Frost line depth in Mellette County, SD
How deep the ground freezes in Mellette County, SD
37 in. Dig to 38 in, just below the frost. Nearest NOAA station: Cedar Butte, 13.2 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.