## 4 Results and Discussion  
### Block Triangulation  
During the satellite triangulation of the tri-stereo scene, TPs were automatically extracted in each Pleiades image. They were homogenously distributed within the forward, nadir and backward images, with standard deviations of the adjusted coordinates for elevations ranging from 1.49 m (2.1 GSD) to 3.49 m (5.0 GSD) (Table 2). The standard deviations of the TP elevation obtained for theWorldView-3 scenes are clearly better relative to the GSD, with a maximum value of 0.74 m (2.5 GSD). The standard deviation of the satellite image block-triangulation was 0.54 pixels for Pleiades images and 0.46 pixels for WorldView-3.  
We evaluate the accuracy of the estimated orientation with respect to the measured GCPs and CPs. The root mean square error values of their coordinates in units of meters and pixels are shown in Table 3. For both sensors the RMSE values of the GCPs for latitude, longitude and elevation are at sub-pixel level, showing that the block adjustment provides precise results for subsequent processing. The small planimetric discrepancies suggest that the GCP identification in images is better than 1/3 rd of a pixel. The highest discrepancy values for the CP height are 0.85 m (1.21 GSD) for Pleiades and 0.28 m (0.93 GSD) for WorldView-3. The Pleiades elevation accuracy (0.85 m) is not as good as the one in [5], which resulted to 0.49 m, but is more favorable than the results in [7], where 1 m RMSE in elevation were reported. For WorldView-3, the vertical accuracy of 0.28 m fits to the results in [12], which reported an elevation bias of less than 0.5 m for 6001 ground LiDAR CPs. The residual errors for Pleiades images at CPs are comparable with the results obtained by [40] reporting RMSEs of 0.44 m, 0.60 m and 0.50 m at 40 GCPs for latitude, longitude and elevation, respectively.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & & & \multicolumn{4}{c}{**Standard Deviation (m/pixels)**} \\ \cline{3-8}
**Sensor Type** & **No. of TPs/Image** & \multicolumn{2}{c}{**Latitude (Along Track)**} & \multicolumn{2}{c}{**Longitude (Across Track)**} & \multicolumn{2}{c}{**Elevation**} \\ \cline{3-8}  & & **min** & **max** & **min** & **max** & **min** & **max** \\ \hline Pleiades & 561/552/582 & 0.16/0.23 & 0.33/0.47 & 0.14/0.20 & 0.27/0.38 & 1.49/2.12 & 3.49/4.98 \\ WorldView-3 & 556/585/554 & 0.09/0.30 & 0.19/0.63 & 0.08/0.26 & 0.20/0.66 & 0.38/1.26 & 0.74/2.46 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Standard deviation of adjusted tie points (TPs) coordinates.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sensor Type**} & \multirow{2}{*}{**No. of GCPs/CPs**} & \multicolumn{2}{c}{**RMSE Values (m/pixels)**} \\ \cline{3-5}  & & **Latitude (Along Track)** & **Longitude (Across Track)** & **Elevation** \\ \hline \multirow{2}{*}{ Pleiades} & 43 GCPs & 0.20/0.29 & 0.19/0.27 & 0.27/0.39 \\  & 50 CPs & 0.44/0.63 & 0.53/0.76 & 0.85/1.21 \\ \hline \multirow{2}{*}{ WorldView-3} & 22 GCPs & 0.06/0.21 & 0.08/0.29 & 0.11/0.37 \\  & 50 CPs & 0.13/0.43 & 0.12/0.40 & 0.28/0.93 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Root mean square error (RMSE) values of ground control point (GCP) and check point (CP) discrepancies.