Ice cover is expressed as a percentage for each lake, using a virtual lattice, or grid, on maps of the Great Lakes as a guide. These data are from the Canadian Ice Service and National Ice Center and are based on various imagery sources with resolutions down to 50 meters per pixel. Imagery sources include but are not limited to ENVISAT, DMSP OLS, AVHRR, and RADARSAT. Ship reports and other observations are also included. From 1973 to 1988, only the Canadian Ice Service was producing ice maps for the Great Lakes. Since 2001, the National Ice Center has been issuing a joint Canadian/U.S. product for the Great Lakes. For more details on this data set, see the following:
UPDATE 08/17/2016: Ice cover data has been re-processed as a discontinuity in the database was discovered. Previously, the pixels containing less than 10% ice were not included in the ice cover totals because ice climatologists consider those areas to be 'open water'. However, since all ice was totaled for the Great Lakes Ice Atlas, we re-calculated totals to keep the database internally consistent.
This time series may look unusual in places where the beginning of the ice season was missed. This was more apt to happen in the 1970's and 1980's.
WANG, J., X. BAI, H. HU, A.H. CLITES, M.C. COLTON, and B.M. LOFGREN Temporal and spatial variability of Great Lakes ice cover, 1973-2010. Journal of Climate 25(4):1318-1329 (DOI:10.1175/2011JCLI4066.1) (2012).
Also see the GLERL Great Lakes Ice Cover webpage