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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B5. Istanbul 2004
makes the geometric reconstruction of the target centre easy
from the software which interpolates the light surface among
the points falling back on it (some hundreds) with accuracy of
some millimetres. At last the cross appearing at the target centre
allows to survey the centre by total station (figure 3).
Figure 3 - Target used for the laser taken and radiometric
answer of the points.
The coordinates of the Ground Control Points (GCP) have been
determined with uncertainty of 1.2 cm in the x coordinates, 1.6
cm in y, 1.1 em in z, according to the presumed accuracy of the
photogrammetric restitution and of the laser survey.
The surveys with laser scanner have been carried out with a
terrestrial laser Optech ILRIS -3D of the Codevintec srl of
Milan. This scanner can work up to 800-1000m in the best
conditions of the surface reflectivity. The field edge is 40? x
40° while the frequency of the points measure is maximum
2000 points/s. At a distance of 100 m the laser trace on the
object is about 30 mm (perpendicular shot) and the accuracy on
the flat surface is about 3-5 mm on the points.
For the Baptistery survey 14 scanning have been carried out: 13
of which at the square level have been done contemporary to the
photogrammetric survey. The last one, taken from the top of the
loggia of Torrazzo at about 100m on the square level, has been
taken after some months.
3. CREATION OF THE SOLID MODEL
In photogrammetry the creation of the solid model is made after
the external orientation of the images and from the consequent
fixing of some characteristic control points. Connecting these
points, we created the primitive meshes of the model. By now,
the photogrammetric algorithms allow, knowing the external
orientation of the images, to find out automatically both of the
characteristic points on the object surveyed (thought the
operators that pick the characteristic points basing on the
chromatic variation of the image) and the homological points on
the other images. For this purpose, in the close range
photogrammetry it's not allow to use the automatically airborne
triangulation programs because the geometry of the image is
much less standard than the airborne case. Anyway it’s already
exists some algorithms based on the epipolar geometry or on the
field of the parallaxes variation that, after calculating with good
approximation the orientation of the images (Sansó, 1973 or
Zeng at al, 1992) helps to calculate the coordinates of the
homologue points and, consequently to determinate the object
points and to improve the external orientation of each image.
In our case it has been used the software Photomodeler
(Www.photomodeler.com), a program with huge spread that
allow to calibrate the cameras, to calculate the external
orientation of the images. to define the coordinates the object
points, to reconstruct the surfaces and the images overlapped
(even if, in this case with some limitations).
It has been defined 507 points observed at least on 2 frames, 17
of that chosen between the GCP with known coordinates; these
allowed to define together the external orientation of the
Mages and the remaining points in a local Cartesian reference
System tied at the total station points. Moreover it has been
491
added other 1274 points close to the particular feature of the
structure: corners, edges, lines, windows and all the points
necessary for the construction of the solid model.
In the end, with AutoCad, it has been reconstructed the surfaces
and assigned at each one an arbitrary texture (figure 4). It has to
point out that the roof of the baptistery has been “invented”
because, as already mentioned, there are no photogrammetric
images about that.
Regarding the processing and the modelling of the laser scanner
data, every scan has got an own reference system relative of the
survey point, therefore the creation of the solid model is
dependent by the merging of all the scans made by different
positions using any one reference system. The software used for
this application and for processing the data is the module
IMAlign by PolyWorks (http://www.innovmetric.com); this
software allow to chose some natural point for linking up
different scans (such as some recognizable feature on the object
surveyed, house corners, identifiable geometry, different
reflectivity of the walls, etc).
ml
Figure 4 — Solid model of the Baptistery generated by the
photogrammetry images, upon that there has been impose some
arbitrary texture
All of this procedure permit to be easier the survey on the field
and at the same time to be faster for the absence of the artificial
targets, used only for the georeference of all the model. The
align of different scans in made individualizing three or more
connection points present on both of the two adjacent scans.
These points, who the position is known in their respective
reference systems, at the beginning, consent to move closer the
two scans; the software, that used ICP algorithm, realises
subsequently an automatic research of the “homologue” points,
performing a spatial rotation and translation without scale
variation respect to the reference system of the adjacent scan
and it estimates the 6 independent parameters: the 3 rotation
around the axis XYZ and the 3 spatial translation.
In this way with the alignment it has been given the parameters
estimation of the orientation and generated a roto-translation
matrix for each scan. This process has been applied on each
scan and permits the creation of the complete solid model of the
object surveyed.
At the end of this process we has been realised a triangulation
(TIN) to the single mesh and it has been made a georeference of
all the model to set it in the reference local system tied to the
points surveyed by the total station. This process has been done
using some artificial targets, above described, positioned on the
surface surveyed.
Once known the spatial position of at least one of this three
