851 
TOWARDS IN-FLIGHT QUALITY ASSESSMENT OF AIRBORNE LASER 
SCANNING 
P. Schaer 3 ' *, J. Skaloud 3 , P. Tomé b 
a TOPO Lab, Ecole Polytechnique Fédérale (EPFL), Station 18, 1015 Lausanne, Switzerland 
b Nemerix AG, Stabile Gerre 2000, 6928 Manno, Switzerland 
KEY WORDS: LIDAR, Real-time, Georeferencing, GPS/INS, Quality, Monitoring, Integration, Point Cloud 
ABSTRACT: 
One of the main problems of today’s ALS is the lack of reliable data quality assessment within or shortly after the airborne survey 
campaign. The paper presents an in-flight quality monitoring tool that allows assessing the quality of the recorded data “on the fly”, 
featuring a real-time processing of the GPS/INS data and the subsequent georeferencing of the laser returns. The tool is capable of 
displaying the scanning progress in real-time and detecting data gaps immediately after terminating the strip. The paper presents the 
adopted strategy for data processing and communication in order to achieve scalable distribution across a network of computers. 
Further we discuss first experiences with this tool within airborne survey projects that demonstrate its successful application in 
practice. 
1. INTRODUCTION 
1.1 Motivation 
Figure 1: Insufficient data coverage detected in ALS point- 
cloud after post-processing due to poor reflectivity 
(left) or insufficient strip overlaps (right) 
With measurement rates up to 167 KHz (Optech, 2008) 
Airborne Laser Scanning (ALS) provides masses of data. As a 
rule of thumb, it takes at least ten to fifteen hours to generate a 
final product for each hour of recorded information. 
Furthermore, as the costs per flight mission are considerable, it 
is essential to ensure that the area of interest is covered 
completely and the requirements, such as precision and point 
density, are satisfied. Although ALS has become well 
established and broadly used technology in the surveying 
industry, it is not uncommon to encounter pitfalls due to 
undetected sensor behaviour, varying data quality, consistency 
and coverage (e.g. Figure 1). These are only detected in post 
processing, in other words, long after the flight. Worse, some 
types of errors can only be quantified by independent and 
expensive ground-based surveying methods. This may result in 
conditions where the data quality control may take an 
overwhelming part in the cost of a mapping product. Besides, 
the employment of such control further increases the time 
between a flying mission and the product delivery to a client. 
Therefore one of the main problems of today’s ALS is the lack 
of reliable data quality assessment within or shortly after the 
airborne survey campaign. 
The arguments raised above call for top-level of automation in 
data processing and a flexible in-flight quality monitoring tool 
that allows assessing the quality of the recorded data “on the 
fly” (Legat et al., 2006). The pre-requisite for such a tool is on 
one side the implementation of real-time (RT) processing of the 
GPS/INS data and the georeferencing of the laser returns; on 
the other side, the analysis of the trajectory integrity together 
with the scanning precision and geometry. This concept has 
already proven its feasibility in post-processing (Latypov, 2002; 
Schaer et al., 2007). This paper will focus on the 
implementation of such strategy in-flight in parallel with the 
data acquisition. 
Figure 2: Handheld mapping system (Scan2map) installed in a 
helicopter 
Our institution (EPFL-TOPO) lead the built up of a system 
called “Scan2map”, that combines GPS, INS, ALS and a 
medium format digital camera in one solid mount (see Figure 2) 
(Skaloud et al., 2006). It can be used in nadir and oblique setup 
within the same flight. However, this flexibility in gaze 
direction as compared to classical ALS systems (i.e. fixed 
installation of the laser head on a plane), involves more 
accurate flight planning and tight control of plan execution to 
guarantee the complete data coverage. These particularities are 
also true for other ALS systems operated from helicopters and 
used for scanning projects in demanding topography at 
relatively low flying heights. Accordingly, the most important 
* Corresponding author. Tel: +41 21 693 27 06 / Fax: +41 21 693 57 40 / email: philipp.schaer@epfl.ch