## 5 Current Challenges and Future Trends  
These submissions show the wide variety of applications for remote sensing and GIS in environmental monitoring. In this section, we show the expected future trends and challenges of this topic. * **Current problems related to the low use of UAVs:** In this Special Issue, we encountered that most of the papers are based on satellite image sources. Nonetheless, we expect that in the near future, drones will have an even more relevant role in remote sensing and the digitalization of different environmental aspects. In fact, UAVs have appeared as a relevant subtopic in remote sensing and environmental monitoring papers in the last three years, as seen in Figure 3. The inclusion of multispectral or hyperspectral cameras in the drones will allow better environmental monitoring. Similar to the inclusion of multispectral cameras, the inclusion of other capabilities such as light detection and ranging (LiDAR) or radar sensing systems might be included in UAVs in the next few years. The last limitation identified through the review of the literature is the low battery time of these devices, especially of the smallest and cheapest ones. Since the efficiency of existing energy harvesting systems does not allow for their inclusion in the UAVs to expand their battery life, new options should probably be studied. The topic of energy limitation on drones was recently studied in [26], in which the authors conclude that drone-to-drone charging or fuel-based drones might be some of the possible solutions. * **Future challenges linked to the higher use of UAVs:** One of the major problems related to UAV use is that due to the great variability of environmental conditions, constraints, and variables, the proposed solutions might only be applied in other cases. Thus, it is almost impossible to develop operation algorithms which can be applied universally under all scenarios [27]. Nonetheless, it is important to follow a systematic approach in order to find the best practices to be applied in other use cases. Combining images from satellites or drones with on-land or underground sensors such as Internet of Things devices will create diverse and heterogeneous networks that will provide data for different uses. The data we will have in the future, characterized by higher resolutions and the joining together of different data sources, will be used for new applications which are currently unexploited. Since we cannot foresee the characteristics or requirements for future applications, identifying the best practices and continuing systematic approaches to the current case studies is essential. Nonetheless, some examples include how remote sensing and on-ground sensors are used for detecting and verifying wildfires [28]. * **The storage problem and its relationship with energy:** One of the future challenges that we will need to face in this area, particularly linked to the use of UAVs, will be the storage requirements needed to save all the generated data. The storage and operational requirements for analyzing vast volumes of monitoring data were already mentioned for GIS systems in [29] in 2016. The use of multispectral cameras in combination with land-based sensors and LiDAR will exacerbate the problems related to storage requirements. It must be noted that storing, processing, and particularly, sending data are activities with high energy consumption. Recently, some scientists have included their carbon footprint in data management [30]. Therefore, we can expect certain sorts of limitations in the future regarding data storage. Minimizing the stored and analyzed data without reducing the information contained in the data will be essential. Some efforts in this area can be seen in [31]. * **GIS and data visualization:** The way in which data is presented to the end-users and the stakeholders is very important. To maximize the impact of the research about remote sensing and GIS on society, the visualization of data in the correct environment is very important.