How can I calculate the surface area from a griddata interpolation?

Hi,

I would like to calculate the surface area from a griddata interpolation ZI =griddata(Voff,Ioff,Eoff,XI,YI,'linear'). So I have a surface form (XI,YI,ZI), all of them are < 167 x 167 double>. Then, I have applied Eoff_rows=trapz(t,ZI,2) where t represents the time variable (< 1 x 167 double >) and finally, Eoff_tot=sum(Eoff_rows) in order to calculate the surface area. I am not sure I have proceeded coorectly. Any suggestion? Thanks