What is the test for determining the value of a fee paid by the owner of a piece of land condemned as an easement under eminent domain?

In cases where the fee, after being burdened with the easement created by the condemnation, will retain a substantial value, then it is held that the 'condemning party is not bound to pay the full value of the land taken, but merely the decrease in market value that is due to the imposition of the public easement; in other words in awarding compensation, the value of the interest in the land remaining to the owner is to be deducted from the fair market value of the land' which will underlie the easement. (18 Am.Jur. 889; see also City of Oakland v. Schenck (1925), 197 Cal. 456, 465, 241 P. 545; 29 C.J.S. Eminent Domain 143, p. 987, note 64.) This rule has been applied frequently in determining the amount of compensation[49 Cal.2d 554] to be paid to the owner of a strip of land over which power or other wire lines are erected. (See 124 A.L.R. 418; 49 A.L.R. 700.)