Abstract. A new Sentinel-1 image-based sea ice classification
algorithm using a machine-learning-based model trained in a semi-automated
manner is proposed to support daily ice charting. Previous studies mostly
rely on manual work in selecting training and validation data. We show that
the readily available ice charts from the operational ice services can
reduce the amount of manual work in preparation of large amounts of
training/testing data. Furthermore, they can feed highly reliable data to
the trainer by indirectly exploiting the best ability of the sea ice experts
working at the operational ice services. The proposed scheme has two phases:
training and operational. Both phases start from the removal of thermal,
scalloping, and textural noise from Sentinel-1 data and calculation of grey
level co-occurrence matrix and Haralick texture features in a sliding
window. In the training phase, the weekly ice charts are reprojected into
the SAR image geometry. A random forest classifier is trained with the
texture features on input and labels from the rasterized ice charts on
output. Then, the trained classifier is directly applied to the texture
features from Sentinel-1 images operationally. Test results from the two
datasets spanning winter (January–March) and summer (June–August) seasons acquired
over the Fram Strait and the Barents Sea showed that the classifier is
capable of retrieving three generalized cover types (open water, mixed
first-year ice, old ice) with overall accuracies of 87 % and 67 % in
winter and summer seasons, respectively. For the summer season, the classifier
failed in distinguishing mixed first-year ice from old ice with accuracy of
only 12 %; however, it performed rather like an ice–water discriminator
with high accuracy of 98 % as the misclassification between the mixed
first-year ice and old ice was between them. The accuracy for five cover
types (open water, new ice, young ice, first-year ice, old ice) in the winter
season was 60 %. The errors are attributed both to incorrect manual
classification on the ice charts and to the semi-automated algorithm.
Finally, we demonstrate the potential for near-real-time service of the ice
map using daily mosaicked Sentinel-1 images.