Understanding Tidal Datums: What Are They and How Are They Set?

Tidal datums are reference points used to measure tide heights, allowing people to interpret tide predictions more accurately. Whether you're a boater, fisherman, or coastal resident, understanding tidal datums is key to reading tide charts correctly and planning coastal activities.

In this article, we’ll explore what tidal datums are, how they’re established, and how you can use them.

Table of Contents

1. What Is a Tidal Datum?
2. How Are Tidal Datums Established?
3. Common Types of Tidal Datums
4. How to Read Tide Measurements with Tidal Datums
5. Using Tidal Datums in Coastal Navigation
6. Conclusion

What Is a Tidal Datum?

A tidal datum is a reference point or baseline used to measure the height of tides. When reading a tide chart, the water levels—both high and low—are measured relative to this datum.

Think of a tidal datum as a baseline or reference level from which tide heights are measured. To establish this datum, tide gauges are installed in coastal locations, such as harbor walls, piers, or specially built monitoring stations. These gauges, which can be mechanical or electronic, measure the water's height relative to a fixed point. Instead of measuring directly from sea level, the tide chart uses this datum as a consistent starting point. This allows for accurate comparisons of tide heights across different times and locations, even though the actual water level can vary due to local conditions.

How Are Tidal Datums Established?

Tidal datums are established based on long-term observations of local sea levels. In the U.S., they are typically calculated by averaging water levels over a 19-year period (known as the National Tidal Datum Epoch). This long-term average helps smooth out variations caused by weather, seasons, and other factors.

In the UK, tidal datums are calculated differently. The most commonly used reference is Ordnance Datum Newlyn (ODN), which is based on sea level measurements taken at Newlyn in Cornwall from 1915 to 1921. This reference level is used as the baseline for elevations on land across Great Britain.

For nautical purposes, the UK uses Chart Datum (CD), which serves as the baseline for tide charts and nautical maps. CD varies by location, but it is generally set at a level that the tide only rarely falls below, making it a useful reference point for navigation in shallow waters.

Common Types of Tidal Datums

There are several types of tidal datums, each used for different purposes. Here are a few of the most common:

1. Mean Sea Level (MSL): The average sea level over a long period of time. In the UK, this is based on Ordnance Datum Newlyn (ODN), which is specific to sea level at Newlyn, Cornwall.
2. Mean Lower Low Water (MLLW): Common in the U.S., this is the average of the lowest low tide recorded each day over a 19-year period. The UK uses Chart Datum (CD) instead of MLLW for nautical purposes.
3. Mean High Water (MHW): The average of all high tides recorded over a long period. It’s useful for determining how far up the shore the tide will reach.
4. Lowest Astronomical Tide (LAT): The lowest tide level that can be predicted under normal astronomical conditions. This is important for navigation, as it helps ensure safe passage in shallow waters.

How to Read Tide Measurements with Tidal Datums

When reading a tide chart, the water levels are expressed in terms of how high or low they are compared to the tidal datum. For example, if the tide is predicted to be “2 feet at high tide,” that means the water will be 2 feet above the datum (often Chart Datum (CD) in the UK).

Here’s an example of what a typical tide chart might show:

• Low Tide: -0.5 feet at 6:00 AM (below the datum)
• High Tide: 4.0 feet at 12:00 PM (above the datum)

If you understand the datum being used (such as CD in the UK or MLLW in the U.S.), you can better gauge how much the water will rise or fall at different times of the day.

Using Tidal Datums in Coastal Navigation

Tidal datums are vital for safe coastal navigation, especially in areas with significant tidal ranges. Mariners rely on tidal datums to understand water depth at low tide, avoiding potential hazards like sandbars or shallow areas.

In the UK, nautical charts use Chart Datum (CD) as the baseline for depth measurements. This ensures that even during the lowest low tides, sailors have accurate information about how deep the water will be, minimizing the risk of running aground in shallow areas.

Conclusion

Understanding tidal datums is essential for anyone relying on tidal predictions, whether for boating, fishing, or planning a day at the beach. Tidal datums provide a consistent reference point for interpreting tide heights and times, making it easier to stay safe and make the most of your time near the water.

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FAQs

Q: How are tidal datums calculated?
A: Tidal datums are calculated by averaging tide levels over long periods (typically 19 years in the U.S.). In the UK, Ordnance Datum Newlyn (ODN) is used for land-based measurements, and Chart Datum (CD) is used for nautical charts.

Q: What is the most commonly used tidal datum for navigation?
A: In the UK, Chart Datum (CD) is used for navigation. In the U.S., Mean Lower Low Water (MLLW) is more commonly used for the same purpose.

Q: Can tidal datums vary by location?
A: Yes, tidal datums are local, meaning they can differ between coastal regions based on historical tidal data and local sea level changes.