For the purpose of this act, undeveloped areas shall mean those areas which:

1. (a) Are in excess of 21/2 acres;
2. (b) have not been platted;
3. (c) are primarily used for agricultural purposes; and
4. (d) have a population density of less than one family per acre.

L. 1969, ch. 95, § 4; L. 1981, ch. 80, § 4; L. 1991, ch. 64, § 3; July 1.