How Tide Predictions Work in Marin

Every tide time on Marin Tides is a prediction from NOAA's Center for Operational Oceanographic Products and Services (CO-OPS). Predictions come from the movements of the moon and sun, which repeat on known cycles. That is why they can be made years ahead.

Reference and Subordinate Stations

A reference station has a long record from a tide gauge. NOAA fits that record to a set of harmonic constituents, one for each regular pull of the moon and sun, and adds them up to predict the water level at any moment. Point Reyes, Bolinas Lagoon and Corte Madera Creek are reference stations.

Most Marin stations are subordinate stations. NOAA predicts their highs and lows by taking the San Francisco (Golden Gate) reference station and applying a time and height offset measured at the spot. Highs and lows at a subordinate station are reliable, but the shape between them is not published. The curves here are drawn with the standard cosine method between each high and low.

What the Heights Mean

Heights are in feet above mean lower low water (MLLW), the average of each day's lower low tide over a 19-year period. Zero is not the lowest the water ever gets. A negative height, a minus tide, means the water is below that average and more of the shore than usual is exposed.

When the Real Tide Differs

Predictions cover the astronomical tide only. Strong onshore winds, low air pressure, storm surge and winter runoff can raise the water above the prediction, and offshore winds and high pressure can lower it. In big winter storms the difference can be a foot or more. For live readings, NOAA's water level gauges at Point Reyes and San Francisco report every six minutes.

Which Station Each Marin Spot Uses