Frost line depth in Ziebach County, SD
How deep the ground freezes in Ziebach County, SD
42 in. Dig to 42 in, just below the frost. Nearest NOAA station: Dupree, 4.9 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.