## 5 Discussion and Best Practice
### Quality Assurance and Metadata Information  
The signal of an object is influenced at different stages before it is stored as a digital value in a data product. In the last sections, we looked at different sensors and their calibration, geometric, and radiometric processing, as well as influences of the environment, i.e., illumination conditions and the atmosphere. The keys to transforming data to useful information are the auxiliary and metadata. They support the interpretation of scientific data, and in general help to ensure long-term usability. Metadata provide a basis for the assessment of data quality and the possibility of data sharing and comparing between scientists [213]. It can be pixel, image (measurement), or scene-specific.  
Important pixel specific metadata includes the signal-to-noise ratio [123] and an approximation of the radiometric resolution [43] that gives an indication of the quality of a pixel value stored in a data product. While both can be derived on the image level during the relative radiometric calibration (c.f. Section 4.3.1), their estimation for pixels in a scene can be complex, since it is modified when, e.g., information of two pixels is composed. For every measurement, the measurement time (to reconstruct the Sun's position) and illumination conditions should be recorded. The latter would include qualitative information on the sky condition (clear or cloud-covered) and a direct-diffuse ratio, which could be derived from a shaded and a non-shaded reference panel. Additionally, the measurement geometry of the FOV or IFOV of every pixel, in case of imaging sensors, needs to be stored, since being in interaction with the illumination conditions, the measurement geometry has a significant influence on the data [24; 42; 161]. In imaging data, this is sometimes visible along the transition of mosaiced images. In this context, the influence of the data-processing scheme also needs to be taken into account [42]. Thus, metadata that describes how the data is processed needs to be generated for every scene. It should include the software and its version, as well as the parameters that were set during the processing. Software packages such as Agisoft Photoscan, Pix4D mapper, and open source tool MicMac [214] generate a report file after processing. These files could be provided as supplementary data in every publication. Other scene-based metadata include the information on method/protocol used to derive top-of-canopy reflectance (c.f. Section 4.4), as well as the sensors used in the study (including their band configuration and model number or manufacturing year, since some of the UAV sensors are manually manufactured and constantly improved, eventually making them unique). As described above, pixel and image (measurement)-specific metadata can improve the interpretability of the data and should be saved with the data as already done in airborne and satellite remote sensing [215; 216]. To the best of our knowledge, a standard procedure for UAV remote sensing does not yet exist, but some researchers have implemented it into their work (e.g., [161; 163] for the viewing geometry; [43] for the radiometric resolution, [162] to calculate uncertainty of the output HDRF observations via the image signal-to-noise ratio and reflectance transformation standard deviation; [42] to trace the information from the individual images into the data product). Scene-specific metadata can be stored in an additional file, similar to ENVI header files. Ideally, quantitative metadata parameters should also have an uncertainty assigned to them. Table 5 summarizes the mandatory and optional metadata for UAV remote sensing. We argue that at least the mandatory scene-based metadata should be stated in every publication or its supplementary material. With increasing resolution, additional factors need to be considered that were not visible in remote sensing data of coarser resolution. One example is wind and wind gusts that can have an influence on the spectral signature. Further studies need to evaluate such effects.