IOCCG Summer Lecture Series 2012

2 – 14 July 2012 - Villefranche-sur-Mer

The International Ocean Colour Coordinating Group (IOCCG) organized the first IOCCG Summer Lecture Series dedicated to high level training in the fundamentals of ocean optics, bio‐optics and ocean colour remote sensing. This was a 2‐week intensive course that took place from July 2nd – 14th at the Laboratoire d'Océanographie de Villefranche (LOV), Villefranche‐sur‐mer, France. A total of 13 renowned lecturers were invited to teach at the course and 17 students from 13 different countries took part in the course. More than 100 students had applied to participate in the course and the 17 remaining applicants were primarily chosen with respect to their motivation and on the basis of their academic background. The majority of them were PhD students and post‐Docs, and some were starting their careers as young researchers. The participants came from a broad range of backgrounds, but all were familiar with at least some domains of ocean colour science and had a solid understanding of ocean colour remote sensing.

Errors and uncertainties in ocean colour remote sensing (2)

by Roland Doerffer, Helmholtz Center Geesthacht /Brockman Consultants, Germany

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Abstract

One of the main questions you will be asked as a remote sensing expert is: how reliable and good is information, which we derive from remotely sensed ocean colour data? Can we trust them? What is the error or uncertainty range of these data? In this section of the IOCCG training course, which consists of 3 lectures and exercises, we will look into this problem.

Lectures
The first lecture will be dedicated to the sources of uncertainties. We have to consider that our observations are the reflectivity in a number of spectral bands, which are measured at the top of atmosphere (TOA) or, in case of an aircraft platform, in a certain height above the water. We try nothing less than to isolate, retrieve and quantify a small effect on these spectra, which is caused by absorption and scattering of e.g. of phytoplankton, from a large number of other effects, of which in particular the atmosphere dominates the TOA spectrum. Problems of this kind may induce large uncertainties. In some cases it might be even impossible to retrieve reliable information of the ocean from remotely sensed reflectance spectra. Thus, one important area of ocean colour research is to analyze sources of uncertainties, to develop methods to quantify uncertainties and finally to find way to reduce uncertainties.
In this lecture we will consider

  • Natural factors, which determine uncertainties, and their variability
  • Uncertainties, which are induced by reducing the manifold of factors to a few dominant wavelength (nm)
  • Radiance (Wm‐2 sr‐1 μm‐1) air molecules different aerosols thin clouds
  • Sky reflectance Sun glint foam floating material chlorophyll
  • Suspended particles different phytoplankton species dissolved organic matter
  • Vertical distribution Bottom reflection contrails
Factors, which determine top of atmosphere reflection spectra, from which try to retrieve e.g. the chlorophyll concentration
  • Errors caused by spaceborne or airborne instruments: calibration, ageing, noise
  • Errors caused by in situ measurements, sampling and procedures
  • Problem of comparing in situ with space borne
In the second lecture we look into procedures, how to determine uncertainties:
  • How to quantify uncertainties: scatter, bias, robustness, stability
  • Validation procedures and strategies
  • Testing of algorithms
  • Round robin exercises
  • Sensitivity studies
  • Determination of uncertainties on a pixel by pixel bases
  • flagging
The third lecture will finally discuss the results of our exercises and will be dedicated to the question, how to reduce uncertainties. This is a wide field, where a lot of research is still needed, and it offers themes for your future work.
  • Detection of spectra / pixels, which are out of scope of the algorithm
  • Masking of clouds and cloud shadows
  • Use of additional information
  • Pre‐classification of water types and use of dedicated algorithms
  • How to produce maps from satellite data, which include information about uncertainties.


Speaker(s) : Roland Doerffer, Helmholtz Center Geesthacht /Brockman Consultants, Germany
Public : All
Date : Tuesday 10 july 2012
Place : Villefranche-sur-Mer