1 map ; 51 x 70 cm "Soundings are in feet and 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 Graham's annual report (no. 200) to the Chief Top. Engineer, dated Chicago September 30th 1857." "Map G no. 50."