## 4 Discussion  
This study demonstrated the feasibility of existing satellite-based techniques for identifying and characterizing EIFs with a focus on hummocks and ice islands. The use of VHR electro-optical images enables identification of EIFs, relying on analyst experience. In addition, the VHR data can be applied to extract EIFs with the height-from-shadow technique. Height from shadow for ice ridge characterization is limited in the case when  
Figure 17: Fine-quad data (FQ11 on (**a**)) and (FQ26 on (**b**)) represented as color composite R (HH)-G (HV)-B(VV) (**a**,**b**); DEM extracted over area shown in the previous figure (**c**). RADARSAT-2 data and products © MDA Ltd. (2013)—all rights reserved. the ridge is aligned with the Sun's illumination direction because, in this case, the shadow is broken into smaller patches and the ridge height cannot be accurately extracted. The most accurate method for topography characterization and identification of EIFs is to apply the VHR electro-optical stereo DEM. This method demonstrated good agreement with other techniques including height from shadow. There are two main obstacles for wide application of VHR electro-optical data including stereo datasets for monitoring large Arctic areas: (i) high cost of imagery and (ii) small size of scene. With the current trends in the rapid development of new satellite constellations (e.g., Planet), the availability of VHR and high-resolution data is expected to improve in the future. In addition, the electro-optical and infrared imagery (VHR and medium resolution) are affected by visibility conditions because of the presence of clouds and fog, as well as the polar night phenomenon when the Sun remains below the horizon in winter months. Medium-resolution Landsat-8 data are an important source of information for detection and identification of EIFs. As was demonstrated, the TIR band provides thermal contrast helpful to identify and detect thicker EIFs (hummocks and ice islands). The usage of the TIR band was limited for summer months when the ice surface warms up and starts melting. Also, TIR band resolution (120 m) is too coarse to observe small ice features. The main advantages of Landsat 8/9 are free data availability for all users, systematic coverage of the Canadian Arctic and the large image swath (185 km) allowing the monitoring of extended AOIs. Therefore, this data source is currently preferred in studying extreme ice features in the Canadian Arctic or the Arctic Ocean. The single-pass InSAR technique using TDM data demonstrated excellent performance in the 3D mapping of hummocks and ice islands. The advantages of this technique are the all-weather and at-night capabilities that allow high confidence in operational data tasking. With an upcoming single-pass SAR mission such as Harmony, the usage of the InSAR method for sea-ice applications will become a commodity [54]. Potentially, this method can become operational for EIF monitoring. The interesting results were achieved with the use of a single high-resolution SAR image for extracting height information from height from shadow and topography from shape using shading techniques. Results comparison with InSAR DEM demonstrated good performance. The main limitation of the shape-from-shading technique is the requirement of low or moderate topography of EIF. It is suitable for ice-island topography characterization but may not be so helpful for hummocks with high peaks. The high peaks problem can be addressed with the height-from-shadow method. These techniques, based on a single image, require VHR or high resolution, which are available with current and upcoming NewSpace constellations (e.g., ICEYE and Capella) and have the potential to become cost-efficient tools for EIF surveillance. However, both height from shadow and topography from shape using shading techniques require additional implementation for fully automated performance. Full- and dual-polarimetric data are proven sources of information for sea-ice typing for many years.