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 3 feet of perpendicular elevation." "Accompanying Lt. Colonel J.D. Graham's annual report (no. 200) to Chief Top. Engineer, dated Chicago, September 30th 1857." "Map G. No. 54." 1 map; 65 x 101 cm