-55- 
selected scan in an external reference system (interactive 
procedure), registration of two adjacent scans (automatic 
procedure). 
4.1 Starting 
The operator introduces some information on the used laser 
scanner (e.g. measurement accuracies, beam divergence), the 
size of the reflecting targets and the value of their minimum 
reflectivity. This value can be based on the reflectivity image 
(see fig. 3): in standard conditions it decreases with the distance. 
The minimum reflectivity should then be tested on the furthest 
targets. This value can sometimes also characterize also other 
object (e.g. vegetation, road signs): in these cases, the operator 
has to previously erase the undesired portion of the scan or 
operate in different portions of the scan with different threshold 
values. 
Markets utilizzati 
Precisione dello strumento f T oieranze á confronto 
Valore di precisione del sensore laser 
Precisione nella valutazione delle distanze 
J 25* cm 
Precisione nella valutazione degli angoli 
j — gon 
Tolleranza di filtratura + 
j 2 ° 
Usa precisioni del Laser Scanner : 
Riegl LPM-25HA 
1 Riegl LMS-Z21Û! 
Figure 10. General information input window 
4.2 Scan selection 
The operator selects one or two scans that must be registered. 
The program operates in ASCII files; as all laser scanner 
devices usually export the data in this format, the LRS software 
can operate independently from the used instrument. 
A preview of the selected files is shown and the operator can 
indicate the data format (e.g. x-y-z-reflectivity or range- 
horizontal angle-elevation angle-reflectivity). 
« Impostazione file ASCII 
File ASCII che si intende aprire. E' necessario fornire la sequenza dei dati 
:;Ì#ASàl Export Fte * ' 
Il Automatically generated by • 3D-R¡SCAN 
fi 3D data size - meas / line: 1006 
fi lines / image: 2871 
fi meas id = measfi + linefi x (meas/line) 
fi coordinates x y z [meter] a [1 ] r[m] pol[gon] azi[gon] 
fi 
•14 24717 898 2 064 40 22 969 94.273 142.800 
•14 239 1 7 888 2 033 51 22.953 94 353 142 800 
-14.215 17 858 2001 56 22.913 94.433 1 42 800 
C coordinates x y z [meter] a [1] r [m] poi [gon] azi [gon] id Time 
C coorcfinates x y z [meter] a [1] r [m] poi [gon] azi [gon] id 
<■ coordinates x ji 2 [meleti a [1| 1 |m| poi Igcfti azi Igor.j 
P coorcfinates x y z [meter] a [1] 
Apri i Se I Annulla 
Figure 11. Data format input window 
4.3 Target selection 
The automatic selection of the reflecting marks runs on the 
selected scans. 
First of all, the procedure extracts all the points that have a 
reflectivity value higher than the threshold fixed at the 
beginning; in a second step, these points are clustered in order 
to group all the pointsthat describe the same reflecting marker. 
This clustering is performed considering the beam divergence 
and the diffraction of the high reflective points (e.g. a square 
target with a 2 cm side is identified by points grouped in a circle 
with a diameter of 4 cm). 
The coordinates of the target, computed as the mean of the 
coordinates of the clustered points, are recorded in a separate 
file which is joined to the scan file for all the subsequent 
elaborations. 
IliLlii i 1,4.1' |g,ff!M№? re 
O Laser Scanner Registratici Orientamento Filtra file Unione di scansioni Converti file Finestra ? 
Punti individuati in modo automatico 
<— Aggiungi punto 
Figure 12. Automatic target selection 
Obviously, this step can only run using the original scans. The 
previously described filtering data noise reduction (see par.3) 
makes it possible to loose the correct position of the reflecting 
targets. 
4.4 Data noise reduction 
This procedure runs for each single scan. The operator has to 
choose the mesh dimensions indicating the angular steps in 
spherical coordinates. The size of the mesh must be at least two 
or three times larger than the acquisition scanning rate in order 
to provide a consistent number of points to the described 
algorithm. 
Figure 13. Data noise reduction window 
4.5 Single scan registration in an external system 
This step simply allows the registration of a single scan in a 
reference system defined by at least three points of known 
coordinates. The known points are selected by the operator on 
the scan and the transformation parameters are estimated; than