What constitutes ice-on? It is hard to know what standards were used in the "old days". Since 1993, we have employed the following standard for determining what is the lake freeze-up date: when the entire lake is skimmed over by ice (regardless of thickness) and remains so for the rest of the winter. In this observation we ignore minor areas that may open up because of warm conditions or winds. This is more or less the standard that is used by observers in other parts of the country.
Ice-off dates are a little more complicated because the melting process tends to be more gradual. Again we rely on the standard that is suggested by ice observers elsewhere - when the lake is "substantially" ice-free (90% or more) it is considered melted. Hence there still may be some floating ice or some wind-driven piles of ice along some shorelines.