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Hydrography is the science underpinning nautical charts. Hydrographers take measurements of different aspects of the seabed and the ocean, including waves, tides (pictured above), and currents.
Waves are created by energy passing over or through water. That energy usually comes from wind. As wind blows across the water’s surface, it pushes the water forward, creating a wave.
Tides raise and lower the ocean’s water level over the course of a 24-hour day. They are really just very big waves that move through the ocean in response to the Earth’s rotation and the gravity exerted by the sun and the moon. While the sun is 27 million times larger than the moon, the moon is 390 times closer to the Earth than the sun — this allows the moon to exert a greater gravitational force on the water. Tides can almost always be predicted for every coastal area on Earth. When hydrographers survey coastal areas of the ocean, they organize them into different tidal zones based on the area’s tidal characteristics.
When hydrographers contribute data and information for nautical charts, they include water depth based on the area’s tidal cycles. This allows navigators to safely plot their courses based on their vessels’ size, so they do not damage the bottoms of their ships or run aground.
In addition to tides, hydrographers are interested in ocean currents’ direction and strength, which are affected by the rise and fall of the tides, winds, the shape of the coastline, water temperature, and many other factors.
An ocean current’s strength has a direct impact on how a vessel navigates through the water. If a vessel is traveling with a strong, smooth ocean current in the open sea, its travel time can be greatly reduced. If the vessel is traveling against a strong ocean current, it will take much longer to reach its destination. Strong currents in shallow or rocky areas can be very dangerous and can cause ships to run aground or possibly sink. These areas are marked on nautical charts to alert mariners that they are areas to be avoided.
In Maine, Eastport's fishermen deal with some of the East Coast’s most extreme tides and currents, with an average range of 18 to 20 ft. At low tide, vessels sit far below the edge of the seawall.
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