From Fixed Gauges to a Global View
Traditionally, measuring tides was a strictly local affair. Tide gauges, instruments that measure sea level relative to a fixed point on land, have been the standard for centuries. While incredibly valuable, they have a fundamental limitation: they only
provide data for a single spot. This leaves vast stretches of the world's coastlines, especially in remote areas, as data deserts. Conventional satellite radar altimetry, which has been in use since the 1990s, improved this by providing open-ocean data but struggled with resolution near the complex and crucial coastal zones. This gap in knowledge is significant, as tide levels can vary by nearly a metre within a single bay, a detail easily missed by sparse gauges or lower-resolution satellite methods.
How Satellites 'See' the Tides
There are two primary ways satellites are revolutionizing tide mapping. The most established method is radar altimetry. Satellites like those in the Jason series send microwave pulses down to the ocean and measure the time it takes for the signal to bounce back. Knowing the satellite's precise altitude allows scientists to calculate the sea surface height with remarkable accuracy, often within centimetres. A newer, groundbreaking method uses decades of optical satellite images from programs like Landsat. Instead of measuring water height directly, this technique tracks the shifting boundary between land and water over thousands of images. By combining this shoreline movement with data on the beach's slope, researchers can calculate the rise and fall of the water level, effectively turning the beach itself into a giant ruler.
Turning 'Noise' into a Signal
What makes the new shoreline-tracking method so innovative is its use of what was previously considered 'noise'. The constant movement of the waterline in satellite images was often filtered out to get a clearer picture of the stable coastline. Now, researchers have shown this 'noise' is actually a valuable signal containing detailed information about local tidal patterns. By analysing more than 40 years of these images, scientists can identify the characteristic repeating rhythms of the tides for every 100-metre segment of a studied coastline. This provides an unprecedented level of local detail, filling in the gaps left by other observation systems and creating hyper-local tide models.
A Revolution for Science and Safety
This wealth of new data has profound implications. For communities living along coastlines, more accurate local tide forecasts can drastically improve flood risk assessment and preparedness, especially for compound flooding events where tides, storm surges, and river discharge combine. For maritime industries, better tidal knowledge means safer navigation and more efficient port operations. For scientists, it allows for the improvement of global climate and ocean circulation models. By better distinguishing tidal movements from long-term changes, researchers can more accurately measure the true impact of sea-level rise on coastal erosion and saltwater intrusion into freshwater systems.
The Future Is Higher Resolution
The technology continues to evolve. The recently launched SWOT (Surface Water and Ocean Topography) mission, a collaboration between NASA and other international space agencies, is set to revolutionise the field again. SWOT uses a Ka-band radar interferometer to map water surfaces in wide swaths, resolving features at a much higher resolution than previous altimeters. This will provide unparalleled detail on coastal tides, eddies, and even the tidal influence on rivers and estuaries. While challenges like the massive computational power needed to process decades of data remain, the trajectory is clear. We are moving towards an ever more complete and dynamic understanding of our planet's vital ocean rhythms.
