CH Tides, Water Levels & Vertical Datums 1 — Questions and Answers

Question 1: What is the primary cause of ocean tides on Earth?

Earth's rotation
Wind patterns
Gravitational forces of the moon and sun (Correct answer)
Ocean currents

Correct answer: Gravitational forces of the moon and sun

The primary cause of ocean tides is the gravitational pull exerted by the Moon and, to a lesser extent, the Sun on Earth's oceans. The Moon's gravity creates bulges of water on both the side of Earth facing the Moon and the opposite side. As Earth rotates, these bulges create the rhythmic rise and fall of sea levels we observe as tides.

Question 2: What is Mean Lower Low Water (MLLW)?

Average sea level over a year
Average of daily low tides
Lowest recorded tide
Average of the lower low tides each day over 19 years (Correct answer)

Correct answer: Average of the lower low tides each day over 19 years

Mean Lower Low Water (MLLW) is a specific tidal datum defined as the average of the lower of the two low tides occurring each day, observed over a specific 19-year National Tidal Datum Epoch. This datum is crucial for hydrographic charting in the U.S. as it represents a conservative reference plane for navigation, ensuring that charted depths are typically deeper than the actual water level.

Question 3: Which instrument is commonly used to measure water levels over time?

Echo sounder
GPS receiver
Tide gauge (Correct answer)
CTD profiler

Correct answer: Tide gauge

A tide gauge is an instrument specifically designed to continuously measure and record water levels over time at a fixed location. These measurements are essential for understanding tidal patterns, establishing vertical datums, and providing real-time or predicted tide corrections for hydrographic surveys. Modern tide gauges often use acoustic or radar sensors to determine water surface elevation.

Question 4: Why are vertical datums important in hydrography?

They reduce water pollution
They determine water temperature
They provide a height reference for soundings (Correct answer)
They improve sonar resolution

Correct answer: They provide a height reference for soundings

Vertical datums are fundamental in hydrography because they establish a common, consistent reference surface from which all measured depths (soundings) are reported. Without a defined vertical datum, depths would be relative to an arbitrary water level, making it impossible to compare measurements, create accurate nautical charts, or ensure safe navigation. They ensure all depths are standardized to a specific plane, like MLLW.

Question 5: Which vertical datum is used for referencing water depths in U.S. nautical charts?

Mean Sea Level
Ellipsoidal Height
Mean High Water
Mean Lower Low Water (Correct answer)

Correct answer: Mean Lower Low Water

In the United States, Mean Lower Low Water (MLLW) is the standard vertical datum used for referencing water depths on nautical charts. This datum represents the average of the lower of the two daily low tides over a 19-year period. Using MLLW ensures that charted depths provide a conservative estimate of the minimum water available for navigation, enhancing safety for mariners.

Question 6: How do tides influence hydrographic survey data?

They alter water temperature
They change sediment composition
They affect GPS accuracy
They require depth corrections to a vertical datum (Correct answer)

Correct answer: They require depth corrections to a vertical datum

Tides cause significant fluctuations in water levels, meaning the raw depths measured by an echo sounder are constantly changing relative to the seafloor. To produce accurate and consistent bathymetric charts, all measured depths must be corrected to a specific, unchanging vertical datum, such as Mean Lower Low Water (MLLW). This correction removes the tidal component, presenting depths relative to a fixed reference plane.

CH Tides, Water Levels & Vertical Datums 1

What is the primary cause of ocean tides on Earth?