# Quality Assessment of Photogrammetric Methods--A Workflow for Reproducible UAS Orthomosaics
## Materials and Methods
### Photogrammetric Processing  
The Metashape software (Agisoft LLC, St. Petersburg, Russia; formerly known as Photoscan) is widely used for UAS image processing. The standard photogrammetric workflow includes the image alignment, the generation of a digital surface model (called Digital Elevation Model in Metashape) and the orthorectification and mosaicing (Figure 2). The image alignment starts with the automatic identification \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Mission**} & \multirow{2}{*}{**Date**} & **Sun** & \multirow{2}{*}{**Angle (\%)**} & \multirow{2}{*}{**Conditions**} & \multirow{2}{*}{**Camera**} & **Area** & **GSD** & **Alt.** & **Overlap,** & **Images** \\  & & & & & & **(ha)** & **(cm/px)** & **(m)** & **F/S (\%)** & **Images** \\ \hline Forest 01 & 2020/07/07 & \multirow{2}{*}{38.1} & \multirow{2}{*}{cloud free} & GoPro & \multirow{2}{*}{7} & \multirow{2}{*}{2.58} & \multirow{2}{*}{50} & \multirow{2}{*}{\textgreater{} 90/75} & \multirow{2}{*}{630} \\  & 11:20 a.m. & & & Hero & & & & & \\ Forest 02 & 2020/07/07 & \multirow{2}{*}{44.33} & \multirow{2}{*}{cloud free} & GoPro & \multirow{2}{*}{7} & \multirow{2}{*}{2.58} & \multirow{2}{*}{50} & \multirow{2}{*}{\textgreater{} 90/75} & \multirow{2}{*}{630} \\  & 11:42 a.m. & & & Hero & & & & & \\ Forest 03 & 2020/07/07 & \multirow{2}{*}{50.33} & \multirow{2}{*}{partially} & GoPro & \multirow{2}{*}{7} & \multirow{2}{*}{2.58} & \multirow{2}{*}{50} & \multirow{2}{*}{\textgreater{} 90/75} & \multirow{2}{*}{630} \\  & 12:11 a.m. & & & cloudy & Hero & & & & \\ Forest 04 & 2020/07/07 & \multirow{2}{*}{56.13} & \multirow{2}{*}{partially} & GoPro & \multirow{2}{*}{7} & \multirow{2}{*}{2.58} & \multirow{2}{*}{50} & \multirow{2}{*}{\textgreater{} 90/75} & \multirow{2}{*}{630} \\ Forest 05 & 2020/07/07 & \multirow{2}{*}{61.76} & \multirow{2}{*}{cloudy} & GoPro & \multirow{2}{*}{7} & \multirow{2}{*}{2.58} & \multirow{2}{*}{50} & \multirow{2}{*}{\textgreater{} 90/75} & \multirow{2}{*}{630} \\ Forest 06 & 2020/07/07 & \multirow{2}{*}{62.28} & \multirow{2}{*}{cloudy} & GoPro & \multirow{2}{*}{7} & \multirow{2}{*}{2.58} & \multirow{2}{*}{50} & \multirow{2}{*}{\textgreater{} 90/75} & \multirow{2}{*}{630} \\ Forest 06 & 13:43 a.m. & & & Hero & & & & \\ \hline Grassland & 2013/06/01 & 41.34 & partially & Sony & \multirow{2}{*}{7.68} & \multirow{2}{*}{3.32} & \multirow{2}{*}{111} & \multirow{2}{*}{70/75} & \multirow{2}{*}{27} \\
2013 & 11:17 a.m. & & cloudy & NEX-SN & & & & \\ Grassland & 2015/05/22 & \multirow{2}{*}{19.99} & \multirow{2}{*}{cloud free} & Sony & \multirow{2}{*}{14.2} & \multirow{2}{*}{3.68} & \multirow{2}{*}{169} & \multirow{2}{*}{75/75} & \multirow{2}{*}{57} \\
2015 & 09:26 a.m. & & & & & & & \\ Grassland & 2017/05/18 & \multirow{2}{*}{13.58} & \multirow{2}{*}{partially} & Sony & \multirow{2}{*}{32.4} & \multirow{2}{*}{3.97} & \multirow{2}{*}{132} & \multirow{2}{*}{75/75} & \multirow{2}{*}{57} \\
2017 & 08:53 a.m. & & cloudy & ILCE-7 RM2 & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Overview of the flight missions used to acquire the two test datasets in forest and grassland environments. The cameras were triggered by time interval. In the forest flights, altitude refers to a uniform height above the canopy. In the grassland flights, altitude referes to a fixed relative height above the take-off point. Overlap referes to both forward (F) and side (S) overlap. of distinct features in the individual images. This process is enhanced and requires less computation time if the individual images already contain GNSS information. Those features which appear in more than one image, the so called tie points, are matched and projected in a 3D space, forming the sparse cloud that is georeferenced using the surveyed GCPs. The georeferenced sparse cloud is subsequently used to compute a digital surface model, either through a dense pointcloud or a mesh interpolation of the sparse cloud (Figure 2a,b). The surface model is finally used for rectifying the georeferenced images.