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Figure 3. Visualisation of the geo-referenced ALS and TLS 
point cloud; Upper part: ALS points (red); Middle part: ALS 
(red) and TLS (grey) points; Lower part: TLS points (black). 
For the study of the simultaneously acquired ALS and TLS 
data, an adequate geo-referencing of the data sets is essential. 
For all of the following steps the commercially available Riegl 
software package (REANALYZE, RiPROCESS and RiWORLD) 
was used (Riegl, 2010). 
As a first step, a decomposition of the acquired FWF-ALS data 
set was performed. This includes the detection of all echoes per 
emitted laser pulse and for each echo the determination of a 
model in order to derive further echo parameters (amplitude and 
echo width). For geo-referencing, the trajectory of the airplane 
has to be determined using the observations from the global 
navigation satellite system (GNSS) and the inertial 
measurement unit (IMU). Based on the trajectory in the global 
co-ordinate system, the detected echoes in the scanner co 
ordinates of all echoes can be derived by direct geo-referencing. 
However, due to the requirements of the study, an advanced 
geo-referencing of the ALS data was essential. Therefore, based 
on the absolutely determined roof faces and additional planar tie 
elements, a strip adjustment of the ALS data was performed 
(Kager, 2004, Riegl, 2010). Within this adjustment, the 
differences to both the planar pass and tie patches were 
minimized 
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