The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B5. Beijing 2008 
b) the estimated data gaps (i.e. zones within the extend that 
were not scanned completely or haven’t reached the 
minimal required point density). 
After generalization of the boundaries using the Deuglas- 
Peucker algorithm (Douglas and Peucker, 1973), the vectorized 
boundaries are passed through socket to HELIPOS, where the 
controller can load the polygons in different layers and get an 
update of the actual data extend and zones with insufficient 
point density after each completed strip. 
Figure 6: ALS density raster (left side) and vectorization of 
data extend and data gaps (right) for two parallel 
strips (Minimum Point density set to 1 point/m 2 ) 
The achievable accuracy of the LIAN analysis is in direct trade 
off with the processing speed. Hence, in function of the project 
requirements and processing power the algorithm can be 
parameterized by: 
• Minimum required point density in pts/m 2 
• Density raster resolution 
• Vector generalization threshold 
• ALS data thinning rate 
The requirements on the computational effort can be specially 
lowered applying a “thinning” of input data to LIAN. Empirical 
tests have shown that reducing the ALS data up to one fifth of 
the initial laser points doesn’t affect the quality of coverage 
analysis (extend and gaps) in LIAN. 
6. IN-FLIGHT MISSION CONTROL (HELIPOS) 
The control module is the graphical interface between the 
operator and the previously described modules. It has two major 
functions: 
1) To provide flight management and guidance of the pilot 
2) To control the system settings and to monitor the data 
quality and coverage 
6.1 Flight management and pilot guidance 
For the purpose of pilot guidance, HELIPOS offers two 
different views within the same instance (see Figure 7): 
• The controller view, where all available information can 
be displayed (i.e. flight-lines, map data, RT position, RT 
swath, data extend and gaps) 
• The pilot view, where only the map data, the actual flight 
line and the current position and heading are displayed 
Figure 7: Standard setup of HELIPOS in the cockpit: 
Controller view (bottom screen), pilot view as an 
extended screen on 8 inch - monitor (top) 
Setting up an extended screen on the controller laptop, the pilot 
view can be transmitted to an external screen (see Figure 7) 
through VGA connection. As both views run within the same 
application, they can be cross-linked. This enables the operator 
to control easily the pilots display by changing the view extend 
(e.g. zoom, pan) or by highlighting or deactivating the flight 
lines. 
6.2 System controlling and data quality monitoring 
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Figure 8: Overview of main functionalities of the HELIPOS 
graphical user interface (GUI) 
Within a mission, all modules described until now run hidden 
form the user. HELIPOS is in charge of collecting all the 
necessary information and transmitting them to the system 
operator. Figure 8 depicts the most important controlling 
features: 
A. Status log: All important status messages from the data 
loggers (GPS/IMU/ALS) are gathered and listed in this 
window. Hence, data logging problems or identified 
hardware failures are communicated and the operator can 
react in consequence. 
B. GPS/INS info: The actual position, speed, heading and 
GPS quality indicators (e.g. DOP, number of visible 
satellites) can be displayed. When the GIINAV module is 
running, the operator has direct access to the integrated 
trajectory including RT attitude (roll, pitch and yaw) of the 
sensor head. 
C. ALS info: The measured distance by the laser and the 
swath coverage (in percent) can be displayed. This