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can be estimated and all the points of a scan can be changed into 
the coordinate system of a scan that has been assumed as the 
reference system. 
If this simple approach is followed, all the scans that describe an 
object can be referenced to a single coordinate system (e.g. the 
first scan coordinate system or an external system defined by at 
least three points). 
1 i . iyi. 1 , i , 1 . iii, 1 . i, 1 .«i 1 1 1 1 1 1 1 1 1 1 
Figure 2. Multi scan for large objects 
From a photogrammetric point of view, this procedure is the 
same as the one that is used in independent model triangulation; 
therefore the same problems arise. Small overlaps between 
adjacent scans lead to systematic errors during the junction of 
adjacent scans; these errors can be overcome with a final 
compensation using at least one or two points for each scan 
surveyed by means of a total station in a terrestrial reference 
system. 
This procedure requires a large amount of human intervention 
to collect homologous tie points inside the overlapping portions 
of adjacent scans. 
2.1 Automatic tie point collection 
The collection of tie points and the search of the homologous in 
adjacent scans can be performed automatically using reflecting 
targets. 
Laser scanner devices record the X, Y, Z point coordinates and 
the average reflectivity of the impact area of the laser beam. 
Buildings are usually made of poor reflective material (e.g. 
stones, clay). If some reflective targets are superimposed onto 
the object, they can easily be found (see fig. 3) simply by 
selecting, from all the acquired points, those which have a 
higher reflectivity than a prefixed value (e.g. the higher 
reflectivity value of the material of the object). 
Figure 3. Digital representation of the recorded reflectivity 
values using the laser scanner 
The reflecting target must be placed on the object in such a way 
that at least three targets can be found in the overlapping portion 
of two adjacent scans. The size of the target must be large 
enough to allow the laser scanner to record it. 
If the usually adopted beam divergence is considered, square 
target sizes of 2 cm x 2 cm can satisfy almost the whole 
application of architectural object recording. The same targets 
can be used for a total station survey to obtain the necessary 
information for the positioning of the scans in an external 
defined reference system. 
Figure 4. Reflecting targets 
The reflecting targets must be placed far from other high 
reflective objects in order to avoid errors in this phase. 
2.2 Homologous tie point correlation 
Let us consider two adjacent overlapping scans. Some reflecting 
targets (at least three) have been placed in the overlapping 
portion and recorded by the laser scanner. All these targets can 
easily be automatically found (see previous paragraph)and the 
coordinates can be recorded. 
The purpose of this procedure is to connect each point of the 
right scan to the homologous point of the left scan; the reference 
system of the latter scan is fixed and only the point recorded in 
the right scan can rotate and translate in space. 
It can be assumed that the Z axis of the two scans are vertical: 
actually all laser scanner devices are placed on the ground on 
top of topographic tripods and stage plates. 
This hypothesis simplifies the search for homologous points: if 
one starts with the two vertical Z axis search for the right set of 
points can rotate around their Z axis and translate in space but 
they cannot rotate around their X and Y axis. 
Z L 
Left set of points Right set of points 
Figure 5. Reflecting targets found in the adjacent scans 
This problem is solved in two subsequent steps. Using the 
points of the left set, the procedure calculates two spherical 
coordinates (range and elevation) of the points considering each 
time one of the points has the origin 0 S j: in the case of figure 5, 
six series of spherical coordinates are determined. 
Using the points of the right set and one of these as the origin 
0 R , the same two spherical coordinates of the remaining points 
are then determined. These coordinates are compared to all 
those of the previously computed six series and the one which 
has the maximum number of equal coordinates (both range and 
corresponding elevation angle is selected). Point 0 R is 
considered to be the homologous of the origin 0 S j of the 
selected spherical coordinate set. This procedure is iterated for 
each point of the right set of points. The equality of the 
coordinates is judged according to the range and angles 
tolerances that are typical for the laser scanner. 
If one of the comparisons gives no equality for at least one 
point, it means that the point Or is not present in the left set of 
points. 
Once the homologous points have been selected, the procedure 
verifies the obtained results by comparing the differences 
between the angular spherical coordinates (elevation and