Can we really gauge underwater visibility using satellite technology? Emma and Gordon Taylor, authors of Snorkelling Britain, uncover the science behind the murk
UKDIVING
Wizards of viz
Can we really gauge underwater visibility using satellite technology? Emma and Gordon Taylor, authors of Snorkelling Britain, uncover the science behind the murk
Co-author Emma Taylor snorkelling in clear water
Ask a group of divers what the visibility was on a dive and their guesses are likely to vary wildly. Estimating distance underwater is surprisingly difficult because of the well-known distortion of light as it passes through our masks. Interestingly, this often means that the actual visibility is better than reported by divers. So to remove the subjectivity we need a standardised method for measuring water clarity. The traditional approach involves lowering
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a white or patterned disk into the water and recording the depth at which it disappears from view; this is known as the Secchi depth after its 19th-century Italian inventor. An alternative is to use a turbidity meter, which applies the catchily named‘ nephelometric principle’ to measure light scattered by suspended particles. So quantitative methods do exist, but their main limitation is that they only provide a snapshot of an ever-changing environment: a single measurement at one place and at one time.
Eyes in the sky
Modern technology has furnished us with constellations of satellites, stealthily charging around in low earth orbit, sensors focused on every aspect of us and our planet at ever increasing resolution. While much of this hardware is in private or military ownership, some are tax-payer funded and make their data available to the public. One such is Copernicus, a European-funded programme that now consists of multiple assets, daily churning out huge data sets to be eagerly consumed by academics and citizen scientists around the world, all with an interest in monitoring the health of our planet.
Estimating Secchi depth from raw satellite measurements involves solving some bewildering orbital physics: atmospheric corrections, masking of land and cloud pixels, correlation with semi-empirical models, calibrating with in-situ measurements from a global network of buoys. Fortunately, this heavy lifting is performed for us by the Copernicus scientists, and the public data set simply