The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B5. Beijing 2008 
3. aluminium disk (<£=12 cm) with a central reflecting 
circular surface of 10 cm and a thickness of 0.5 cm 
(no. 7); 
4. aluminium disk (<£=8 cm) of dark colour with a 
central reflecting circular surface of 3.5 cm and a 
thickness of 0.3 cm (no. 10). 
The 3-D coordinates of target centres have been measured by a 
total station Leica TCA2003 into the GRS; results have shown a 
std.dev less than ±0.4 mm. 
During this test, the scanner Riegl LMS-Z420/ has been 
positioned on a fixed stand-point and the target framework has 
been moved along the range direction at different distances (10, 
50, 100, 200 and 300 m). At each distance step the panel has 
been rotated of 30 deg w.r.t. the vertical plane (configuration 
“v30”), and of 30 and 45 deg w.r.t. the horizontal plane (“h30” 
and “h45”). Estimated accuracy of these rotations is in the order 
of ±4 deg. The whole framework has been scanned at the 
maximum angular resolution (0.004 gon) and the target have 
been recognized, scanned and measured by Riscan Pro software. 
3.2 Test 2: close-range measurements 
The second group of tests has been setup to evaluate systematic 
errors in range (r), horizontal (<p) and vertical (0) angles 
corresponding to the measured target centres. In a 12x6x3 m 
room of the Politecnico di Milano university, 38 square RRTs 
of size 5 cm have been glued on the walls and measured by a 
total station Leica TCRA1200 whit a std.dev less than ±lmm. 
To distinguish between two different tests that have been 
carried out, these have been referred to: 
• Ex2.T. the laser scanner has been positioned in a central 
stand-point (n. 300 in Figure 1) and 9 scans have been 
repeated for each target in order to check the repeatability; 
• Ex2.2: the laser scanner has been positioned over two 
stand-points (n. 100 and 200 in Figure 1) at a relative 
distance of ~7 m. From each stand-point, 3 different scans 
rotated in the horizontal plane of 120 deg have been 
captured. 
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Figure 1. The room of Test 2 with TLS stand-points and target 
positions for both Ex2.1 and Ex2.2 
3.3 Test 3: Error modeling 
The last test has been setup to evaluate the precision of target 
measurement in relation to laser beam angle of incidence and 
range. In this case a square RRTs of side 5 cm has been glued in 
the centre of a circular laminate (Figure 2) that can rotate 
around both vertical and horizontal axes without changing the 
target’s centre position. The target was scanned using the 
automatic procedure implemented in Riscan Pro software. 
The range of measurement was selected in function of the 
algorithm adopted by Riscan Pro to scan the target, as described 
in sub-section 2.1. In the considered range (4-35 m) and for a 
target size d= 5 cm, the horizontal spatial resolution is given by 
s H ~ dl20=2.5 mm, while the vertical resolution s v always 
depends on the maximum angular scan resolution. The size of 
the scan window is about 23x23 cm. The following tests have 
been performed: 
H 
7 
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Figure 2. Target with its rotation axes 
• Ex3.1: the target has been scanned at every 0.3 m distance 
step from 4 to 20, and from 20 to 35 m at 1 m steps. The 
frame has been oriented to face the scanner; 
• Ex3.2: the target has been placed at distances of 9, 13.5 
and 18 m from the scanner, and rotated of 10 deg steps 
from -70 to 70 deg w.r.t. the vertical and horizontal 
target’s axes, with an accuracy of about ±1 deg. In 
addition, for the range of 13.5 m the target has been also 
simultaneously rotated in both directions. This has 
resulted in 3-D incidence angles (estimated std.dev ±0.01°) 
of 18.5, 31.4, 47.6, 61.3, 75.5, 85.6 deg. 
4. ANALYSIS OF RESULTS 
4.1 Algorithms for automatic RR target measurement 
When a target is scanned, the acquisition software calculate 
automatically the centre of it by applying an alghoritm for its 
measurement. Nevertheless, the algorithms implemented in 
commercial softwares are unknown for the most. 
In Lichti et al. (2000) three different methods are described. 
The first defines the centre of each target as the position with 
the maximum radiance. The second defines the centre by the 
mean position of the radiometric centre of the 4 strongest 
returns. The third algorithm defines the centre of the target as 
the radiometric centre of all returns. These methods will be 
referred to henceforth as “maxrad”, “maxrad4” and “radcent”. 
In Valanis & Tsakiri (2004) some other algorithms are 
presented and tested using a Cyrax 2500 TLS in laboratory 
conditions. The method used was based on the fuzzy clustering 
technique introduced by Bezdek (1981). In the present work, 
results obtained by an algorithm based on the same method 
(“fuzzypos”) will be presented, even though it has been 
modified to work with RRTs adopted in tests described in Sec.