Yueqin Zhou 
  
It is obvious that the residuals both at the control points and at the checkpoints follow the normal distribution with zero 
mean, which means the errors estimated by (7) are the unbiased estimates of the true errors. Therefore it is reasonable to use 
the polynomial to model errors contained in the D-InSAR products. Using (6) and (7), the D-InSAR height change model is 
then calibrated. 
5. DISCUSSION 
In the procedure of error modelling, the levelling measurements are used as the ground truth. In fact, the height values of 
the levelling points are not original measurements but obtained by interpolation. The interpolation actually decreased the 
accuracy of control points. In addition, the coordinate tran sformation can also result in errors. Thus, the standard derivation 
at the control points and the RMS at the checkpoints with the values of 1.46 cm and 1.21 cm, respectively, do not represent 
the true accuracy of error modelling but contain extra errors caused in the procedure of error modelling. Therefore, the 
accuracy of error modelling by the polynomial needs to be further investigated. 
6. CONCLUSIONS 
The method of integrated use of interferometric SAR data and levelling measurements for monitoring land subsidence was 
discussed in the paper. The method was tested in Tianjin area. The preliminary results show that: (1) D -InSAR can detect 
the cm-level subsidence occurred in the urban area where the conherence is high enough; (2) the errors between the 
subsidence values measured by D-InSAR and those by levelling can be quantitatively modelled, and thereafter, the accuracy 
of D-InSAR products can be improved. The further work is to design a levelling network containing a limited number 
points but to be strong enough to control the whole covered area. This will be useful in practical applications to reduce the 
costs while the accuracy is not decreased. 
ACKNOWLEDGEMENTS 
The authors would like to knowledge Prof. L.R. Huang, First Land Deformation Monitoring Center, Earthquake Bureau of China, and 
Mr. Ke Hu, Tianjin Bureau of Surveying and Mapping, for their providing the ground survey dada. We would also like to thank 
Dr.Theo Bouloucos for the helpful discussion and suggestions. This work was supported by "The Science and Technology 
Development Program (No.9940)" from Tianjin City Constructional Commission, China . 
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358 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B1. Amsterdam 2000.