So, this is how you can describe the wave elevation at a given location taking into the directional aspects any doubts. Shall I proceed? Now equation sometimes is employed in numerical simulation naught sometimes it is mostly employed in simulation of 3 D waves. In such a case a m can be represented as shown here. Already I have explained this is how you represent amplitude as a function of spectral density or the spectral density is nothing but half amplitude of amplitude square. So, the determination of a m by equation 15 is incorrect from view point of wave variability. Because that is a general equation that when you set x and y equal to 0 then this equation 13 and 14 for a fixed location without losing its general description can be rewritten as shown here.