FU Classification - Algorithm Specification

Introduction

This algorithm is used for deriving the hue angle and FU (Forel-Ule) value from remote sensing reflectance. The data is used with the weighted linear sum of the remote sensing reflectance in which visual bands plus a correction term for the specific band-setting of each instrument. The algorithm is validated by a set of 603 hyperspectral measurements from inland, coastal and near-ocean waters. The processor supports the the following sensors:

The algorithm is described in the following articles which are available online.

An introduction to the SNAP FU-processor and examples of potential applications
Van der Woerd, H.J., Wernand, M., Peters, M., Bala, M., Brockmann, C., 2016. True Color Analysis of Natural Waters with SeaWiFS, MODIS, MERIS and OLCI by SNAP. Proceedings of the Ocean Optics XXIII, Victoria, BC, Canada, pp. 23?28.
A worldwide investigation of the spectral characteristics of FU- and hue properties.
Pitarch, J., Van der Woerd, H.J., Brewin, R.J.W., Zielinski, O., 2018. Optical properties of Forel-Ule water types deduced from 15 years of global satellite ocean color observations (https://doi.org/10.1016/j.rse.2019.111249)
The derivation of the hue angle for the instruments SeaWiFS, MODIS, MERIS and OLCI are described in:
Van der Woerd, H.J., Wernand, M.R., 2015. True colour classification of natural waters with medium-spectral resolution satellites: SeaWiFS, MODIS, MERIS and OLCI. Sensors 15, 25663-25680. doi: 10.3339/s151025663 (Open Access)
The algorithms to convert instruments like MSI on Sentinel 2:
Van der Woerd, H.J., Wernand, M.R., 2018. Hue-Angle Product for Low to Medium Spatial Resolution Optical Satellite Sensors. Remote Sens. 2018, 10, 180. doi:10.3390/rs10020180
The conversion of the hue angle to the Forel-Ule number and the corresponding colour legend is based on:
Novoa, S., Wernand, M.R, Van der Woerd, H.J., 2013. The Forel-Ule scale revisited spectrally: preparation protocols, transmission measurements and chromaticity. JEOS RP 8, 13057. DOI: http://dx.doi.org/10.2971/jeos.2013.13057 (Open Access)
The conversion of the global CCI ocean colour set to hue angle and FU:
Pitarch, J., Bellacicco, M., Marullo, S., and van der Woerd, H. J.: Global maps of Forel?Ule index, hue angle and Secchi disk depth derived from 21 years of monthly ESA Ocean Colour Climate Change Initiative data, Earth Syst. Sci. Data, 13, 481?490, https://doi.org/10.5194/essd-13-481-2021
This algorithm connects satellite ocean colour observations to historical in-situ colour observations:
Wernand, M.R, Van der Woerd, H.J., Gieskes. W.W.C, 2013. Trends in ocean colour and chlorophyll concentration from 1889 to present. Plos One. http://dx.plos.org/10.1371/journal.pone0063766 (open access)