Fast terrain modeling for hydrogeological risk mapping and emergency management: the contribution of high resolution satellite SAR imagery

Geomatic tools for fast terrain modelling play a relevant role both for hydrogeological risk mapping and for hydrogeological emergency management. Among these tools, Synthetic Aperture Radar (SAR) satellite systems may give important contributions in terms of Digital Surface Models (DSMs) and Digital Elevation Models (DEMs), considering their complete independence from logistic constraints on the ground (as for airborne data collection), illumination (daylight) and weather (clouds) conditions. Here we focus on the present potentialities of high resolution SAR satellite imagery for DSMs generation with the radargrammetric stereo-mapping approach and with the well-known interferometric technique. Therefore, the goal of this paper is just methodological, devoted to illustrate both the fundamental advantages of radargrammetric approach and also its drawbacks respect to the InSAR technique. As regards the pros, it is worth to mention the independence from image coherence (unlike interferometric approach), the parsimony (it can work with just a couple of images), and therefore the short time required for imagery collection (from tens of minutes to few hours) also thanks to the mentioned independence from illumination and weather. Concerning the cons, the well known deformations of SAR imagery (layover and foreshortening) may cause remarkable difficulties with complex morphologies and have to be duly accounted for the acquisition planning. A suite for the DSMs generation through the radargrammetric approach has been implemented in SISAR (Software per Immagini Satellitari ad Alta Risoluzione), a scientific software developed at the Geodesy and Geomatic Division of the University of Rome La Sapienza. In order to demonstrate the radargrammetric mapping potentialities of high resolution SAR satellite imagery, several tests were carried out using data with different features (SpotLight, StripMap) coming from different platforms (COSMO-SkyMed, TerraSAR-X). Here we discuss the results obtained with a COSMO-SkyMed SpotLight stereo-pairs related to the San Francisco area, which is characterized by mixed morphology and land cover; we mainly focused on urban areas and zones covered by bare soil and rocks. Overall, accuracies range from 3 m over bare soil to about 4 m over more complex morphology, whereas they decrease in presence of remarkable image deformations.