The oldest record of ocean color measurements consists of visual comparisonsto a standardized color scale, the Forel-Ule scale (FU). Analysis of FU archived dataallows the construction of a century-long time series. In situ protocols of FUmeasurements require the perceived color to be estimated over the water column above aSecchi disk (SD) at half of the depth where it goes out of sight, whereas satellites retrieveFU over the water column alone. I show in this article that these two methodologies leadto different FU readings and thus, merging both kinds of data will create artificial trends.In case 1 waters, radiative transfer simulations show that measuring over a SD shifts FUbetween 0 and + 2 in respect to no SD, and there exists no possibility to relate the two ina univocal fashion. A univocal relationship is found if color is expressed in terms of thehue angle, which can be calculated from light spectra or RGB images.
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