Abstract: The transformation model under special datum employed in this paper is the similarity transformation model with seven transformation parameters;this transformation can keep the steadiness of the relative position in the deformation block and can express the relative movement information of transformed displacement vectors in the block.The diagnosis of abnormal displacements is performed using quasi-accurate detection method(QUAD).After properly selecting the quasi-accurate observations,this method can make the real errors of displacement take on the distinct hive off phenomena.The results of a simulated example show good effect of linear similarity transformation method and QUAD method.