Tidal datums

If you want to participate in any activity on the ocean or coast, especially one requiring navigation, it’s important to understand tides and how they impact water levels. Tides raise and lower the ocean’s water level due to the Earth’s rotation and the forces exerted by the moon and the sun. But how high a tide rises or how low it falls can vary depending on its location, the time of day, and the day of the month.

When the tide is low, the water is at its shallowest, so mariners must be careful not to hit any submerged structures or sunken objects or to run aground. When the tide is high, large ships traveling under bridges must make sure they have enough room, or clearance, to pass. Understanding how the tide impacts water depth is critical for safe navigation.

To create accurate nautical charts with water depth information, we rely on something called a tidal datum. In general, a datum, or a set of data, is a reference point used to help measure heights or depths. A tidal datum is the average height of the water level during a specific stage in the tidal cycle for a particular area. This average height serves as a reference value to help measure water levels in that area.

Tidal datums are created based on long-term observations and are tied to local reference points, known as bench marks. Since tidal datums are connected to specific bench marks, they can only be used to measure water levels in the areas where those bench marks are located. There are hundreds of tidal datums used throughout the world. NOAA’s Center for Operational Oceanographic Products and Services (CO-OPS) maintains several tidal datums, which are the official datums shown on nautical charts.

Beyond measuring depths, tidal datums also play a key role in defining boundaries, such as where national waters end and international waters begin.