## 8 Challenges and Promising Approaches
Since global mapping efforts need to comply with internationally-agreed upon standards, only a substitution over the high latitudes, which is to some extent compatible with the global maps, can be made. Existing classes need to be re-named and subdivided. The same sub-class may have to be assigned to different parent classes eventually. This is, e.g., the case for wetlands. Alternatively, they could be provided as a separate information layer for the purpose of up-scaling of fluxes. Global maps are in general based on optical data. Only SAR-based lake mapping has been so far considered in one case (CCI Landcover, [139]). SAR can, however, deliver similar thematic information content (e.g., [73; 32]), especially when combinations of different polarizations are used. It provides additional information on surface roughness [76], moisture [78] and vegetation structure [140; 141]. These parameters reflect information on wetlands [79], shrubs and soil properties which is required for high latitude applications. The thickness of mosses represents to some extent soil organic carbon content, and it relates to wetlands. C-band SAR has been shown to provide information on both in tundra areas. L-band SAR is also suitable for wetland mapping. A number of L-band maps already exist with a forest or wetland mapping focus for Northern Eurasia [113] and America [114]. They may need to be reviewed for consistency and harmonized, as well as gap filled across the Arctic. Such data could complement the traditional optical data-based retrieval methods. So far, mostly unfrozen period SAR data have been used. Recent studies demonstrated the potential of winter  
Figure 6: Comparison of the number of classes and spatial resolution from land cover classifications north of the treeline for different applications. (sources: [9; 11; 13; 23; 25; 26; 35; 40; 42; 47; 50; 52; 56; 59; 63; 64; 66; 67; 69; 73; 75; 96; 97; 100; 101; 103; 110]). acquisitions [76; 79; 140; 141] for land surface characterization. The advantage is the exclusion of pixel to pixel variation, the difficult to model backscatter contributions due to differing dielectric properties when unfrozen [76]. Passive microwave data, such as AMSR-E, provide coarse resolution information on the water fraction [128], but also different land cover classes [115], including wetland complexes. Their resolution is similar to climate models, and wetland information is of need for land surface modeling in high latitudes [36]. Such maps would be therefore useful, but not applicable beyond. Medium resolution satellite data may, however, provide sufficient spatial resolution for soil properties up-scaling over peatlands [52]. On the local scale, X-band SAR, as well as UAVs become more and more important (e.g., [142; 143; 144]). Such investigations can complement regional mapping efforts in a nested approach in order to reflect the fragmented nature of permafrost regions and to identify the subtle changes at the land surface. The majority of publications reviewed in this paper deal with retrieval techniques. Only about 40 of them make use of the classification results for, e.g., habitat mapping, soil carbon and flux upscaling or periglacial features detection (Figure 6). Most local and regional maps are not shared on online repositories. Exceptions are the CAVM [5], the Lena Delta map [40; 145] and also the Kuparuk basin maps. This fosters their usage, but usually one of their co-authors is directly involved in new applications in case of regional maps. The CAVM has been used beyond the original development team due to its circumpolar coverage.