This polygon shapefile contains ice observations in the Arctic region for April 30, 1975. This layer is part of the Arctic Climate System (ACSYS) Historical Ice Chart Archive. The earliest chart in the data set comes from 1553, when Sir Hugh Willoughby and Richard Chancellor, commanders of two expeditions sent out by the Company of Merchant Adventurers, recorded their observations of the ice edge. Early charts are irregular and infrequent, reflecting the remoteness and hostility of the region. The frequency of observations generally increases over time, as the economic and strategic importance of the Arctic grew, along with the ability to access, observe and record information on sea ice. The Norwegian Meteorological Institute in Tromsø used a combination of satellite imagery and in situ observations to produce daily digital charts each working day. These show not only the ice edge, but also detailed information on the range of sea ice concentrations and ice types. The Norwegian Meteorological Institute is continuing this series, and more recent charts may be obtained from this source. The ACSYS Historical Ice Chart Archive presents historical sea-ice observations in the Arctic region between 30ºW and 70ºE. The earliest chart dates from 1553, and the most recent from December 2002.
