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

Hendrik Van Der Woerd,Marcel Wernand

doi:10.3390/rs10020180

Abstract

In the European Citclops project, with a prime aim of developing new tools to involve citizens in the water quality monitoring of natural waters, colour was identified as a simple property that can be measured via a smartphone app and by dedicated low-cost instruments. In a recent paper, we demonstrated that colour, as expressed mainly by the hue angle (α), can also be derived accurately and consistently from the ocean colour satellite instruments that have observed the Earth since 1997. These instruments provide superior temporal coverage of natural waters, albeit at a reduced spatial resolution of 300 m at best. In this paper, the list of algorithms is extended to the very first ocean colour instrument, and the Moderate Resolution Imaging Spectroradiometer (MODIS) 500-m resolution product. In addition, we explore the potential of the hue angle derivation from multispectral imaging instruments with a higher spatial resolution but reduced spectral resolution: the European Space Agency (ESA) multispectral imager (MSI) on Sentinel-2 A,B, the Operational Land Imager (OLI) on the National Aeronautics and Space Administration (NASA) Landsat-8, and its precursor, the Enhanced Thematic Mapper Plus (ETM+) on Landsat-7. These medium-resolution imagers might play a role in an upscaling from point measurements to the typical 1-km pixel size from ocean colour instruments. As the parameter α (the colour hue angle) is fairly new to the community of water remote sensing scientists, we present examples of how colour can help in the image analysis in terms of water-quality products.

Highlights

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]
The entries for Medium Resolution Imaging Spectrometer (MERIS), Moderate Resolution Imaging Spectroradiometer (MODIS), Sea-viewing Wide Field-of-view Sensor (SeaWiFS), and Ocean and Land Colour Instrument (OLCI) on Sentinel-3 have already been published in Van der Woerd and Wernand [1], and the MERIS result serves here as a benchmark for the new calculations
Validation by a set of over 600 spectra from North Sea and Dutch inland waters confirmed this point, and the hue angles could be well reconstructed from a limited set of narrow spectral bands, mimicking the band setting of MERIS, MODIS, SeaWiFS, and OLCI

Summary

Introduction

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. Colour was chosen in the European Citclops (Citizens’ observatories for coast and ocean optical monitoring) project [4] as a primary parameter to involve citizens in the monitoring of natural waters [5]. Involvement of the general public in the collection of observations of the environment has gained momentum under the term “citizen science”. These observations might help scientists in their understanding of local processes, and create awareness and commitment in the environmental stewardship of the general public [6,7,8]

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Conclusion