## 3 Experimental Section
### Study areas and dataset  
The investigation was carried on in the heart of Provence (France), in the triangle of Arles, Avignon and Marseille (Figure 5), in the Mausanne les Alpilles and Salon de Provence C-SAP test sites. For Mausanne les Alpilles, an area of about 650 km\({}^{2}\) was selected as study area (centre image coordinates: 4,844,944 m North and 644,291 m East, projection: UTM, datum: WGS84). Here ground elevation ranges from 47m in the south-west to 633m in the north-east and the territory presents a mixture of rural areas (60%), green forests (35%) and urban areas (5%). For Salon de Provence, an area of about 600 km\({}^{2}\) was selected as study area (centre image coordinates: 4,847,651 m North and 666,697 m East, projection: UTM, datum: WGS84). Like Mausanne les Alpilles, the test field is hilly, with elevations ranging from 83 m in the south-west to 710 m in the north-east and presents a mixture of rural and urban areas. Figure 5 shows an overview of the two study areas.  
The standard C-SAP dataset of Mausanne les Alpilles delivered to the study teams was composed of a 2.5 m Cartosat-1 stereo pair collected by ISRO on 31 January 2006, a set of 32 high-precision GPS measured GCPs (Figure 6) and a 2 m ground resolution DSM purpose-made with a Leica ADS40 digital camera (Figure 7). Both GCPs and the DSM were provided by the Principal-Investigator European Commission-Joint Research Centre. Similarly, the standard C-SAP dataset of Salon de Provence was composed of a 2.5 m Cartosat-1 stereo couple collected by ISRO on 6 February 2006, a set of 22 GCPs derived from the French DB ORTHO(r) database (Figure 8) and a medium-resolution digital terrain model (DTM) with 25 m ground resolution and part of the French MNT DBTOPO(r) DTM. Both GCPs and the DTM were provided by the Principal-Investigator French Institut Geographique National.  
The Cartosat-1 stereoscopic data were delivered by ISRO as GeoTIFF Stereo Orthokit product with a location accuracy of 220 m (three-sigma tolerance) [20,21]. ISRO also supplied to the study teams predetermined Rational Polynomial Coefficient (RPCs), based on its own orbit and attitude model for the acquisition, and metadata for the correction of the imagery geometric distortions [22-25].  
Figure 5: French C-SAP test fields of Mausanne les Alpilles and Salon de Provence (view made with NASA World Wind®).  
Regarding the vertical accuracy of reference data, the European Commission-Joint Research Centre supplied for Masuanne les Alpilles GCPs with a vertical accuracy of 0.05 m and a high-resolution DSM with a root mean square error (RMSE) of 0.6 m [15]. The French Institut Geographique National  
Figure 6: Cartosat-1 Aft image collected over Mausanne les Alpilles (France) with superimposed the high-precision GPS measured GCPs provided by the Principal-Investigator European Commission-Joint Research Centre.  
Figure 7: High-resolution raster DSM for Mausanne les Alpilles (France), purpose-made with a Leica ADS40 digital camera and provided by the Principal-Investigator European Commission-Joint Research Centre.  
supplied for Salon de Provence GCPs with a vertical accuracy better than 2.5 m (geolocation error less than 1.5 m) and a medium-resolution DTM with a 1 m RMSE, evaluated over the whole France [17].  
In addition to this dataset, we also used for comparison the radar DSM generated from the NASA's SRTM mission and available free of charge from the NASA JPL's ftp server [19]. In terms of linear error at 90% confidence level (LE90), the SRTM DSM generated for Eurasia is accredited of an absolute elevation accuracy of 6.2 m and an absolute geolocation error of 8.8 m [26].