For the purpose of determining the revenue-rates, the settlement officer may divide any area into units and in forming such units, he shall have regard to the physical features, the agricultural and economic conditions and trade facilities and communications; and shall then determine the revenue-rates for different classes of lands in each unit in the manner and according to the principles prescribed and in particular, in the case of agricultural land, to the profits of agriculture, to the consideration paid for leases, to the sale prices of land and to the principal monies on mortgages and in the case of non-agricultural land, to the value of the land for the purpose for which it is held.