## 4 Results
### The Number of Points Detected on Raster Images
Using the SURF detector, the largest maximum numbers of tie points were detected for raster images in the spherical and Mercator projections (about three times more than for orthoimages). Rasters in the Mercator projection were characterized by the largest mean numbers of detected characteristic points, with a value three times larger than for other projections. Due to the lack of texture and characteristic geometrical shapes for test sites III and IV, the algorithms detected and matched a considerably smaller number of tie points. In the case of Test site IV (a long and narrow shop in the shopping mall), the sufficient number of points was obtained only for raster images in the spherical projection and for orthoimages. Resuming, the greatest number of tie points the greatest number of tie points was detected for Test site II. While the analysed object was not geometrically complicated and had the smooth ceiling and oak parquet, due to painting on the walls tie points were best detected and matched by detectors. In the case when the office or industrial objects are analysed (having poor texture and low geometric diversification) it is recommended to use a FAST detector and raster images in spherical projection and orthoimages. At the first stage of tie point's detection, the process of tie matching points in pairs consisted of the analysis of the descriptor, which was based on the reflection intensity of the laser beam. Due to the fact that analysed scans were acquired under different angles and from different distances from surfaces being measured, the laser beam reflectance intensity was not uniform and it was characterised by the high diversification of tonal values. For such raster images, the algorithms developed for image matching did not always operate correctly and they required another approach to filtration of possible tie points. Another factor that influenced incorrect detection and matching of tie points included \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{3}{*}{**Statistic**} & \multirow{3}{*}{**Detector**} & \multicolumn{6}{c}{**Percentage no. of Incorrectly Detected and Matched Keypoints**} \\ \cline{3-8}  & & & \multicolumn{3}{c}{**Test Site I**} & \multicolumn{3}{c}{**Test Site II**} \\ \cline{3-8}  & & & \multicolumn{3}{c}{**Cartographic transformation**} & \\ \cline{3-8}  & & Spherical & Orthoimage & Mercator & Spherical & Orthoimage & Mercator \\ \hline \multirow{2}{*}{**Avg**} & FAST & 93.3 & 78.6 & 61.0 & 62.3 & 64.9 & 61.0 \\  & SURF & 95.1 & 87.1 & 66.2 & 66.8 & 58.0 & 66.2 \\ \hline \multirow{2}{*}{**Min**} & FAST & 82.5 & 67.2 & 19.6 & 42.3 & 31.0 & 19.6 \\  & SURF & 92.1 & 79.3 & 46.1 & 42.3 & 44.0 & 46.1 \\ \hline \multirow{2}{*}{**Max**} & FAST & 100.0 & 95.0 & 98.3 & 95.7 & 96.6 & 98.3 \\  & SURF & 99.3 & 99.5 & 83.2 & 83.7 & 68.8 & 83.2 \\ \hline \multirow{2}{*}{**Statistic**} & \multirow{2}{*}{**Detector**} & \multicolumn{6}{c}{**Test Site III**} & \multicolumn{6}{c}{**Test Site IV**} \\ \cline{3-8}  & & & \multicolumn{3}{c}{**Cartographic transformation**} & \\ \cline{3-8}  & & Spherical & Orthoimage & Mercator & Spherical & Orthoimage & Mercator \\ \hline \multirow{2}{*}{**Avg**} & FAST & 17.6 & 17.3 & 17.9 & 19.3 & 20.0 & 15.9 \\  & SURF & 17.8 & 17.5 & 16.7 & 19.0 & 19.4 & 20.0 \\ \hline \multirow{2}{*}{**Min**} & FAST & 16.7 & 16.7 & 16.7 & 14.3 & - & - \\  & SURF & 16.7 & 16.7 & 17.6 & 14.3 & 14.3 & - \\ \hline \multirow{2}{*}{**Max**} & FAST & 20.0 & 20.0 & 20.0 & 25.0 & - & - \\  & SURF & 21.1 & 20.0 & 20.0 & 25.0 & 25.0 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The percentage number of incorrectly detected and matched keypoints for particular projections of point clouds for all test sites (both scenarios). geometric deformations (\"distortion\"), which occurred in raster images in the spherical projection and in the Mercator projection. Therefore the RANSAC algorithm with parameters presented in Figure 2 was applied for initial detection of corresponding tie points.