On the logic of remote detection of plastic litter in the aquatic environments: A revisit

Remote detection of plastic litter in both marine and freshwater environments using satellite measurements has become a hot research topic in the past decade, where numerous papers have shown “successful” algorithm development and applications. However, many of these results appear to need some revisits because, in logic, the causality of A to B (i.e., A  => B) does not lead to the inference of B => A unless A is the only reason to cause B. In practice, even though plastics can lead to a certain type of signal anomaly (e.g., spectral, spatial, backscattering) from controlled experiments, the same anomaly detected from the natural environments cannot be used to infer plastics unless other possible reasons can all be ruled out. This is especially true when considering that non-plastic floating matters are much more ubiquitous in the aquatic environments. Unfortunately, this logic has been missing in many, if not most, publications. Here, using spectral reflectances of various types of floating matters and through demonstrations of several examples, I show why such logic is critical in remote detection of plastic litter and why pixel averaging and subtraction are necessary steps to spectrally discriminate the signal anomaly in multi-band optical remote sensing imagery. It is argued that unless other possibilities are ruled out using imaging spectroscopy or other means, it is premature to attribute the detected signal anomaly to plastic litter. After all, not every anomaly pixel is necessarily due to litter, and not every litter pixel is necessarily due to plastics unless proven otherwise.