## 5 Discussion and Best Practice
### Geometric Processing
#### Ground control points (GCPs)  
GCPs are traditionally used for a non-direct georeferencing solution, where a collection of clearly visible targets is distributed on the study site. A real-time kinematic (RTK) GNSS survey of the GCP markers (accurate to within 2-4 cm of absolute accuracy depending on the baseline distance to the base station) or total station survey (accurate to within 1 cm or better) is generally used to obtain accurate coordinates for the GCPs. The GCPs are identified in the imagery, and a geometric image transformation is performed preferably as part of the bundle adjustment through rigorous \begin{table}
\begin{tabular}{c c c c c} \hline
**Year** & **Description (Novelies)** & **Sensor Type** & **Sensor** & **Content** & **Reference** \\ \hline \multirow{4}{*}{2014} & Self-assembled pushbroom system & \multirow{2}{*}{Pushbroom} & \multirow{2}{*}{Self-assembled} & \multirow{2}{*}{C, L P} & [40,126] \\  & Orientation of image lines with a combination of GNSS/INS and aerial & & & & \\ \hline \multirow{4}{*}{2014} & First pushbroom system on multi-rotor & \multirow{4}{*}{Pushbroom} & \multirow{4}{*}{Micro-Hyperspec VNIR} & \multirow{2}{*}{C, I} & [38] \\  & UAV for ultra-high resolution imaging & & & & \\  & Spectroscopy & & & & \\  & Comprehensive description of calibration & & & & \\  & procedures & & & & \\ \hline \multirow{4}{*}{2014} & Uncertainty propagation of the & \multirow{4}{*}{Rikola FPI} & \multirow{4}{*}{C, P} & [162] \\  & hemispherical directional reflectance & \multirow{4}{*}{spectral 2D imager} & \multirow{4}{*}{Cubert Firefleye} & \multirow{4}{*}{C, L P} & [43] \\  & observations in the radiometric & & & & \\  & processing chain & & & & \\ \hline \multirow{4}{*}{2015} & Hyperspectral 3D models & \multirow{4}{*}{D snapshot 2D} & \multirow{4}{*}{Cubert Firefleye} & \multirow{4}{*}{C, L P} & [43] \\  & Quality assurance information & & & & \\  & integration & & & & \\ \hline \multirow{2}{*}{2015} & Multi-angular measurements with UAV & Point spectrometer & Ocean/Optics STS-VES & C, L P & [24] \\ \hline \multirow{2}{*}{2016} & Multi-angular measurements with UAV & Pushbroom & Self-build (H)TMSY) & P & [187] \\ \hline \multirow{2}{*}{2016} & SWIR 2D imaging from UAV & Sequential band 2D & \multirow{2}{*}{Tunable FPI SWIR} & \multirow{2}{*}{P} & [65] \\  & Implementation and calibration of & Multi-camera spectral & \multirow{2}{*}{Self-assembled} & \multirow{2}{*}{C, I} & [38] \\  & multi-camera system on UAV & 2D imager & & & \\ \hline \multirow{4}{*}{2017} & Measuring sun-induced fluorescence in & \multirow{4}{*}{Point spectrometer} & \multirow{4}{*}{Ocean/Optics USB4000} & \multirow{4}{*}{C, I, P} & [25] \\  & the O2A band & & & & \\ \cline{1-1}  & Comprehensive description of calibration & & & & \\ \cline{1-1}  & procedures & & & \\ \hline \multirow{2}{*}{2017} & Toolbox for pre-processing drone-borne & Sequential spectral 2D & \multirow{2}{*}{Rikola VNIR} & \multirow{2}{*}{C, P} & [68] \\  & hyperspectral Data & & & & \\ \hline \multirow{2}{*}{2017} & BRDF measurements with UAV & Sequential band & \multirow{2}{*}{Rikola FPI} & \multirow{2}{*}{P} & [70] \\  & speedful 2D imager & & & & \\ \hline \multirow{4}{*}{2018} & Theoretical considerations to & \multirow{4}{*}{2D imagers in general} & \multirow{4}{*}{Cubert Firefleye} & \multirow{4}{*}{C, P} & [42] \\  & comprehend imaging spectroscopy with & & & & \\ \cline{1-1}  & 2D imagers & & & & \\ \cline{1-1}  & Explanation of differences between & & & & \\ \cline{1-1}  & imaging and non-imaging data & & & & \\ \hline \end{tabular}
\end{table} 
Table 2: _Cont._photogrammetric modeling or the typical SfM workflow. For 2D imagers and SfM orthomosaics, relatively few GCPs are needed. Typically, 5-13 GCPs are sufficient for most scenes, which are distributed along the outside of the scene, with at least one GCP in the center [94]. The advantage of this approach is that it does not require complex and expensive GNSS/IMU hardware on the UAV. The disadvantages are as follows.