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| 
A QUALITY ASSESSMENT OF REPEATED AIRBORNE LASER SCANNER 
OBSERVATIONS 
E. Ahokas*, H. Kaartinen, J. Hyyppä 
Finnish Geodetic Institute, Geodeetinrinne 2, 02430 Masala, Finland 
Eero.Ahokas@fgi.fi, Harrı.Kaartinen@fgi.fi, Juha.Hyyppa(@fgi.fi 
KEY WORDS: Comparison, LIDAR, DTM, accuracy, quality, planimetric error, repeatability 
ABSTRACT: 
This paper describes the height and planimetric errors of repeated ALS (airborne laser scanning) strips with a deeper focus on 
building extraction. Measurements with Toposys Falcon airborne laser scanner were arranged in May 2003 in Espoo, Southern 
Finland. A 5 km” test area, consisting of urban settlements and forests, were collected from the altitude of 400 m resulting in 
measurement density of about 10 points per square metre. One 4 km long and about 100 m wide strip was collected five times 
allowing the analysis of the repeatability of the laser scanning- One strip was used as a reference and inter-strip comparisons were 
made. Point wise comparison methods were also used to characterize the differences. Additionally, target models were compared 
against each other. Real Time Kinematic (RTK) GPS and also tachymeter measurements were used as ground reference. Extraction 
of building vectors from laser scanner data was performed using interactive methods implemented in the TerraScan software. The 
accuracy of the vectorization is also reported. Mean height errors for elevation points were —2 to 1 cm and standard deviations were 
mainly £3-4 cm. In planimetry, mean errors of the centre points of the buildings were less than 30 cm for the first and also for the last 
pulse data when compared with the buildings on the map. The standard deviations varied between «11-28 cm (first pulse) and +14- 
18 cm (last pulse) for extracted buildings using repeated observations. Mean errors were between 3-8 cm and standard deviations £3- 
6 cm using last pulse data of repeated observations and extracted ridge information. Extracted buildings were systematically larger 
from first pulse data than from last pulse data. 
1. INTRODUCTION 
Airborne laser scanning (ALS) produces 3D information about 
the object, giving both the terrain elevations and 3D target 
models. The original output of the ALS is a point cloud 
containing x, y and z coordinates and intensity values of the 
points. The main applications of airborne laser scanning are 
digital elevation models, but it can be used successfully for e.g. 
3D city modelling, power line corridor mapping, forest 
mapping, urban planning, water resource management and 
railway surveying. More and more applications will appear as 
the ALS equipment and analysis methods improve all the time. 
Quality of airborne laser scanning has been studied for many 
years (e.g. Crombaghs et al, 2002; Maas, 2002). Rónnholm 
(2004) has studied the repeatability of laser scanning strips by 
analyzing the shifts of local laser point clouds with the 
interactive orientation method  (Rónnholm et al., 2003). The 
measurement accuracy of the laser points for different sensors is 
reported in (Baltsavias, 1999). Building modelling accuracy in 
position and in elevation has been reported by Steinle et al., 
(2000). Maas et al. (1999) used raw laser altimetry data and 
two methods, the intersection of planar faces and the analysis of 
invariant moments methods, to model buildings. The invariant 
moments method yielded a precision of 0.1-0.2 m for the 
building dimensions. 
2. MATERIAL AND METHODS 
2.1 Airborne laser scanner data 
Toposys Falcon airborne laser scanner measurements were 
carried out in May 14”, 2003 in Espoonlahti area, Southern 
Finland. A 5 km? test area, consisting of urban settlements and 
forests, were collected from the altitude of 400 m resulting in 
nominal measurement density of about 10 points per square 
metre. One 4 km long and about 100 m wide strip was collected 
five times. First and last pulse data with intensity values were 
recorded. Technical details about the Toposys Falcon laser 
scanner system can be found in (www.toposys.com) and are 
summarized in Table 1. A constant elevation adjustment for the 
whole strip was done by Toposys. In their method the value of 
the correction was determined at the overlaps of the flight strips 
in the profiles that were running across the flight strips. The Z 
correction was —0.03 m for strips ID number 2, 3 and 4 and — 
0.01 m for strips 5 and 6. Strips 3 and 5 were flown to the 
southeast direction and strips 2, 4 and 6 to the northwest 
direction. 
  
  
  
  
  
  
  
  
  
  
  
Sensor type Pulsed fibre scanner 
Range 1600 m 
Distance resolution 0.02 m 
Scan width 14° (47°) 
Scan rate 653 Hz 
Laser pulse rate 83 000 Hz 
Pulse length 5 ns 
Laser wavelength 1560 nm 
Data recording First and last echo, intensity 
  
Table 1. Technical parameters of the Toposys Falcon airborne 
laser scanner.