## 2 Test Site and Data  
The study area is located in the south-west of Germany, north of the city of Karlsruhe within the federal state of Baden-Wurttemberg and has a size of 924 ha (Its coordinates in the _Gauss-Kriger_ system are: top left: 3,456,300,00/5,436,100,00 and bottom right: 3,458,400,00/5,431,700,00). A vegetation height model from ALS and a color-infrared (CIR) orthophoto of the study area is shown in Figure 1.  
The study site is characterized by flat terrain (min. height: 101 m a.s.l., max. height: 123 m a.s.l.) and various forest stands (pure and mixed forest) with different species and age classes. However, Scots pine (_Pinus sylvestris_) dominates the study site with 51% (in total 56% coniferous trees). Four spectral channels were acquired during leaf-on conditions in July 2008 by TopoSys GmbH using a multispectral line scanner. One advantage of this sensor is that shadowing effects only occur orthogonal to the flight line [16]. The single flight strips were rectified and geo-referenced using a surface model, which was filtered from laser scanner data (6-7 points/m\({}^{2}\)) and acquired at the same time as the optical data. Important flight and technical parameters of the line scanner are listed in Table 1.  
Full-waveform laser scanner data were measured in November 2009 by Milan Geoservice GmbH using the IGI Litemapper 5600 system with a Ri \begin{table}
\begin{tabular}{l l} \hline \hline
**Parameter** & **Value** \\ \hline Flying height & 700 [m] AGL \\  & Blue: 450–490 [nm] \\  & Green: 500–580 [nm] \\ Spectral channels & Red: 580–660 [nm] \\  & Near infrared: 770–890 [nm] \\ Viewing angle & 21.6 [ ] \\ Pixels per line & 682 \\ Ground sampling distance & 0.4 [m] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Flight and technical parameters of the flight campaign in summer 2008 using a multispectral line scanner integrated in the “Falcon II system” (AGL = above ground level). Figure 1: **(a)** Vegetation height model from ALS and **(b)** Color-infrared orthophoto from multispectral line scanner data of the study area near Karlsruhe, Germany. To obtain a high point density (\(>\) 20 rays/m\({}^{2}\)), the study area was flown twice, first in north-south and then in east-west direction. Flight and system parameters are shown in Table 2.  
The full-waveform data was processed using the software \"RiANALYZE 560\" and was delivered in ASCII format with 3D coordinates of the reflections and additional information such as target number, number of targets in beam, echo signal amplitude and the echo pulse width. An \"Active Surface Algorithm\", implemented in the software \"TreesVis\" [17] was used to compute a terrain and a surface model (DTM and DSM) with 1-m resolution from the reflections. The algorithm employs the general technique of matching a deformable surface to the laser points by means of energy minimization. To estimate a model of the bare earth (DTM), the surface will be fitted to the points that are considered to be terrain points, which are frequently the lowest points within a defined neighborhood [18]. For the computation of a model including vegetation and buildings, the surface will be fitted to the highest reflections. A normalized DSM (nDSM), in forests often referred to as canopy height model (CHM), was derived as the difference model of DSM and DTM:  
\[\text{nDSM(x,y)}=\text{DSM(x,y)}-\text{DTM(x,y)}\]  
Field measurements were carried out in winter 2009 and spring 2010. Nine square field plots with a size of 30 \(\times\)30 m were established within forest stands dominated by Scots pine. Some selected stands have a low proportion of further tree species such as red oak (_Quercus rubra_), sessile oak (_Quercus petraea_), beech (_Fagus sylvatica_) and birch (_Betula pendula_). Figure 2 shows a 3D view (DSM with CIR texture) of a typical pine stand in the test site with one of the square field plots.