A copy of the field notes of the original survey of each county by the United States shall be procured, and a map of the county shall be constructed in accordance therewith on a scale of not less than one inch to a mile, and laid off in townships and sections. Such map and field notes shall be deposited in the office of the county clerk, and be by him preserved. Whenever the boundaries of any county are changed, the necessary alteration in such map may be made, or a new map of the county may be made if the county board so directs.