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Errors in the DEM contributed about 9 mm of noise to the coseismic offset estimates obtained by Massonet et al. ( 1993 ) , where rms vertical errors in the topographic data were estimated after the fact to be about + / -24 m. Systematic errors in selected areas can , of course , be much larger than this . By this we mean that , within a given pixel , the position of radar scatterers has not changed to any significant degree , but the ensemble of scatterers ( i.e ., the entire ground surface within that pixel , as well as some adjacent ones ) has moved up , down , or sideways in some correlated fashion . The DEM for the Mojave region is representative of digital topographic data in existence for most of the Northern Hemisphere ( 90 - by 120-m pixels , nominal vertical accuracy of 30 m ) ; unfortunately , much of it is classified and not available for scientific research . A critical question is whether or not SAR images obtained at different times from heavily vegetated or snow-covered volcanoes can be correlated , so that interferograms that measure ground deformation can be constructed . The January 17 , 1994 Northridge , California earthquake is now estimated to have been the most costly seismic disaster in U.S. history . Massonnet et al. ( 1993 ) tested a number of satellite passes before finding two with optimum characteristics , then adjusted the orbital parameter estimates to minimize coseismic offset far from the fault . Houseplants infected by a root rot often appear wilted and fail to recover after watering . The extent of vertical lowering ( related to the volume of material removed ) can be estimated with accurate topographic change data monitored over years to decades .