# From Analogue to Digital Photogrammetry: Documentation of Padise Abbey in Two Different Time Stages
## 4 New Survey and Processing  
Since the survey in 1991, Padise Abbey has been partly renovated. The renovation of the south wall of the abbey, in which we were interested, is already in an advanced stage and the state of the wall is very different on the first sight. After deeper inspection, it was obvious that during the renovation, original stones of the wall were preserved. It was even possible to find the carved markers (M1-M4) from the original survey. In 2017, we carried out a new photogrammetric survey. The new images were processed using structure from motion method and point cloud and orthophoto was created to have the possibility to compare the different state and condition of south wall of the abbey.  
For capturing the images, we used a digital non-metric camera Nikon d610 with Nikon AF-S DX Nikkor 35 mm f/1.8G lens and we captured 30 images with resolution \(6016\times 4016\) pixels from different positions at different angles. The images were captured 18-39 m from the object with average GSD 4.8 mm/pix.  
Structure from motion method was used to process the images and Agisoft Metashape was chosen as a software for SfM processing. After computation of relative orientation, the model was georeferenced to have the same coordinate system as we had in the photogrammetric model made by re-processing. As a control and check points, we used the markers (M1-M4) from the original survey, which were visible and identifiable also in new digital images. We also used significant corners of the stones (P1-P6) of the wall which were possible to identify in images captured in 1991 and in the new images captured in 2017 (see, Figure 7). The coordinates of the markers and significant points we measured in the photogrammetric model made by re-processing. In total, four control points equally distributed in-model were used for georeferencing.  
Then, refinement of the orientation was carried out. After the refinement, we measured six check points in images, and we compared their calculated coordinates to the coordinates calculated in the photogrammetric model of re-processing. In Table 4, there are presented differences in coordinates, Euclidean distance, and root mean square errors (RMSE) for each axis. The largest differences are in Y axis, which is due to the configuration of images. RMSE in X and Z axis (plane XZ is approximately parallel to the south wall) are under 0.01 m. In our case, the RMSE values represent only the accuracy of georeferencing of the new survey model to the model of the re-processing. The accuracy of the  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Stereo-Pair** & **Measured d (m)** & **Calculated d (m)** & **Difference (m)** \\ \hline A1–B1 & 2.060 & 2.066 & 0.006 \\ A1\_15–B1\_15 & 2.060 & 2.067 & 0.007 \\ A2–B2 & 1.849 & 1.852 & 0.003 \\ A3–B3 & 1.676 & 1.678 & 0.002 \\ A4–B4 & 3.444 & 3.423 & 0.021 \\ A5–B5 & 1.923 & 1.919 & 0.004 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Differences between measured distances between images of each stereo-pair and distances calculated in photogrammetric model.  
model of the new survey itself is represented by reprojection error in pixels on control and check points (Table 4). None of the reprojection error exceed 1 pixel. RMS reprojection error of the tie points was 0.4 pix.  
After the accuracy check on check points, point cloud, 3D model, and orthophoto with 0.005 m/pix resolution were generated. The resolution of the orthophoto was chosen according to the average GSD (4.8 mm/pix) and according to the fact that RMS reprojection error and reprojection errors on control points and check points do not exceed 1 pixel.