The soundings are in feet & tenths of a foot. "The figures on land express the elevations above the lake surface in feet and decimals. The horizontal curves or contour lines are drawn at intervals of 5 feet of perpendicular elevation." "Accompanying Lt. Colonel J.D. Graham's annual report (no. 161) to the Chief Topographical Engineer, dated Chicago, November 15th 1857." "Map G. No. 47." 1 map; 66 x 96 cm