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If you use ACad civil engineering, C3D, it automatically creates contour lines and surfaces from the grid data. One way would be to draw circles from the point, P to the 4 grid points surrounding it and measure their radius, say r1,r2... then a good estimate is $Z_P= \frac{Z1r1+Z2r2+Z3r3+Z4r4}{r1+r2+r3+r4}$