Abstract: Based on the error propagation law,the theoretical accuracy model of exterior orientation elements to vertical parallax is proposed in this paper firstly. Then,two sets of actual data at different image scales were used for experiments. The empirical results show that when the relativities between the exterior orientation elements are considered,the theoretical accuracy of vertical parallax calculated can embody the actual accuracy of image exterior orientation elements exactly. But without considering the relativities,the results of theoretical accuracy are rather poor,existing obvious systematical error,which can not reflect the accuracy of exterior orientation elements obtained Via POS properly. Therefore,to use image exterior orientation elements obtained Via POS for photogrammetry properly,the covariance matrix of image exterior orientation elements should be provided together.