## 2 Data Set and DSM Preparation
### Remote Sensing Data
The acquisition times of these two datasets are very close, which leads to similar spectral information of the tree leaves. WorldView-2 is a commercial Earth observation satellite that provides very high resolution space borne images. Being Digital Globe's third satellite, it was launched on 8 October 2009. It provides 0.5 m panchromatic images and 2 m resolution eight band multi-spectral images, adding to the usual four bands (red, green, blue, and NIR1) a set of four new bands (coastal, yellow, red edge and NIR2). WorldView-2 is a pushbroom sensor able to collect large areas of stereo-view or multi-view imagery in a single pass by adjusting the viewing angle of the camera by steering the satellite. The average revisit time of the satellite can be up to 1.1 days by using also off-nadir viewing [25]. For the satellite stereo sensors, DSMs were generated from ALOS/PRISM (Figure 2d), Cartosat-1 (Figure 2e), RapidEye (Figure 2f) and WorldView-2 (Figure 2g) stereo imagery using the Semi Global Matching (SGM) algorithm implemented at DLR [33]. The DSM generation and gap filling procedures are described in detail in [34] and [4]. It has to be mentioned that the DSM from Cartosat-1 is a fused result from the three available pairs of stereo imagery [4]. Among these sensors ALOS/PRISM, Cartosat-1 and WorldView-2 can provide along-track stereoscopy data, resulting in a less complex image matching, as the two stereo images are captured with the same weather and illumination conditions. The DSMs from ALOS/PRISM and Cartosat-1 are sampled to a resolution of 5 m, while the DSM generated from WorldView-2 is characterized by a resolution of 1 m. Limited by the lower image resolution and across-track stereoscopy (acquired at different dates), the lower density of original matched pixels for RapidEye data only enables a 10 m resolution DSM. * Satellite SAR data  
TanDEM-X is a German satellite mission and consists of the two X-band SAR satellites TerraSAR-X and TanDEM-X, which are nearly identical in construction and fly in a close formation (see also [26]). The TanDEM-X mission provided for the first time single pass interferometric SAR data from space. The main objective of the mission is the acquisition of data for a global DSM with a spatial resolution of 12 m and an absolute height accuracy of 2 m and 4 m, respectively. The nominal revisit time of TanDEM-X is eleven days. However, by changing the incidence angle revisit times up to three days are possible. TanDEM-X provides a wide range of different acquisition modes, while usually it operates in strip map mode with VV (vertical transmit and vertical receive) polarization, for experimental issues a dual-pol mode is provided and in a special experimental mode even quad-pol acquisitions are feasible. Additionally, the TanDEM-X mission provides a ScanSAR, a spotlight, and a sliding spotlight imaging mode which can be operated at different combination of polarizations [26]. The TanDEM-X2013 DSM (Figure 2i), was calculated with an experimental software package developed by the DLR-Microwaves and Radar Institute (HR) and has an original resolution of 6 m. Calculation steps for the processing of interferometric data are well described in [35], including a description of the parameters affecting the expected accuracy. The TanDEM-X specific interferometric configuration and its performance issues are extensively described in [26]. As radar penetrates into the vegetation layer (for instance in forest areas), the DSM obtained represents a height somewhere between the top of the vegetation layer and the underlying ground surface [36]. * Aerial image data  
One of the airborne stereo datasets was captured by an UltraCam Eagle camera on 16 June 2012 as part of the regularly scheduled aerial survey of Bavaria [37]. The original color images have a ground sampling distance (GSD) of around 0.20 m, while the aerial data from 2003 was captured by the High Resolution Stereo Camera (HRSC) with GSD between 0.15 and 0.25 m [38].