2003 Tyrrhenian International Workshop on Remote Sensing INGV Digital Elevation Model of the Alban Hills (Central Italy) from ERS1-ERS2 SAR data Andrea Arturi, Andrea Minchella, Giovanni Schiavon DISP - Tor Vergata University (Rome) Salvatore Stramondo Istituto Nazionale di Geofisica e Vulcanologia (INGV)

2003 Tyrrhenian International Workshop on Remote Sensing INGV SAR Interferometry The interferogram is the “image” of phase differences of two SAR observations (separated in space, in time or both) over the same spot on the ground Interferometric phase

2003 Tyrrhenian International Workshop on Remote Sensing INGV The interferometric phase Topographic phase contribution

2003 Tyrrhenian International Workshop on Remote Sensing INGV General criteria: Temporal separation (baseline) Spatial baseline Meteorological data Ascending and descending images SAR data selection

2003 Tyrrhenian International Workshop on Remote Sensing INGV Decorrelation Factors Temporal Decorrelation Spatial Decorrelation Thermal Decorrelation Temporal separationSurface change System parameters (, B w, R, ) Target location (B n,  )

2003 Tyrrhenian International Workshop on Remote Sensing INGV Short temporal baseline: 1 and 6 days temporal span between the acquisitions Period of acquisition: from November to April (vegetation, water vapor etc.) Data selection criteria

2003 Tyrrhenian International Workshop on Remote Sensing INGV Perpendicular baseline range: 120 m <B n < 300 m Sensitivity to topography Phase unwrapping problems in high slope relief areas Short altitude of ambiguity Data selection criteria

2003 Tyrrhenian International Workshop on Remote Sensing INGV Detection of meteorological condition using informations from Aeronautica Militare Italiana and Meteosat Data Absence of precipitations and snow during at least 24 hours preceding the passage of the satellite; in such way the backscatter coefficient should be the same between the two acquisitions, specially for tandem couples. clear or few cloudy sky, to limit the presence of atmospheric artifacts in the interferograms condition of moderate wind (11-16 knots or 5,5-7.9 m/s) to avoid further losses of coherence (in case of strong wind, the scattering of vegetated areas could be very different) Data selection criteria

2003 Tyrrhenian International Workshop on Remote Sensing INGV SatellitePass directionOrbitData acquisition E1Descending /03/1994 E1Descending /03/1994 E1Ascending /04/1996 E2Ascending /04/1996 E1Ascending /11/1999 E2Ascending /11/1999 Selected data InterferogramsB n (m)h a (m) 13781_ _ _

2003 Tyrrhenian International Workshop on Remote Sensing INGV - Implementation of precise orbits provided by Delft Institute for Earth-oriented Space Research (DEOS) - Iterative phase unwrapping procedure - Extraction of Ground Control Points (GCP) from a 20m pixel size map-derived DEM by defining no-data mask SAR data processing and DEM generation block diagram

2003 Tyrrhenian International Workshop on Remote Sensing INGV Phase Unwrapping Algorithm Phase Unwrapping procedure

2003 Tyrrhenian International Workshop on Remote Sensing INGV GCP selection Low coherence areas mask  ≤ 0.3  ≥ 0.3

2003 Tyrrhenian International Workshop on Remote Sensing INGV Urban (buildings, manufactures) and layover mask from intensity (I) values GCP selection

2003 Tyrrhenian International Workshop on Remote Sensing INGV GCP selection Final mask GCP selection

2003 Tyrrhenian International Workshop on Remote Sensing INGV 5-6/04/ /03/ /11/99 Single pair SAR DEMs

2003 Tyrrhenian International Workshop on Remote Sensing INGV SAR DEMs merging procedure The resulting SAR DEM is obtained by weighting for the coherence each single-pair SAR DEM

2003 Tyrrhenian International Workshop on Remote Sensing INGV SAR DEM Map derived DEM A median filtering procedure has been applied to the residual no-data areas of the image; each center pixel is replaced by the median value within the 3x3 pixel kernel-size

2003 Tyrrhenian International Workshop on Remote Sensing INGV Map of residual SAR DEM Map derived DEM

2003 Tyrrhenian International Workshop on Remote Sensing INGV 1250 m 0 m No data Profile 2 Profile 1 Topographic profiles

2003 Tyrrhenian International Workshop on Remote Sensing INGV Topographic profile in flat areas The map-derived DEM is obtained from cartography with equal altitude lines every 10m Due to interpolation of digitized isohypses the map-derived DEM is stepped in flat areas.

2003 Tyrrhenian International Workshop on Remote Sensing INGV Hypsometric curves Spikes in the diagram below correspond to altitude values multiple of 10m

2003 Tyrrhenian International Workshop on Remote Sensing INGV Slope histograms

2003 Tyrrhenian International Workshop on Remote Sensing INGV Conclusions SAR Interferometry technique allows to produce reliable DEMs, at relatively low cost, characterized by a good resolution and covering large areas In the Alban Hills due to the high slope a single image pair is unable to completely recover topography. The use of more data with appropriate spatial and temporal baseline allows to improve results, reducing coherence loss. GCP have been opportunely chosen masking urban and no-data areas The final SAR DEM is weighted by each single image-pair coherence The comparison with map-derived DEMs points out how SAR data well reconstruct topography and avoid artifacts due to map-derived DEM generation procedures Due to their properties SAR DEMs can be a useful tool for geomorphological analysis