You have already used griddata! It looks like you have found a code snippet, but have no clue as to what it does or why it works. So read the help for griddata.
Now you wish to interpolate at some general point. You can either use griddata again at that general point, or since you have apparently now formed a regular grid you COULD use interp2.
The latter seems logical, but it is in fact worse, because then you have two sources of interpolation error. So just use griddata, as you did already.