Satellite images reveal tide shifts along New Zealand coast

Researchers at Technische Universität München and the University of Oxford have developed a way to measure tides along coastlines from satellite images. The work found that tidal heights can differ sharply over short stretches of New Zealand's coast.

The study drew on more than 40 years of Landsat imagery and analysed shoreline movements across Pacific coastlines to estimate tides at 100-metre intervals. Rather than measuring sea height directly from the images, the researchers used the position of the waterline on sloping beaches to infer changes in sea level.

This approach allowed the team to examine local variation often missed by tide gauges and conventional radar-based satellite measurements, which tend to provide coarser coverage near shore. Tide gauges record conditions at specific points, while offshore satellite systems usually operate at resolutions of tens of kilometres.

New Zealand was a prominent test case because of its large tidal range and complex tidal patterns. The researchers found that tidal height varies by almost one metre across the roughly 90km span of South Taranaki Bight.

Around Christchurch, the analysis showed another sharp contrast. Tides in Pegasus Bay, east of the city, were about 40cm higher than those on beaches farther south near the Rakaia River.

Those differences matter because flood risk can change significantly from one beach to the next. A storm surge combined with a higher local tide may push one stretch of coast into flooding while a nearby area remains below a critical threshold.

Local detail

The findings point to a gap in existing coastal observations. Many current forecasts and models rely on measurements taken some distance from the specific beach or estuary being assessed.

Dr Thomas Monahan of the Department of Engineering Science at the University of Oxford said local variation in tides has practical implications beyond academic oceanography.

"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 neighbouring regions being safe or flooded in the same storm. As these methods mature, they could contribute to much more localised tide forecasts, telling people not simply what the tide is doing 'near here', but what it is doing at their beach," Monahan said.

The method relies on one of the longest continuous records of Earth observation from space. Landsat imagery has captured coastal shorelines for decades, giving researchers repeated snapshots of where the sea meets land under changing tidal conditions.

By combining those observations over time, the team identified repeating tidal rhythms for individual coastal segments. Once established, those patterns can be used to estimate tides at those locations across the historical record and to study how they shift.

Broader use

The study suggests the technique could help fill gaps in the global ocean observing system, especially where tide gauges are sparse. Better coastal measurements could also improve models used to assess compound flooding, sea-level variability and saltwater intrusion.

Lead author Dr Michael Hart-Davis of Deutsches Geodätisches Forschungsinstitut said the coastal zone remains under-observed despite the importance of tides to a wide range of activities.

"Tides are a key driver of the ocean and the broader climate system, and have been studied for centuries. However, the coastal zone, where they have the greatest impact on navigation, pollution spills, safety and flooding, remains a gap in oceanographic knowledge. By showing that satellite images of the shoreline can be used to monitor ocean tides, our study opens up important applications, from improving scientific models and forecasts to coastal protection and helping communities become more resilient in a changing environment," Hart-Davis said.

New Zealand's geography helps explain why it featured strongly in the research. The tidal wave rotates around the country's islands, so opposing coasts can experience very different stages of the tide at the same moment. An amphidromic point between the North and South islands also creates an area of minimal tidal range around which the wave turns.

That setting creates strong variation in places such as Cook Strait and South Taranaki Bight, making the country useful for testing whether satellite-derived shoreline observations can capture changes that standard systems may smooth over.

Hart-Davis said the method could also support broader efforts to study tides and sea level in finer coastal detail.

"We find that satellite imagery can be used to study tides along coastlines globally, allowing us to observe and predict tides along every 100 metres of coast. This study also presents the first use of imagery for sea-level science, which, thanks to more than 40 years of satellite imagery, could allow us to build deeper insights into both tides and sea level at unprecedented scales.

"New Zealand was chosen because its tides are highly interesting. The tidal wave rotates around the islands, meaning one coast can experience high tide while the other experiences low tide at the same time. Between the North and South islands is what is called an amphidromic point, or a region of no tides, around which the tidal wave rotates. In the context of this research, this is where we would expect the highest tidal variability, which is demonstrated in our figures around Cook Strait and along South Taranaki Bight.

"Accurate knowledge of tides, and how they vary along coasts, means we are better able to predict the tides and, therefore, downstream effects such as coastal flooding, for which tides are a major driver, as well as impacts on navigation, surfing and even simple beachgoing," Hart-Davis said.