## 3 Methods and Results
### Electro-Optical and Infrared
#### 3.1.2 Stereo-Photogrammetry  
Ice islands and their fragments rise up to 10 m above the ocean surface, and this freeboard may be captured by any remote sensing technology capable of extracting 3D information (topography) at high resolution. The most common satellite-based method of extracting a digital elevation model (DEM) (a 3D representation of a terrain's surface elevations) is the stereo-photogrammetric method. In addition, the method called height from shadow [48] can be useful to validate the DEM height of ice features [27].  
Stereo datasets acquired by VHR optical and SAR satellites were analyzed. Stereo SPOT 6 imagery was acquired over Axel Heiberg Island. Stereo images allow sea-ice topography to be estimated after extracting DEM. For a sea-ice environment that also includes a snow layer, the DEM includes the snow surface and may also be referred to as a digital surface model (DSM). PCI Geomatica Orthoengine (now called CATALYST Professional) and the ERDAS photogrammetric module were used to generate the DEM.  
Testing different software parameters on SPOT 6 stereo images demonstrated that a DEM of sea ice and large icebergs can be extracted. DEM generation over ice fields in the Canadian Arctic with along-track SPOT5 HRS stereo data has previously been described in [31]. The results demonstrated that stereoscopy can perform well over ice and snow-covered surfaces. The DEM accuracy is acceptable for estimating the relative elevation of extreme ice features with respect to each other or the surrounding ice, but to estimate absolute elevation with respect to sea surface height, ground control points (GCPs) must be collected. Figure 3a shows SPOT 6 imagery (acquired on 27 April 2014) over EIFs including ice islands, ice-island fragments, ridges, new forming hummocks and MYHFs. Figure 3b shows an example DEM corresponding to an area in the SPOT 6 imagery. It can be seen that the height over the ice surface (including snow) of hummocks varies from 3 to 4 m for MYHF and up to 25m for forming hummocks. The height of ice islands is about 5 to 7 m, which is consistent with the freeboard of Ward Hunt Island, where many of the ice islands are expected to originate from. The DEM is a valuable source of ground-truth information for EIF analysis.  
Figure 3: A fragment of one of stereo SPOT 6 imagery (**a**) over Axel Heiberg Island used to generate DEM (**b**) over the corresponded area. SPOT 6 image © Airbus Defence and Space 2014.  
Stereo imagery and the produced DEM were analyzed to understand the maximum height and possible thickness of EIFs, which allows for better EIF identification and quantitative characterization. The height information provided by a DEM is useful for identifying smaller EIFs. For example, the small MYHF in Figure 4 appears to be similar to the rubble field of Figure 4c in the WorldView-2 image (acquired on 29 April 2014). However, using the DEM, these features can be clearly distinguished because MYHFs have higher freeboard than rubble fields (compare Figure 4b with Figure 4d). The height of MYHF above the sea surface (including snow) spans from 2 m to 4 m (Figure 4b), whereas a rubble-field elevation (Figure 4d) varies from 0 m to a maximum of 1.6 m (the mean is below 1 m).  
A DEM is used as an additional source of ground-truth (verification) information for EIF identification and can also be used to generate a topographic map of sea ice and snow surface.