Hydrolysis of 1,4-alpha-D-galactosiduronic linkages in pectate and other galacturonans. [(1->4)-alpha-D-galacturonosyl](n) + H2O = [(1->4)-alpha-D-galacturonosyl](n-1) + alpha-D-galacturonate Belongs to the glycosyl hydrolase 28 family.