## 2 Datasets and Method
### Description of Datasets
#### 2.1.2 ISPRS Vaihingen Datasets  
The ISPRS Vaihingen datasets, which are provided by the German Society for Photogrammetry, Remote Sensing and Geoinformatics, are for object classification and building reconstruction. The LiDAR data of the Vaihingen in Germany was acquired using a Leica ALS50 sensor (Leica Geosystems, Heerbrugg, Switzerland). The LiDAR point density varies between 4 and 7 ppm that is equivalent to GSD of 0.4 m-0.5 m. The datasets include IR aerial true orthoimages and DSMs with resolution of 0.09 m (Figure 6). Furthermore, they also include label data of geo-coded land cover classification with six classes, namely, impervious surface, building, low vegetation, tree, car, and clutter/background, as shown in Figure 7[19; 20; 21]. Figure 8 is 3D perspective view of the LiDAR data.  
Figure 4: Examples of DALES datasets: (**a**) LiDAR data; (**b**) semantic segmentation label data; (**c**) object categories with 8 class labels.  
Figure 5: 3D perspective view of the LiDAR data shown in Figure 4a.  
Figure 6: ISRPS Vaihingen datasets for 2D semantic labeling overlaid on IR orthoimage.  
The standard LiDAR data format is LAS (i.e., LASer file format). The LAS specification was developed and is maintained by the American Society for Photogrammetry and Remote Sensing (ASPRS). The LAS format includes 3D coordinates (i.e., X, Y, and Z) of each point, and additional attributes are provided: intensity, return number, number of returns, classification values, edge of flight line, RGB values, GPS time, scan angle, and scan direction. Table 1 shows an example of the return number and number of returns of the LiDAR data in LAS format (see Figure 1).  
In this paper, the number of return attributes plays a crucial role in extracting buildings. Two returns of the LiDAR data with the LAS format are dominantly generated along the building boundaries. \"Return number\" and \"number of returns\" are confused frequently. An emitted laser pulse can have several returns depending on the features it is reflected from and the capabilities of the laser scanner used to collect the data. The first return will be flagged as return number one, the second as return number two, and so on, with the LAS format. On the other hand, the number of returns is the total number of returns for a given pulse. For example, a laser point may be return two (i.e., return number) within a total number of returns [22].  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Laser Beam**} & \multirow{2}{*}{**X**} & \multirow{2}{*}{**Y**} & \multirow{2}{*}{**Z**} & \multicolumn{2}{c}{**Return**} & \multicolumn{1}{c}{**Number**} \\  & & & & & **Number** & **of Returns** \\ \hline \multirow{3}{*}{1} & 514,519.07 & 5,447,023.33 & 95.39 & 1 & 2 \\  & 514,520.24 & 5,447,031.85 & 91.71 & 2 & 2 \\ \hline \multirow{3}{*}{2} & 514,519.10 & 5,447,023.81 & 96.14 & 1 & 2 \\  & 514,520.23 & 5,447,023.02 & 95.26 & 2 & 2 \\ \hline \multirow{3}{*}{3} & 514,519.13 & 5,447,023.63 & 100.20 & 1 & 4 \\  & 514,519.33 & 5,447,024.32 & 97.76 & 2 & 4 \\ \cline{1-1}  & 514,519.41 & 5,447,024.63 & 96.76 & 3 & 4 \\ \cline{1-1}  & 514,519.65 & 5,447,025.46 & 92.81 & 4 & 4 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Example of return number and number of returns of LiDAR data.  
Figure 8: 3D perspective view of the LiDAR data shown in Figure 7b.  
Figure 7: Examples of ISPRS Vaihingen datasets for 2D semantic labeling: (**a**) IR true orthoimages; (**b**) LiDAR data; (**c**) DSM; (**d**) land cover label data (i.e., ground truth); (**e**) land cover categories with 6 class labels.