Determining Ocean Depth with Tidal Correction
In hydrographic surveys, one of the biggest challenges is determining ocean depth accurately, even though sea level is constantly changing due to tides. This requires a consistent reference system to standardize depth data from various times and locations.
Basic Measurement Principles
The illustration shows how to determine depth at a point in the ocean, for example, Point A. On the left side is a tide staff, which is used to directly read the sea level at the time of the observation. Because sea level is never constant rising with high tide and falling with low tide this reading is crucial for recording water level conditions at a specific time.
The dotted curved line at the top represents the sea level position at time T, the time of the observation. Because this data is highly dynamic, a long-term reference standard is needed.
Mean Sea Level (MSL)
To normalize the data, a horizontal line called the Mean Sea Level (MSL) is used. MSL is the average sea level calculated from long-term tidal data (usually 19 years in international standards, or several months to years for practical surveys). With MSL, we have a general overview of the average sea level, thus facilitating the correction process for depth data.
Main Components in Correction
From the illustration, there are several important components:
R → Sea level height read from the dipstick at the time of observation.
MS → Vertical distance from the lowest low tide datum (Chart Datum/Lowest Astronomical Tide) to MSL.
Zo → Sea level correction to the low tide datum. This correction is important because every depth observation must be converted to the same reference system.
AEA (Depth Datum/Chart Datum) → The baseline for all depth measurements. With Chart Datum, all depth data can be corrected to a single reference, ensuring consistency.
Depth Determination Process at Point A
The steps for determining depth at Point A are:
Measure the vertical distance from the sea surface during observation to the seabed using acoustic equipment such as a Single Beam Echo Sounder (SBES) or Multibeam Echo Sounder (MBES).
Concurrently, read the sea level height (R) using a tide staff or automatic tidal sensor.
Correct the depth measurement results using Zo, which is the difference in height between the sea level during observation and Chart Datum.
The final result is a standard depth referenced to Chart Datum (AEA), allowing for comparison with other measurements at different times or locations.
The Importance of Tidal Correction
Without this correction, depth data will be biased due to tidal changes. For example, measurements at high tide may yield shallower depths than at low tide, even though the seabed is at the same point. Correction to datum ensures consistent data, is safe for navigation, and can be integrated into nautical charts and bathymetric surveys.
Benefits in Practice
Navigation: Ships can sail safely because depth charts always refer to the same datum.
Infrastructure Planning: Accurate data for docks, dams, bridges, or reclamation projects.
Environmental Management: Monitoring changes in seabed morphology, sedimentation, and coastal erosion.
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