Hydrographic Surveying: Depth Determination
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1
Depth determination
One of the main tasks in hydrographic surveying
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Bathymetry
Representation of underwater topography
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Sounding
Process of measuring water depth at a particular point
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Echosounding
Indirect method that determines depth using sound travel time
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Velocity of sound
Speed at which sound travels through water
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Travel time interval
Time between transmission of a sound pulse and reception of its echo
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Reduced Level (R.L.)
Height or depth of a point above or below a reference datum
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Chart datum
Reference surface to which soundings are reduced for charting
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Tide gauge
Instrument used to monitor changes in water level
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Mean Lower Low Water (MLLW)
Commonly adopted navigation datum stated in the lesson
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Measured sounding
Sounding measured directly from the water surface
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Reduced sounding
Sounding corrected to a specified datum
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h'
Inclined distance traveled by the sound pulse on a one-way path
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d
Horizontal separation between the transmitter and receiver
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Graduated sounding rod or pole
Shallow areas, approximately 5 m
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Marked leadline
Areas deeper than approximately 5 m
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Echosounder
Deeper seas or river estuaries
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Transmitter
Sends the sound pulse.
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Receiver
Detects the returning echo.
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Velocity of sound (v)
Speed of sound in water.
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Travel time (t)
Total time for the pulse to travel down and return.
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Transmitter-receiver separation (d)
Distance between the separate transmitter and receiver.
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•h′ = (v × t) / 2
Where:
•h′ = inclined one-way distance
•v = velocity of sound in water
•t = total round-trip travel time
Where:
•h′ = inclined one-way distance
•v = velocity of sound in water
•t = total round-trip travel time
Basic depth formula
24
•h = √[(h′)² − (d/2)²]
Where:
•h = corrected vertical depth
•h′ = inclined one-way distance
•d = separation between transmitter and receiver
Where:
•h = corrected vertical depth
•h′ = inclined one-way distance
•d = separation between transmitter and receiver
Corrected vertical depth formula
25
•h = ac − D
Where:
•h = reduced sounding
•ac = measured sounding
•D = water gauge reading above the datum
Where:
•h = reduced sounding
•ac = measured sounding
•D = water gauge reading above the datum
Reduced sounding formula
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•Correction = Gauge reading at datum − Gauge reading at sounding time
•Corrected sounding = Measured sounding + Correction
•Corrected sounding = Measured sounding + Correction
Gauge correction formula
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Positive correction
gauge reading at the datum is greater than the actual gauge reading at sounding time.
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Negative correction
gauge reading at the datum is less than the actual gauge reading at sounding time.
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