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INTEGRATED USE OF INTERFEROMETRIC SAR DATA AND LEVELLING 
MEASUREMENTS FOR MONITORING LAND SUBSIDENCE 
: Yuegin Zhou', Martien Molenaar , Deren Li" 
"International Institute for Aerospace Survey and Earth Sciences (ITC), the Netherlands 
zhouyq G'itc.nl, molenaar G itc.nl 
^ Wuhan Technical University of Surveying and Mapping, China 
dli ?hp827s.wtusm.edu.cn 
KEY WORDS: Land subsidence, Monitoring, Levelling, Differential SAR interferometry, Integration. 
  
  
ABSTRACT 
Land subsidence caused by withdrawal of groundwater has become one of the most serious hazards in many parts of the 
world, particularly in densely populated and economically developed coastal areas. Levelling is widely applied, as one of 
the most accurate geodetic techniques, in the world for monitoring land subsidence. But, when levelling is applied over 
areas of hundreds of square kilometers, the discrete point technique is not effective to represent deformation unless very 
densely distributed points are measured, which in turn costs a lot of money and time. 
Differential SAR interferometry (D-InSAR) is a remote sensing technique that can be used for monitoring small surface 
movements, such as co-seismic and post-seismic movements, land subsidence, land slide, ice motion etc. However, the 
quality of D-InSAR products are largely degraded by many factors, which in turn makes the accuracy of D-InSAR is not 
high enough to measure cm-level subsidence. 
An optimal solution to solve the above problems is to integrate D-INSAR with levelling. Using the sparse levelling 
measurements as ground control and ground truth, the D -InSAR products can be quantitatively assessed; and furthermore, 
the errors can be modelled. This integrate method has been tested in Tianjin area. The preliminary results show that: (1) D - 
InSAR can detect the cm-level subsidence 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 thus the accuracy of D- 
InSAR products can be improved. 
1. INTRODUCTION 
With the rapid development of economy, which increases the requirements of fresh water, land subsidence caused by 
groundwater extraction has become serious hazard in many parts of the world, particularly in densely populated and 
economically developed coastal areas. Even though this kind of subsidence is often very slow (e.g. a few centimeters per 
year), it can have serious damage on the infrastructure since it is often large spatial extent, and can be up to several meters 
after long-term acceleration. 
Levelling is widely applied, as one of the most accurate geodetic techniques, in the world for monitoring land subsidence. 
The applications of the technique may sometimes be limited by cost since it can only provide subsidence information at a 
spatially discrete set of locations, hence a large amount of point measurements are required in order to cover a large area to 
obtain the spatial distribution in detail. And also interpolation is needed to get a continuous two -dimensional coverage. 
Differential SAR interferometry (D-InSAR) is a new remote sensing technique for monitoring small surface movements, 
such as co-seismic and post-seismic movements, land subsidence, land slide, ice motion etc. Unlike levelling, the technique 
provides movement information with continuous, non-interpolated coverage. This characteristic makes it very effective for 
measuring surface movements over large areas. D-InSAR is effective for measuring co-seismic movements that are sudden 
events. That has been demonstrated in many applications, among which is the Landers (California) earthquake in 1992 
(Massonnet et al., 1993; Zebker et al., 1994). The applications of D-InSAR to measure cm-level subsidence have been 
investigated (Kooij, 1995a, 1995b, 1997; Massonnet, Holzer and Vadon, 1997). However, there are many factors that 
degrade the accuracy of the differential interferometric results. It is only possible to use D-InSAR to measure cm-level 
motion if the errors are effectively corrected. One existing method to correct the errors is to average multiple products, an 
alternative approach is to use empirical models to model the effects. The use of these two methods may sometimes be 
limited by data availability. 
An optimal solution to solve the above problems is to integrate D-InSAR with levelling. Using the sparsely distributed 
levelling measurements as the ground truth, the D -InSAR products can be quantitatively assessed; and furthermore, the 
errors can be modelled. In this paper, we focus on the integrated use of interfermetric SAR data and levelling measurements 
for monitoring slow land subsidence over large areas. The main objective is to improve the accuracy of differential 
interferometric results by using levelling measurements. 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B1. Amsterdam 2000. 353