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At the left margin, the combination of the river, low coherence fields and the image edge produces a very dark area. This may be subsidence, as those fields are using pumped water, but there's clearly an error component included. There's a similar confusion on the right margin of Figure 4. These areas must be considered as having a lower confidence. In Figure 6, the information in these two images is combined to aid interpretation, providing a geocoded subsidence map shown in the context of a local DEM, also created from TerraSAR-X InSAR images. In creating this, pixels with a coherence below 0.35 were considered unreliable and colored gray. These indicate the trace of the river through the scene's center, particularly in the agriculture/residential areas at the image's east and west margins. This image map shows good correspondence with the image in Figure 1. It's worthwhile to consider the shape of the DInSAR-mapped subsidence, noting its irregularity. There's no way that several point measurements, taken as discussed, could give a picture of the sub- sidence in this area. Indeed, it would be quite pos- sible to miss the feature entirely. This subsidence pattern also indicates the subsurface structure, providing engineers an additional glimpse into the formation they're mining. Author's Note: I would like to thank Astrium Services/Infoterra GmbH for providing 20 TerraSAR-X strip-map mode scenes, which served as input data for this work. In addition, I'd like to acknowledge the cooperation of the Department of Geophysics at the Arizona Department of Water Resources. References Arizona Department of Water Resources. July 2011. "McMullen Land Subsidence Feature," azwater.gov/ AzDWR/Hydrology/Geophysics/McMullenSubsidence.htm. Crosetto, M.; Castillo, M.; and Arbiol, R. 2003. "Urban subsidence monitoring using radar interfer- ometry: Algorithms and validation," Photogrammetric Engineering and Remote Sensing, Vol. 69, No. 7, pages 775-783. Maruya, M.; Yoshimoto, S.; Miyawaki, M.; Yamaguchi S.; Kiselevskiy, E.; Korviakov, P .; Sergeev D.; Baranov, Y.; Ando, G.; and Kawai, M. 2009. "Monitoring Surface Deformations over Siberian Gas Deposit Areas Using ALOS PALSAR Interferometry," PIERS Proceedings, Moscow, Aug. 18-21, 2009. Thompson, T. 2006. "Hydrogeology and Disaster Part 1: Groundwater and Land Subsidence," Wisconsin Ground Water Association Newsletter, Vol. 20, No. 3, wgwa.org/articles/landsubsidenceandgroundwater.pdf. Derrold W. Holcomb is radar subject-matter expert at Intergraph; e-mail: derrold.holcomb@intergraph.com. Figure 6. A color-coded subsidence image map was overlain on a DEM. Figure 5. A close-up image shows correspondence of low coherence (left) and poor displacement measurement (right). FEBRUAR Y 2O12 / WWW . GEOPLA CE . COM 25