Author(s): Mikko Suominen; Jakke Kulovesi; Mikko Lensu; Jonni Lehtiranta; Pentti Kujala
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Abstract: The determination of along track ice thickness from a moving ship is a regular activity during research cruises to ice covered seas. Traditionally, ice thickness has been estimated as a part of visual ice observations from ice pieces broken and turned to upright position by the ship. As the observations are commonly conducted from the bridge, the distance between the observer and the broken ice is large in comparison to the ice thickness. Therefore, the observations are considered indicative. The accuracy can be improved by determining the thickness of the pieces from video images instead. For this purpose a new stereo camera system has recently been developed at Aalto University. The two cameras look downwards over the side of the ship and take simultaneous photos of the broken ice pieces which enables the producing of 3D-images and accurate determination of the ice thickness. Another suitable method for thickness measurements is electromagnetic device (EM device). The EM device seeks to measure the total ice thickness to which may contribute unconsolidated rafted layers or ice block mass of ridge keels. Therefore, the EM values are generally larger than the visually observed values that mostly refer to the consolidated upper layer. During the ice trials of S. A. Agulhas II, in the Baltic Sea in March 2012, the ice thickness was measured simultaneously with the stereo camera system and EM device and the ice conditions were estimated visually. In this paper, the measures of descriptive statistics of ice thickness determined with the three methods are compared. The correlations between EM and stereo camera thicknesses are weak as the methods target to different aspects. The comparison of the stereo camera measurements and visual observations showed that the observers tend to overestimate the ice thickness. The total thickness estimated from observed thicknesses and ridging parameters agrees in part well with the EM thickness. The results are indicative and the correspondence could be improved by better methodology for the visual observations.
Year: 2014