## 2 Data Set and DSM Preparation
### Remote Sensing Data  
The remote sensing datasets used in this paper were collected in different years for different purposes, and are summarized in Table 1. The main characteristics and technical parameters of each sensor are reported. The data are captured in a time span ranging from 2001 to 2013 and include four satellite stereo sensors--(ALOS/PRISM, Cartosat-1, RapidEye and WorldView-2), one satellite InSAR sensor (TanDEM-X) and two aerial stereo camera systems (HRSC and UltraCam), as well as two LiDAR data sets. Most of the nine DSMs displayed in Figure 2 have been derived using data acquired during leaf-on season. * Airborne LiDAR data  
LiDAR data acquired in 2001 and 2010 with different point density were provided by the Bavarian Administration for Surveying (Bayerische Landesamt fur Digitalierung, Breitband und Vermessung, LDBV). The LiDAR data from 2001 were acquired in February with a point density of around 0.5 points/m\({}^{2}\). The 2010 dataset was acquired from March to April 2010, and exhibits a much higher density of 5.65 points/m\({}^{2}\). The DSM and DTM used in this paper were prepared by using the filtering method implemented in the software TreesVis [(30)], and are also described in [(4)]. In the following text, the generated LiDAR DSMs are referred to as LiDAR2001 and LiDAR2010, respectively (see Figure 2a,b). * Satellite optical data  
Data from four optical satellite systems are involved in this research, including ALOS/PRISM, Cartosat-1, RapidEye and WorldView-2. The Panchromatic Remote-Sensing Instrument for Stereo Mapping (PRISM) is one of the three instruments on board of the Japanese satellite ALOS (Advanced Land Observation Satellite). ALOS was launched on 24 January 2006 and stopped acquiring data on 21 April 2011. PRISM provided 2.5 m resolution stereo imagery with up to 70 km swath width and a revisit time of 2 days [(25; 31)]. The ALOS data used in this research was captured in June 2008. Cartosat-1 was launched on 5 May 2005 by the Indian Space Research Organization (ISRO). It carries two panchromatic cameras that obtain stereo imagery with a forward view of +26\({}^{\circ}\) (Band F) and a nadir view of \(-5^{\circ}\) (Band A). The panchromatic images obtained have a spatial resolution \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multirow{2}{*}{**Name**} & \multirow{2}{*}{**Sensor**} & \multirow{2}{*}{**Acquisition Date**} & \multicolumn{2}{c}{Resolution (m)} & \multirow{2}{*}{**Platform**} & \multirow{2}{*}{**Sensor Type**} \\ \cline{3-4} \cline{6-7}  & & & & **PAN** & & **MS** & \\ \hline
1 & LiDAR2001 & LiDAR & February/2001 & n.a. & n.a. & Aerial & Active Optical \\
2 & HRSC2003 & HRSC & July/2003 & – & 0.2 & Aerial & Passive Optical \\
3 & ALOS2008 & ALOS-PRISM & June/2008 & 2.5 & & Satellite & Passive Optical \\
4 & Cartosat2008 & Cartosat-1 & February, July, & & & & \\
5 & RapidEye2009 & RapidEye & May/2009 & – & 6.5 & Satellite & Passive Optical \\
6 & LiDAR2010 & LiDAR & March–April/2010 & n.a. & n.a. & Aerial & Active Optical \\
7 & WorldView2012 & WorldView-2 & June/2012 & 0.5 & 2 & Satellite & Passive Optical \\
8 & Aerial2012 & UltraCam & June/2012 & – & 0.2 & Aerial & Passive Optical \\
9 & TanDEM-X2013 & TanDEM-X & May/2013 & 12 & & Satellite & Active InSAR \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of the remote sensing datasets. of 2.5 m and cover a swath of 30 km. The satellite has a maximum revisit time of five days [25]. Three pairs of Cartosat data captured in February, July and November 2008 were available for this study. The RapidEye satellite system, launched at the end of 2008, provides data in five bands, namely blue, green, red, red edge and NIR, with a spatial resolution of 6.5 m. The RapidEye sensor is not designed to provide stereo data but has a very frequent revisit interval of 2-3 days, allowing across-track stereo matching [32]. The RapidEye data captured in 17 and 23 May 2009 are used as stereo pair in the DSM generation procedure.