Depth Determination
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1
Depth Determination
process of measuring the depth of water at a particular point with reference to a reduced datum.
2
Bathymetry
measurement and representation of underwater depths and the shape of the seafloor.
3
Reduced Level (R.L.)
height or depth of a point above or below a reference datum.
4
Datum
fixed reference surface from which elevations and depths are measured.
5
Chart Datum
reference level used in nautical charts to represent water depths.
6
Mean Lower Low Water (MLLW)
commonly adopted chart datum in navigation.
7
Graduated Rod/Pole
used to directly measure water depth in shallow areas.
8
Graduated Rod/Pole
Shallow Areas (~5 meters)
9
Leadline
graduated line with a weight attached at the lower end to measure water depth.
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Leadline
Areas Deeper than 5 Meters
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Echosounding
indirect method of determining water depth using sound pulses and reflected echoes.
12
Echosounding
Deeper Areas (Seas and River Estuaries)
13
Echosounder
instrument that determines water depth by measuring the time taken by a sound pulse to travel to the bottom and return.
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h = vt / 2
Where:
• h — water depth
• v — velocity of sound in water
• t — time interval between transmission and reception
Where:
• h — water depth
• v — velocity of sound in water
• t — time interval between transmission and reception
Depth Formula:
15
Transmitter/Receiver Distance (d)
horizontal separation between the transmitter and receiver installed separately on the vessel.
16
Inclined Distance (h')
distance traveled by the sound pulse, calculated using the velocity of sound and travel time.
17
h' = vt / 2
Inclined Distance (h')
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h = √[(h')² − (d/2)²]
Where:
• h — actual vertical depth
• h' — inclined distance
• d — distance between transmitter and receiver
Where:
• h — actual vertical depth
• h' — inclined distance
• d — distance between transmitter and receiver
Actual Depth Correction Formula:
19
Water Level Fluctuation
continuous change in water level that causes the reference surface for sounding to change.
20
Tide Gauge
instrument used to measure and record changes in water level.
21
Tide Gauge
Determine the reduced level of the water surface at the time of sounding.
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Reduced Sounding
measured depth corrected with reference to a fixed datum.
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Chart Datum
fixed reference surface used to reduce soundings for nautical charts.
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Mean Lower Low Water (MLLW)
Common Chart Datum:
25
h = ac − D
Where:
• h — reduced sounding
• ac — measured sounding
• D — gauge reading
Where:
• h — reduced sounding
• ac — measured sounding
• D — gauge reading
Reduced Sounding Formula:
26
Gauge Reading Correction
adjustment applied to measured soundings when the actual water level differs from the assumed datum reference.
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Synchronization
coordination of gauge readings with the sounding team's observation times.
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Correction = Gauge Reading at MLLW − Gauge Reading at Time of Sounding
Gauge Reading Correction Formula:
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Positive
when the gauge reading at MLLW is greater than the gauge reading at the time of sounding.
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Negative
when the gauge reading at MLLW is less than the gauge reading at the time of sounding.
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Corrected Sounding = Measured Sounding + Gauge Reading Correction
Corrected Sounding:
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