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The Cryosphere An interactive open-access journal of the European Geosciences Union
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 30 Aug 2019

Submitted as: research article | 30 Aug 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal The Cryosphere (TC).

Accuracy and Inter-Analyst Agreement of Visually Estimated Sea Ice Concentrations in Canadian Ice Service Ice Charts

Angela Cheng1,2, Barbara Casati3, Adrienne Tivy1, Tom Zagon1, Jean-Francois Lemieux3, and Bruno Tremblay2 Angela Cheng et al.
  • 1Canadian Ice Service, Environment and Climate Change Canada, Ottawa, Ontario, Canada
  • 2Department of Atmospheric and Oceanic Sciences, McGill University, Montréal, Québec, Canada
  • 3Environment Canada, Recherche en Prévision Numérique Environnementale, Dorval, Québec, Canada

Abstract. This study compares the accuracy of visually estimated ice concentrations by eight analysts at the Canadian Ice Service against three standards: i) ice concentrations calculated from automated image segmentation, ii) ice concentrations calculated from automated image segmentation that were validated by the analysts, and iii) the modal ice concentration estimate by the group. A total of 76 pre-defined areas in RADARSAT-1/RADARSAT-2 imagery are used in this study. Analysts over-estimate ice concentrations when compared to all three standards, most notably for low ice concentrations (1/10–3/10). The spread of ice concentration estimates is highest for middle concentrations (5/10, 6/10), and smallest for 9/10. The over-estimation in low concentrations and high variability in middle concentrations introduces uncertainty in the ice concentration distribution in ice charts. The uncertainty may have downstream implications for numerical modeling and sea ice climatology. Inter-analyst agreement is also measured to determine which classifier’s ice concentration estimates (analyst or automated image segmentation) disagreed the most. It was found that one of the eight analysts disagreed the most, followed second by the automated segmentation algorithm. This suggests high agreement in ice concentration estimates between analysts at the Canadian Ice Service. The high agreement, but consistent overestimation, results in an overall accuracy of ice concentration estimates in polygons to be 39 %, 95 % CI [34 %, 43 %] for an exact match in ice concentration estimate with calculated ice concentration from segmentation, and 84 %, 95 % CI [80 %, 87 %] for +/- one ice concentration category. Only images with high contrast between ice and open water, and well-defined floes are used: true accuracy is expected to be lower than what is found in this study.

Angela Cheng et al.
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Angela Cheng et al.
Angela Cheng et al.
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Publications Copernicus
Short summary
Sea ice charts by the Canadian Ice Service (CIS) contain visually estimated ice concentration produced by analysts. The accuracy of manually derived ice concentrations is not well understood. The subsequent uncertainty of ice charts results in downstream uncertainties for ice charts users, such as models and climatology studies, and when used as a verification source for automated sea ice classifiers. This study quantifies the level of accuracy and inter-analyst agreement for ice charts by CIS.
Sea ice charts by the Canadian Ice Service (CIS) contain visually estimated ice concentration...