The average natural slope of a property shall be determined on the basis of measurements of areas to be devoted to residential use and any open space provided. Any open space to be purchased by a public agency, or any areas devoted to commercial or other nonresidential use, shall be excluded from the acreage to be measured. (Ord. 1512 § 2, 1973; Ord. 1212 § 1, 1969; prior code § 33.601 (A)(7)(2)(1)).