With thanks to Larry R. Newett of Natural Resources Canada. From The Canadian Surveyor, Vol. 34, No. 4. December 1980: Newitt, L.R. and Dawson, Edward. "Kingston Harbour Magnetic Anomaly."
(28) Local magnetic disturbances.-- Differences from normal variation of from about 006 W to 007 E have been observed at numerous locations throughout Lake Ontario. Differences of up to 37 have been observed in the approach to Kingston, Ont., on the N side of the head of the St. Lawrence River. The locations of these anomalies are shown on NOS chart 14500.
Using autopilot heading east (toward Fort Henry) 200m off the shore out of Portsmouth is liable to be very exciting. Just off the water plant, expect your boat to steer sharply toward the rocks! Whoopie!