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New satellite method reveals tides changing sharply from one beach to the next.
The tide is not the same tide everywhere. Along a single stretch of coast, it can strengthen, weaken or arrive at a different time within just a few kilometres.
Now scientists have found a way to watch those changes almost beach by beach, using more than 40 years of satellite images originally collected for an entirely different purpose. By turning the shifting shoreline in Landsat photographs into a record of the sea’s rise and fall, researchers have mapped tides at 100-metre intervals around parts of the Pacific Rim — exposing fine-scale coastal behaviour that tide gauges and conventional satellites can struggle to see.
The breakthrough starts with something satellites have photographed for decades: the line where the ocean meets the beach.
Normally, researchers using satellite imagery to study erosion try to remove the influence of tides. A high tide pushes the waterline inland; a low tide pulls it seaward. That movement can look like the beach itself has changed.
But Michael Hart-Davis of the Technical University of Munich and his colleagues approached this from the opposite angle. Instead of treating that tidal movement as unwanted noise, they used it as the signal. They used these images to study the tides themselves.
The team analyzed CoastSat shoreline records built from Landsat images taken between 1984 and 2026 across wave-dominated Pacific coasts, including New Zealand, Australia, Japan, Hawaii and the western Americas. The system tracks where the waterline crosses virtual lines spaced every 100 meters along the coast. Researchers then use the beach slope to translate that sideways movement into an estimate of changing water level.
There was, however, a catch. Landsat may revisit the same location only every 16 days, while tides rise and fall within hours.
Yet the timing of those satellite passes creates repeating patterns that scientists can mathematically disentangle. For instance, the the lunar component responsible for two high and two low tides across most coastlines can be recovered from about 191 days of Landsat observations. With decades of imagery, they had enough to paint a comprehensive picture of tides.
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But a beach is not a ruler, and it doesn’t sit still.
The waterline shifts with the tide, but also with breaking waves, storms, river flow and the slow reshaping of sand. Even the slope of the beach changes — and that slope is crucial, because the researchers use it to convert the waterline’s sideways movement into an estimate of how much the sea has risen or fallen.
The bigger challenge comes when looking for changes over decades. The researchers found apparent trends in tidal strength at several sites, but they were far larger than changes measured by tide gauges or conventional satellite altimetry.
Even so, the study conclusively showed that in short distances, the same tide can look very differently, and that’s an important find.
Tide predictions can look deceptively precise. In places like bays or gulfs, where tides can have a big impact, people schedule their activities based on these predictions. But these predictions are misleading.
A tide gauge measures one location, and conventional satellite altimeters have historically struggled close to land. Large stretches of coastline therefore remain poorly observed. So the forecasts lack detail, and especially during a story, there can be real risks.
“Our research shows that tides can vary substantially over relatively short distances. This isn’t just important for activities such as fishing or surfing, but it has big impacts on coastal flooding. Short-scale variations in tides can mean the difference between two neighboring regions being safe or flooded for the same storm. As these methods mature, they could contribute to much more localized tide forecasts, telling people not simply what the tide is doing ‘near here,’ but what it is doing at their beach,” concludes Monahan.
The study was published in the journal Communications Earth & Environment.

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