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 Chief Top. Engineer, dated Chicago, November 15th 1856." "Map G. No. 40." 1 map; 54 x 99 cm This map is in the public domain. Please attribute access and use of this digitized map to the Stephen S. Clark Library, University of Michigan Library.