The method of combining digital terrain models based on the results of photogrammetric processing optoelectronic stereo photographs and interferometric one of radar images developed in the view of the least squares principle is substantiated in the article. The necessary formulas for assessing the impact of systematic errors in jointly handled data sets are given. As a result of analyzing the systematic errors influence in setting the beginning of the height calculation in alternative models on the values of the combined relief model parameters, it is shown that it is impossible to estimate and completely exclude systematic errors of the desired characteristics of the combined relief model without involving additional measurement information. The methodology of the approach to the suppression of the mentioned mistakes in determining the required data is theoretically justified with the necessary mathematical expressions for the implementation of the stages of the proposed techniques based on a comparison of the height matrices obtained through the least squares method and the materials of space laser altimeters