## 6 Conclusions--From Revolution to Maturity of UAV Spectral Remote Sensing  
About five years ago, the lack of suited UAV multispectral cameras along with shortcomings related to UAV remote sensing including high initial costs, platform reliability, lack of standardized procedure to process large volumes of data, and strict aviation regulations were limiting the usability of UAV sensing systems. Today, several of these shortcomings have been overcome, or solutions have been proposed. UAV spectral sensing systems that can capture spectral data in high spectral, spatial, and temporal resolution are now available for a wide audience. We can conclude that the new era in remote sensing with unmanned flying robots is here and ready for widespread use in various application areas. Overall, we expect that UAV spectral sensing systems will become common in the toolbox of researchers in quantitative remote sensing, forestry, agriculture, field phenotyping, ecology, and other fields that rely on environmental monitoring. We identified two trends. On one hand, sensors such as 2D snapshot multi-cameras systems Parrot Sequoia, MicaSense RedEdge and Maia S2 that are available below \(\$4000\), \(\$65000\) and \(\$15,000\), respectively, are built to be consumer-friendly. In combination with commercially built fixed-wing or rotary-wing UAVs, these cameras are becoming a powerful tool for researchers, as their rapid adaption shows, but also for service providers, breeding companies, and even farmers. On the other hand, the complex and potentially more flexible, configurable, and expensive sensors are still mostly used by people coming from the \"core\" remote sensing community. However, with companies such as HySpex and Specim formally focused on the development of airplane sensors entering the UAV market, we hope that professional systems will also drop in price. In the future, we hope to get a more detailed view on the developments with a survey on the state-of-the-art in UAV remote sensing ([[http://www.phenofly.net/uav-survey/](http://www.phenofly.net/uav-survey/)]([http://www.phenofly.net/uav-survey/](http://www.phenofly.net/uav-survey/))) supported by the OPTIMSE community ([[https://optimise.dcs.aber.ac.uk/](https://optimise.dcs.aber.ac.uk/)]([https://optimise.dcs.aber.ac.uk/](https://optimise.dcs.aber.ac.uk/))). Overall, we think that UAV sensing systems will not replace very high-quality plane or satellite systems, but rather complement them. UAV sensing systems allow us to go beyond classical remote sensing approaches, such as customized BRDF sampling or spectral 3D sensing. We are confident that more examples of such exciting UAV applications will appear in the future. Additionally, they provide us with information in higher spatial and temporal resolutions. The potentials of such data are yet to be explored, and novel approaches are needed to handle and analyze these data in more robust and meaningful ways. Three-dimensional hyperspectral data at high spatial and temporal resolution has the potential to provide exciting new insights into ecosystem composition and functioning, and address major research questions in these areas. The important tasks now are to standardize procedures, develop algorithms, and explore the potential to make use of the large amounts of multi-dimensional, high spatial, temporal, and spectral resolution UAV data. In this review, we showed that many approaches exist, and identified best practice procedures to derive calibrated spectral data from UAV sensing systems. On the other hand, some uncertainties will always remain in data collection and processing. While the data quality might be sufficient for many use cases if the best practice calibration procedures are followed, for some applications, the absolute accuracy might be insufficient. Thus, it is critical to make metadata about the data quality traceable throughout all of the steps of data capturing and processing. In addition, the normalization of the dynamic illumination conditions in drone remote sensing still poses a problem.