Abstract:The difficulties and uncertainties caused by reference datum changes in land subsidence estimation are discussed. In order to solve the difficulty,the method in which the velocity datum is determined by the observed elevation variations time series combined with the repeated leveling data at several GNSS stations is presented. Some practical examples are given for the comparison between this method and the traditional one,it is proved that the new method has some advantages.On the basis of these results, some recommendations about the land subsidence monitoring and the evaluation method in the Tianjin area are proposed. These recommendations may have some helps for land subsidence monitoring and evaluation in other similar areas.