The errors of surface profile measurements using grid-illumination Makyoh topography is analysed using computer simulations. It is shown that two regions of imaging exists: at smooth surfaces and small screen-to-sample distances, the error is determined by the pixel reading error, while at less smooth surfaces and larger screen-to-sample distances, the errors of summation approximation dominates. The pixel reading error component can be decreased by increasing the screen-to-sample distance. It is demonstrated that the described evaluation algorithm gives maximum relative errors of roughly 1.5%.