The soundings are in feet & decimals of a foot. "Accompanying Lt. Colonel J.D. Graham's annual report (no. 161) to the Chief Topographical Engineer, dated Chicago, November 15th 1857." "Map G. No. 46." 1 map; 51 x 61 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.