In order to assess accuracy of deformation monitoring, we should consider not only measurement accuracy of instrument, but also influences of field condition errors. All these influences exist with random noise during the whole deformation monitoring. We can separate it from observations with wavelet transformation, and assess accuracy of deformation monitoring as random error. Some experiments are done with actual observations, and system error and random error caused by influences of instrument error and random noise are considered. Therefore we can evaluate the quality of deformation monitoring objectively.