How to calculate the volume under a gridded surface in Matlab

How to calculate the volume under a gridded surface in Matlab

I have a non-uniformly spaced x,y,z data (Eastings, Nothings and
thickness) which I have gridded and plotted as a surface using 'meshgrid',
'griddata' and 'surf'. However, I would like to calculate the volume under
this surface and am struggling to apply other answers to this question to
my dataset and code in question (I am a complete Matlab beginner). Any
ideas how to do this (on Matlab version 2011b)? Thanks in advance.
load E.txt, load N.txt, load Z.txt;
[xi, yi] = meshgrid(25.351639:0.00025:25.426483, 36.363799:0.0005:36.458845);
zi = griddata(E,N,Z, xi,yi, 'linear');
surf(xi,yi,zi)