d. The 
ifferent 
objects 
3 of the 
can be 
igh the 
ertices, 
r. 
ic and 
with a 
of 90? 
of the 
est the 
have a 
L The 
ilicone 
]hesive 
  
3. THE PROCEDURE 
Many laser scanners acquire both range and intensity data, so it 
is possible to detect the points belonging to the bases, by using 
a radiometric filter (only the points with an intensity greater 
than a fixed value are selected). In this way, the white base 
rings are detected. : 
A geometric filter is applied, and the points having a distance 
from the first “white” one less than the base diameter are 
selected. With this procedure, the first white ring is isolated 
(Figure 2). 
Figure 2. Point cloud of a target base ring 
The same technique is used to obtain the other rings. The point 
clouds of every single base are so separated. In case a 
multiresolution scanner is available, a high resolution scan can 
be performed for every target. If the surveyed object presents 
highly reflecting points different from the target ones, a 
supervision is necessary, to avoid wrong detections. If the 
coordinates of the target vertices are known (i.e. through a 
topographic survey performed for the orientation), an automatic 
control on the distances among the targets can be executed. 
To find the vertices of the targets, a cross correlation between 
the points belonging to the conical zone and the theoretical 
cone surface can be executed. Thus, the following operations 
are performed. 
a) For each base, the interpolating plane, and its normal 
direction (approximately the cone axis direction) are obtained. 
The approximated position of the ring center is found. 
  
  
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part BS. Istanbul 2004 
b) A geometric filter is applied to the whole set of data: the 
points with coordinates differences respect to the center 
coordinates, less than a fixed value (depending on the target 
dimensions), are selected. In this way, a point cloud for each 
target is isolated (Figure 3). 
  
Figure 3. Point cloud of a target 
c) The equation of the cone is very simple in a coordinate 
system with the z-axis parallel to the cone axis, and the origin in 
the cone vertex. 
For this purpose, a rotation is performed (xy plane parallel to 
the base ring plane — z axis parallel to the cone axis). The point 
with maximum z value (approximately the vertex) is chosen; a 
translation is then performed: the point selected in this way lies 
in the new origin. 
d) The points of the base, (which distance from the origin is 
greater than the cone apothem) are excluded; the remaining 
points should belong to the conical zone of the target (Figure 
4). 
e) The equation of the conic target surface, in the above 
described local reference system, should be: 
x+y2-22 =0 (1) 
so a least square procedure is applied. 
The new coordinates of the points, x’, y’, Z' will be obtained by 
the equation 
[X'] 7? [R] IX] * [8X] (2) 
where: 
[X] is the vector of the new coordinates ; 
[R] = is the rotation matrix; 
[6X] = is the translation vector. 
The components of [9X] are the coordinates of the "very" 
vertex of the target in tlie local reference system (Figure 5). 
f) ^ reverse rototranslation allows to obtain the coordinates of 
the “very” vertex, referred to the laser scanner acquisition 
reference system.