You've photographed the marker for what the Canadian Hydrographic Service (CHS) calls a horizontal control point, or "station" - in other words, a place on the surface of the earth whose position has been precisely established using surveying techniques. Once in place, with its identifying number (which CHS officials can look up in their database to find the station's latitude and longitude, among other data), such a mark is used as a point of reference for establishing other positions, most often for creating or updating navigation charts.

The first official survey of Canada's navigable waters was established for Georgian Bay and Lake Huron in 1883, following the sinking of the steamship Asia - apparently by grounding on an uncharted Georgian Bay shoal - with the loss of 150 lives. Today's CHS, a division of Fisheries and Oceans Canada, continues to survey and update about 1,000 charts that cover our vast expanses of coastal and inland waterways, using equipment such as multibeam echo sounders and the Canadian Coast Guard's Differential Global Positioning System to pinpoint positions.

By the way, we know where you were when you took this picture. Your marker is called Station OBRI, and it's located on the largest of the O'Brien Islands, a tiny island group southwest of Pointe au Baril in Georgian Bay. According to the surveyor's accompanying sketch, you were about 10 metres from the water's edge, and six metres from a clump of cedars. To be precise, you were at latitude 45°33'01.998" N, longitude 80°31'16.636" W.