Frost line depth in Douglas County, SD
How deep the ground freezes in Douglas County, SD
39 in. Dig to 39 in, just below the frost. Nearest NOAA station: Armour, 5.4 mi.
Your building department sets the legal frost line depth (IRC Table R301.2(1)); footings must go below it (R403.1.4.1). Call them before you dig.