## 4 Discussion
### Prospective Applications of UAV Surveys in the System of Environmental Coastal Monitoring  
It is impossible to resolve problems of prompt thematic mapping and comprehensive monitoring without the use of UAVs. This is due to the opportunities to obtain spatial data of extremely high resolution (up to sub-centimeters) and to perform repeated surveys. UAV-derived ultra-high resolution data cannot always be substituted by detailed satellite imagery because the sub-meter spatial resolution of even multi-spectral and hyper-spectral data does not allow identification of specific objects and features in coastal environments under some thematic tasks. The diurnal timetable of surveys from orbital satellites or satellite constellations is also poorly suited for monitoring objects that change their parameters within a day or even hours. Issues of large-scale and accurate detection of shoreline position are also a task solely for UAVs and other unmanned aerial platforms. Those problems, which can and should be resolved solely through the use of UAVs in the sphere of shore and shore processes research, can be clearly arranged into two categories, namely: prompt responses and consistent observation within a general framework of regular monitoring. In turn, these blocks can be sub-divided according to types of survey equipment and minimal spatial resolution necessary for resolving specific thematic tasks for identifying the main processes having a negative environmental impact on coastal areas and waters. In case of the Lake Sevan, a special task to be resolved using the only ultra-high resolution UAV-data was the detection of point sources of water pollution. This task was divided into three sections with each utilizing data of different spatial resolution (obtained from UAV surveys at different flight levels) for:  
1. Detection of small domestic wastes and plastics. To resolve this task, UAV-derived data obtained at very low survey altitudes (100 m and less) should be used as the recognition of such features is possible only with imagery of sub-decimeter spatial resolution. The surveys identified construction waste in coastal zones as well as clusters of abandoned fishing nets in waters. These nets are traces of illegal, uncontrolled fishing and often become places of active organic decay of macro-biota (including both fish and bird fauna caught in them). 2. Monitoring of camping sites or temporary stay by tourists as sources of coastal littering. Tourist activities along the southern and eastern coasts of Lake Sevan have generated spontaneous and uncontrolled miniature landfills that are not included in waste disposal systems of settlements and static tourist accommodation sites. 3. Detection of points of illegal sewage discharge into the lake. Some recreational housing in the coastal zone comprises temporary structures lacking adequate sewage treatment system, often with all waste waters discharged by the pipeline directly into the lake. At present, the most promising tasks are those that are resolved with the help of UAV-based multi-spectral and hyper-spectral data, including thermal imagery. The analysis of the state and structure of vegetation in nearshore waters and of submerged aquatic vegetation (phyto-indication using multi-spectral and hyper-spectral UAV-derived data) can be already considered as a specific thematic branch of coastal monitoring [15; 16; 17; 20]. In terms of tasks for regularly monitoring anthropogenic impacts on coastal ecosystems and waters and assessing the effectiveness of conservation and environmental measures, the thermal survey may move to the fore. Discharge of industrial and domestic wastewaters can be especially well detected in the thermal spectral range. It is possible to use such thermal UAV images as evidences of environmental violations in case they are supplemented with data acquired simultaneously or in near-time from field hydro-chemical sampling or ground hyper-spectral remote sensing.