    width frequency together with a wide beam width
    frequency are used at the same time to overcome
    this problem.

53
Soundings - Echo Sounder
  • dual frequency echo sounder enables
  • an accurate depth to be obtained below the vessel
  • check on the presence of any high points to the
    side of the vessel.
  • lane widths can be widened compared with those
    for a single narrow beam

54
Soundings - Echo Sounder
  • Calibration
  • velocity of the pulse varies with the temperature
    and salinity of the water
  • time taken for the outward and return pulse will
    vary and the depth indicated will vary

55
Soundings - Echo Sounder
  • Calibration
  • three methods of calibration
  • Calculation
  • use the temperature and salinity of the water to
    calculate the velocity of sound within the water
  • Direct Calibration
  • when the bottom is smooth and level, the recorder
    may be calibrated by by comparison with direct
    sounding by lead line

56
Soundings - Echo Sounder
  • Calibration
  • third method of calibration
  • Bar Check
  • a metal bar is lowered under the transducers to
    known depths below the water
  • the echo sounder is adjusted until the correct
    depth is measured
  • checks are made at other depths

57
Soundings - Echo Sounder
  • Calibration
  • Squat
  • when a vessel moves through the water at
    different speeds it settles differently in the
    water
  • the amount of squat will depend on
  • the speed
  • the loading
  • hull shape

58
Soundings - Reduction of Echo Sounder Traces
  • Calibration
  • Squat
  • determined by
  • sailing over an area of known depth at different
    speeds - the different depth readings record the
    differences due to squat
  • a staff is set up at the bow and the stern of the
    vessel - the vessel is sailed past at different
    speeds and a level used to read the staves - the
    squat is determined from the differing levels

59
Soundings - Reduction of Echo Sounder Traces
  • The soundings at each fix are not reduced
    separately.

60
Soundings - Reduction of Echo Sounder Traces
  • Note the three level lines drawn on the diagram
  • Level Datum - tidal datum

Water line
Transmission Line
  • Water line is the water surface level at any
    moment in time i.e. changes as the tide changes -
    can be above or below the datum line

Level Datum
  • Transmission Line (TZ) transducer position above
    or below the water line

61
Soundings - Reduction of Echo Sounder Traces
Squat
Transmission Line
MLWS
Difference obtained from tide board readings
62
Soundings - Reduction of Echo Sounder Traces
Water line
Transmission Line
MLWS
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