Frost line depth in Brule County, SD
How deep the ground freezes in Brule County, SD
44 in. Dig to 44 in, just below the frost. Nearest NOAA station: Chamberlain 5 S, 11 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.