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Meta-classification of remote sensing reflectance to estimate trophic status of inland and nearshore waters

M . Wether, E. Spyrakos, S. Simis, D. Odermatt, K. Stelzer, H. Krawczyk, O. Berlage, P. Hunter, A. Tyler (2021). Meta-classification of remote sensing reflectance to estimate trophic status of inland and nearshore waters. ISPRS Journal of Photogrammetry and Remote Sensing, Volume 176, June 2021, Pages 109-126, 

Abstract: Common aquatic remote sensing algorithms estimate the trophic state (TS) of inland and nearshore waters through the inversion of remote sensing reflectance (Rrs (λ)) into chlorophyll-a (chla) concentration. In this study we present a novel method that directly inverts Rrs (λ)) into TS without prior chla retrieval. To successfully cope with the optical diversity of inland and nearshore waters the proposed method stacks supervised classification algorithms and combines them through meta-learning. We demonstrate the developed methodology using the waveband configuration of the Sentinel-3 Ocean and Land Colour Instrument on 49 globally distributed inland and nearshore waters (567 observations). To assess the performance of the developed approach, we compare the results with TS derived through optical water type (OWT) switching of chla retrieval algorithms. Meta-classification of TS was on average 6.75% more accurate than TS derived via OWT switching of chla algorithms. The presented method achieved 90% classification accuracies for eutrophic and hypereutrophic waters and was 12% more accurate for oligotrophic waters than derived through OWT chla retrieval. However, mesotrophic waters were estimated with lower accuracy from both our developed method and through OWT chla retrieval (52.17% and 46.34%, respectively), highlighting the need for improved base algorithms for low - moderate biomass waters. Misclassified observations were characterised by highly absorbing and/or scattering optical properties for which we propose adaptations to our classification strategy.


Citizen science with colour blindess: A case study on the Forel-Ule scale

O. Burggraaff, S. Panchagnula, F. Snik (2021). Citizen science with colour blindness: A case study on the Forel-Ule scale.  PLOSE ONE 16(4): e0249755,

Abstract: Many citizen science projects depend on colour vision. Examples include classification of soil or water types and biological monitoring. However, up to 1 in 11 participants are colour blind. We simulate the impact of various forms of colour blindness on measurements with the Forel-Ule scale, which is used to measure water colour by eye with a 21-colour scale. Colour blindness decreases the median discriminability between Forel-Ule colours by up to 33% and makes several colour pairs essentially indistinguishable. This reduces the precision and accuracy of citizen science data and the motivation of participants. These issues can be addressed by including uncertainty estimates in data entry forms and discussing colour blindness in training materials. These conclusions and recommendations apply to colour-based citizen science in general, including other classification and monitoring activities. Being inclusive of the colour blind increases both the social and scientific impact of citizen science.


Validation of S3 OLCI observations
S. Peters, S. Ghezehegn, S. Lazaros, M. Laanen & A. Hommersom. (2020). Validation of S3 OLCI observations using one year of semi-continuous WISPstation measurementsin the high dynamic area of the Eems Estuary. Zenodo.


Abstract: The Eems Estuary is a very dynamic area featuring highly variable turbidity and Chlorophyll-a values. There is an interest to decrease theturbidity and monitoring is being put into place to observe the current status and changes. Remote sensing using Sentinel 3 OLCIobservations is a candidate monitoring technique but should provide robust and validated results. Obtaining high quality turbidity estimates starts with validated Bottom of Atmosphere reflectances. Read more...

Construction of the Solar-tracking Radiometry platform (So-Rad)

A. Wright & S. Simis. (2021, February 1). Construction of the Solar-tracking Radiometry platform (So-Rad) (Version 1.0). Zenodo.


The purpose of the Solar-tracking Radiometry platform is to maintain optimal viewing angles of radiance sensors recording water-leaving reflectance (water colour), avoiding sun glint and platform shading even from moving platforms such as ships or buoys. The system is developed to operate autonomously, with low power consumption, integrating commercially available (ir)radiance sensors and providing remote connectivity. All hardware and software are open-source, through this repository and the associated software repository ( Their use is licensed under a creative commons non-commercial license.

iSPEX 2: A universal smartphone add-on for portable spectroscopy and polarimetry

O. Burggraaff, A. Perdujin, R. van Hek, N. Schmidt, C. Keller and F. Snik (2020). "A universal smartphone add-on for portable spectroscopy and polarimetry: iSPEX 2"

Abstract: Spectropolarimetry is a powerful technique for remote sensing of the environment. It enables the retrieval of particle shape and size distributions in air and water to an extent that traditional spectroscopy cannot. SPEX is an instrument concept for spectropolarimetry through spectral modulation, providing snapshot, and hence accurate, hyperspectral intensity and degree and angle of linear polarization. Successful SPEX instruments have included groundSPEX and SPEX airborne, which both measure aerosol optical thickness with high precision, and soon SPEXone, which will fly on PACE. Here, we present a low-cost variant for consumer cameras, iSPEX 2, with universal smartphone support. Smartphones enable citizen science measurements which are significantly more scaleable, in space and time, than professional instruments. Universal smartphone support is achieved through a modular hardware design and SPECTACLE data processing. iSPEX 2 will be manufactured through injection molding and 3D printing. A smartphone app for data acquisition and processing is in active development. Production, calibration, and validation will commence in the summer of 2020. Scientific applications will include citizen science measurements of aerosol optical thickness and surface water reflectance, as well as low-cost laboratory and portable spectroscopy.

Biases from incorrect reflectance convolution

O.Burggraaff (2020). "Biases from incorrect reflectance convolution" Optics Express, Vol. 28, Issue 9, pp 13801-13816.

Abstract : Reflectance, a crucial earth observation variable, is converted from hyperspectral to multispectral through convolution. This is done to combine time series, validate instruments, and apply retrieval algorithms. However, convolution is often done incorrectly, with reflectance itself convolved rather than the underlying (ir)radiances. Here, the resulting error is quantified for simulated and real multispectral instruments, using 18 radiometric data sets (N = 1799 spectra). Biases up to 5% are found, the exact value depending on the spectrum and band response. This significantly affects extended time series and instrument validation, and is similar in magnitude to errors seen in previous validation studies. Post-hoc correction is impossible, but correctly convolving (ir)radiances prevents this error entirely. This requires publication of original data alongside reflectance.

O. Burggraaff, N. Schmidt, J. Zamorano, K. Pauly, S. Pascual, C. Tapia, E. Spyrakos, and F. Snik, (2019). "Standardized spectral and radiometric calibration of consumer cameras," Opt. Express  27, 19075-19101. 

Abstract: Consumer cameras, particularly onboard smartphones and UAVs, are now commonly used as scientific instruments. However, their data processing pipelines are not optimized for quantitative radiometry and their calibration is more complex than that of scientific cameras. The lack of a standardized calibration methodology limits the interoperability between devices and, in the ever-changing market, ultimately the lifespan of projects using them. We present a standardized methodology and database (SPECTACLE) for spectral and radiometric calibrations of consumer cameras, including linearity, bias variations, read-out noise, dark current, ISO speed and gain, flat-field, and RGB spectral response. This includes golden standard ground-truth methods and do-it-yourself methods suitable for non-experts. Applying this methodology to seven popular cameras, we found high linearity in RAW but not JPEG data, inter-pixel gain variations >400% correlated with large-scale bias and read-out noise patterns, non-trivial ISO speed normalization functions, flat-field correction factors varying by up to 2.79 over the field of view, and both similarities and differences in spectral response. Moreover, these results differed wildly between camera models, highlighting the importance of standardization and a centralized database.


Book chapter

Ceccaroni, L., & Piera, J. (2018). Stakeholder engagement in water quality research: A case study based on the Citclops and MONOCLE projects. In Hecker S., Haklay M., Bowser A., Makuch Z., Vogel J., & Bonn A. (Eds.), Citizen Science: Innovation in Open Science, Society and Policy (pp. 201-209). London: UCL Press.