Frost line depth in Grant County, SD
How deep the ground freezes in Grant County, SD
57 in. Dig to 57 in, just below the frost. Nearest NOAA station: Milbank, 8.2 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.