There are alternatives to griddata() such as scatteredInterpolant evaluated upon a grid. Some of them allow you to adjust the interpolation method. But all of them are going to use multiple values to deduce the value at any one location. If you need to be more precise then you could increase the resolution, producing a larger matrix.
Perhaps what you would like is to use scatter() with a small pointsize and with the c (color) value set according to the z coorinate. There are various ways to capture the result into a matrix, including getframe() and print() and saveas()
