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