The precision measurement of ultra smooth surface roughness is most important part of the precision manufacture. In order to study how does the topography apparatuses influence on the measurement result, the K9 glass material selected is polished in float polishing method. Using four different principle topography apparatuses, the ultra smooth surface of the workpiece is measured. Comparing to the different measurement results, the practical precision of the workpiece is got. By analyzing the principle of different topography apparatuses, decision factor of the resolving power, result of the error and the influence on the roughness measurement, the roughness measurement criterion and the principle of how to select the topography apparatus are got.