True Colour Classification of Natural Waters with Medium-Spectral Resolution Satellites: SeaWiFS, MODIS, MERIS and OLCI

Abstract

The colours from natural waters differ markedly over the globe, depending on the water composition and illumination conditions. The space-borne “ocean colour” instruments are operational instruments designed to retrieve important water-quality indicators, based on the measurement of water leaving radiance in a limited number (5 to 10) of narrow (≈10 nm) bands. Surprisingly, the analysis of the satellite data has not yet paid attention to colour as an integral optical property that can also be retrieved from multispectral satellite data. In this paper we re-introduce colour as a valuable parameter that can be expressed mainly by the hue angle (α). Based on a set of 500 synthetic spectra covering a broad range of natural waters a simple algorithm is developed to derive the hue angle from SeaWiFS, MODIS, MERIS and OLCI data. The algorithm consists of a weighted linear sum of the remote sensing reflectance in all 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. We conclude that the hue angle is a simple objective parameter of natural waters that can be retrieved uniformly for all space-borne ocean colour instruments.

Hue-Angle Product for Low to Medium Spatial Resolution Optical Satellite Sensors

“…As ocean colour satellites do not provide hyperspectral coverage, the full spectrum should be first reconstructed, for example by linear interpolation, based on the Rrs measured at the spectral bands available. It was shown in Van der Woerd and Wernand [1] that for ocean-colour instruments, for example MERIS with nine bands, Equation (1) can be approximated by Equation (2). …”

“…In the (x, y) chromaticity plane, the coordinates are transformed to polar coordinates with respect to the white point, and the hue angle is derived. The hue angle (α) lies between the vector to a point with coordinates (x − x w , y − y w ) and the positive x-axis (at y − y w = 0), giving higher angles in an anti-clockwise direction (see Figure 2 in Van der Woerd and Wernand [1]). Note that the definition of (α) is different in Wang et al [18].…”

“…In Van der Woerd and Wernand [1], the hue-angle algorithm was based on the 500 simulated Rrs spectra for SeaWiFS, MODIS, MERIS, and OLCI. First, the weights M(i) of Equation (2) were precalculated based on CMFs and the central wavelengths of the bands of each sensor.…”

“…First, the weights M(i) of Equation (2) were precalculated based on CMFs and the central wavelengths of the bands of each sensor. We refer to Van der Woerd and Wernand [1] for details. It was essential that these sensors have 10-nm narrow bands, making sure that each band reliably measures the actual Rrs at that wavelength, since the variations of Rrs within the band are small.…”

“…Although ocean colour satellite sensors only measure the visual radiation in a limited number of narrow spectral bands, they are perfectly capable of measuring the colour of natural waters [1]. Colour is a sensation that originates in the human perception of radiation between the wavelengths of 380-720 nm.…”