## 4 Permafrost Subsurface and Land Surface Features  
The application of land cover classifications has also proven significant for the retrieval of permafrost-related features. Specifically, thermokarst-related phenomena are studied, but periglacial processes in mountainous terrain have also been of interest. Many of these applications combine remotely-sensed terrain data with land cover information, e.g., [60; 61; 62]. Vegetation coverage provides information on process activity (erosion). Changes in hydrology due to permafrost degradation can be obtained from water body classifications [63]. Related features are in general rather small; the availability of high spatial resolution satellite data has therefore been the main driver for recent studies. Long-term studies are still limited. General process domains can however be captured using medium resolution datasets [62]. [PERSON] et al. [60] performed morphometric analyses of lakes based on Landsat-7 ETM+ in the Lena Delta. Landsat-7 ETM+ has also been used for the quantification of thermokarst-affected terrain types [64]. [PERSON] et al. [65] used a combination of Landsat ETM+ (15 classes) and CHRIS-Proba (six classes) for spectral characterization of periglacial surfaces and retrieval of geomorphological process areas. Landsat 5 TM was applied by [PERSON] et al. [66] in 2014 for identifying relations between catchment land cover characteristics and lake hydrology, concluding that remote sensing data alone cannot answer the related questions. Aeolian soil erosion, which can also be responsible for erosional landforms, or deflation patches have been studied in Kangerlussuaq, West Greenland [67], as well as the Yamal peninsula in Russia [68]. In both cases, high resolution satellite data were analyzed (WorldView-2 and GeoEye-1, respectively). [PERSON] et al. [69] concluded that only high resolution satellite data can be used to map changes of so-called lithalsas. These are low, circular or oval features around 10-30 m wide and up to 150 m long that result from frost heave and are therefore permafrost-related surface features. High spatial detail is also required for ice wedge mapping. [PERSON] et al. [63] investigated high resolution satellite data from IKONOS, Corona, Gambit KH-7, QuickBird, WorldView-1/-2, SPOT 6/7 and aerial photographs to study ice wedge degradation across the Arctic, where changes over long time periods could be quantified. The discrimination of lakes has been identified as a major challenge. Optical data can be easily used to identify lakes that are larger than the sensor's spatial resolution. [PERSON] et al. [35] used CHRIS-Proba and Landsat-7 ETM+ to investigate subpixel-scale heterogeneity with special emphasis on Evapotranspiration (ET) estimation. Environments with inundated low centered polygons have been shown to be only detectable with high spatial resolution data. The majority of lakes are however smaller than, e.g., Landsat or MODIS, which are used for global maps [6; 35], and frequent cloud cover limits the mapping of these highly dynamic features. SAR data have been suggested to overcome the latter problem. Large areas could be theoretically covered, but wind action, which is common in the Arctic, roughens the lake surface and reduces the detectability [36]. In the case of low wind conditions, higher spatial resolution X-band SAR data (2 m) have been shown to be applicable to capture small lake objects [70]. They provide more realistic representation of lakes than with Landsat as used for the regional map of the entire Lena Delta (see Table 1[40]). Land cover is also used to indirectly map sub-ground permafrost properties. ALT estimates have been made using different approaches, both by evaluation of land cover classifications, as well as by analysis of reflectance values themselves. In this context, there is also potential for X-band SAR applications to infer ALT from the radar backscatter, since the backscatter signal is strongly related to land cover [71]. [PERSON] et al.