Submitted to: International Journal of Remote Sensing
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/1/1998
Publication Date: N/A
Interpretive Summary: Soil-water information is critical for many applications. Measurement of soil water can be made accurately for very small areas, but many of the applications require information over much larger areas. Synthetic Aperture Radar (SAR) is an instrument that has the potential to measure soil-water content over large areas from space. One limitation of SAR is that the signal is affected by the slope of the land surface. This can be corrected using topographic data. We have observed that corrections made using the widely available United States Geological Survey (USGS) topographic data appear to introduce systematic distortions. We performed this study to determine if those distortions result from the fact that SAR data, which is collected in 12.5 m pixels, has a higher resolution than the topographic data, which is in 30 m pixels. We compared corrections using higher resolution topographic data with those from the USGS data and found that the higher resolution data provided a considerable (about 1.5 times) improvement over the USGS data. We then modified the SAR data using filtering and aggregation techniques, both of which improved corrections considerably. However, we could find no technique that would bring the USGS-based corrections up to the level of the high resolution data. We also found that, even with high resolution data, there was
Technical Abstract: Topography in high relief mountainous areas masks the signal variation in airborne Synthetic Aperture Radar (SAR) data caused by soil moisture, surface roughness and vegetation. It also affects the quality of image calibration and registration. Good quality calibration and registration are required for the use of SAR in the estimation of soil water. To address the problem of topographic effects, the widely available, standard 30x30 m United States Geological Survey (USGS) Digital Elevation Model (DEM) has been incorporated into calibration and registration programs. The topographic resolution of SAR imagery relative to the USGS DEM was examined by comparing the correlation between incident angle (I) and SAR backscatter (S) in a high resolution DEM (mapped at 1:4800 and 1:600 from aerial photography for two small areas) to that in the USGS DEM (mapped at 1:24000). We found that the SAR resolves topographic features not resolved by the USGS DEM. Filtering and aggregation techniques were applied to reduce speckle, the apparent noise due to small topographic features resolved by SAR but not resolved by the USGS DEM, and also to reduce the registration error. Increasing the filter window from 3x3 to 5x5 to 9x9 and the cell size from 6x12 m to 30x30 m to 90x90 m, reduced the unexplained variability in backscatter by 0.5...