T.O.F. LASER SCANNER FOR THE SURVEYING OF STATUES: A TEST ON A REAL CASE

The contribution regards the surveying of two statu es of famous contemporary sculptors that have been placed in the central zone of Cosenza, which has been transformed in an open air m useum. To realize a 3-D representation of the museu m, different methodologies have been used, based on classical su rveying (total stations and GNSS), image data and r ge data. The increasing performances of the new models of Ti me Of Flight (T.O.F.) laser scanners allow to build accurate models also for medium-size objects; on the other hand, the recent techniques of 3D modeling enable the processing of large amount of data and the effective removal of noises. Thus, if an extreme ac curacy is not required, one can think to use the T. O.F. laser scanner, also for the surveying of statues. For the acquisition of the su rfaces of the statues, two different types of laser scanning have been used: the Leica Scan StationC10, based on Time Of Flight, and the Mi nolta VIVID 300 triangulation scanner. In the pape r, the comparison between the results obtained by using the different techniq ues is described.


INTRODUCTION
In the last years, the request for digital documentation of works of art has increased exponentially.Multi-resolution 3D models are more and more used for the representation of complex sites.This is allowed thanks to the evolution of new sensors, along with the increasing computing power and the evolvement of techniques for the management of very large data sets.The most used methodologies are basically based on image data (Remondino and El-Hakim, 2006) and range data (Beraldin et al., 1997).Total Stations and GNSS receivers are still used for georeferencing and to obtain control points.Different methodologies are ever more integrated, to take advantage from each technique (El-Hakim et al., 2004;Remondino et al., 2009).For the image data, featureless approaches are currently used (Hammoudi and Dornaika, 2011), whilst for range data the classical ICP method is still used by the most diffused software (Besl and McKay, 1992;Chen and Medioni, 1992;Zhang, 1994).The influence of materials should be anyway taken into account (Godin et al., 2001).The use of T.O.F.laser scanners for the surveying of very large statues or of regular parts of large statues is not new.These surveyings are generally integrated by close range scans, obtained by triangulation instruments, for the irregular surfaces (Ressl, 2007).The surveying of a complex statue has been generally performed by using several scans, retro-reflecting targets and images.The most diffused software give automatically low precision models.The effort for obtaining more accurate results is generally very high (Vozikis et al., 2004).In the last years, the increasing performances of the new models of TOF laser scanners allow to build accurate models also for medium-size objects; on the other hand, the recent techniques of 3D modeling enable the processing of large amount of data and the effective removal of noises.Thus, if an extreme accuracy is not required, one can think to use the TOF laser scanner both for city modeling and for the surveying of statues, by avoiding the use of triangulation instruments.In the framework of the activities in the field of Cultural Heritage in Southern Italy, several monuments and works of art have been surveyed by the Geomatics Lab of the University of Calabria, Italy (Artese et al., 2003(Artese et al., , 2005(Artese et al., , 2006)).The most recent initiatives concern a series of statues of famous contemporary sculptors that have been placed in the central zone of Cosenza, Italy, in order to create an open air museum.In the following, the surveying of two statues, having different characteristics, is described; the surveying has been performed by using both T.O.F. and triangulation scanners.The comparison between the results obtained by using the different techniques is described.

The City
Sited in Calabria, southern Italy, Cosenza was known in antiquity as the Athens of Calabria, thanks to its cultural past: the Accademia Cosentina, e.g., was one of the very first academies to be founded in Europe (1511).The city is located at the confluence of two rivers, the Busento and the Crathis, and is surrounded by seven hills.Alaric, king of the Goths, died in 410 in Cosenza and his body was, according to legend, buried under the riverbed of the Busento.The stream was temporarily diverted from its course while the grave was dug wherein the Gothic chief and some of his most precious spoils were interred (Jordanes,552).An urban open-air museum is a peculiar site, where the works of art are surrounded by buildings; the size, shape and textures of buildings contributes to create the museum, which fusion of the works of art and of the surrounding architecture.

The
For this reason, the surveying cannot be limited to the works of art, but it must regard a portion of the city, the streets and the squares, the single buildings and finally the statues.

The Statues
Two statues are taken into account: the first one, the "Wolf Sila", which author is Mimmo Rotella, is a characterized by flat surfaces; the second one, "St.George and the Dragon" by Salvador Dalì, is characterized by the use of bronze and brass, which allowed to differentiate in a particular way St. George, the horse and the dragon.Consagra), are a donation of the patrons Carlo Bilotti and Enzo used for documentation, air museum is a peculiar site, where the works of art are surrounded by buildings; the size, shape and textures of buildings contributes to create the museum, which is the fusion of the works of art and of the surrounding architecture.
is reason, the surveying cannot be limited to the works of art, but it must regard a portion of the city, the streets and the squares, the single buildings and finally the statues.
Two statues are taken into account: the first one, the "Wolf of Sila", which author is Mimmo Rotella, is a marble object, characterized by flat surfaces; the second one, "St.George and the Dragon" by Salvador Dalì, is characterized by the use of bronze and brass, which allowed to differentiate in a particular .The Wolf of Sila The Wolf of Sila has been realized with green dimensions (lenght, width and height) are 147x38.5x120cm for the statue and 180x88x 17 cm for the basement.to the artist during one of his holidays in the mountains of Sila, at the request of the collector Enzo Bilotti.wooden prototype, shortly before his death, and decided that the specimen final product would be in green The animal, symbol of the city, is represented by the artist while howling.It's simple and linear in shape, completely lack the physical features of the beast, but the whole body seems to be a movement path made with engraved lines and sinuous curves.It is a work that shows not only artistic merit, but also socio-anthropological value, identification of cultural roots.The statuary group of St. George and the Dragon realized by using league of bronze and brass alloy dimensions of the group are 120x76x are 136x82x8 cm.St. George rides his horse and with a spear hits the dragon, caught between the paws of the horse in the background, behind the main scene, a small female figur saved.It is a sculpture that blends wonderfully the three subjects (the Holy One, his horse and the Dragon) in a plastic movement that refers to the Christian tradition of the life of St. George and the saint is depicted without a face, as if to r the dummies of de Chirico.The f immortality, because its function is to transport the immortals, the character who stabs him is a mysterious creature who takes possession of such immortality by defeating the fair.Looking at the composition it can be seen that the color of the bronze changes in the various plans to scale it up, as if the lower level lacked any glimmer of light that figure that destroys the dragon.

SURVEYING AND DATA P
In order to represent a large and complex site several models, having different Level Of Details must be The Wolf of Sila has been realized with green marble.The dimensions (lenght, width and height) are 147x38.5x120cm for the statue and 180x88x 17 cm for the basement.The idea came the artist during one of his holidays in the mountains of Sila, at the request of the collector Enzo Bilotti.Rotella created the wooden prototype, shortly before his death, and decided that the specimen final product would be in green marble.
symbol of the city, is represented by the artist while howling.It's simple and linear in shape, completely lack the physical features of the beast, but the whole body seems to be a movement path made with engraved lines and sinuous hat shows not only artistic merit, but also anthropological value, identification of cultural roots.
of St. George and the Dragon has been eague of bronze and brass alloy.The dimensions of the group are 120x76x148 cm, the base's ones St. George rides his horse and with a spear hits the dragon, of the horse in the background, behind the main scene, a small female figure is the princess who is blends wonderfully the three subjects (the Holy One, his horse and the Dragon) in a plastic movement that refers to the Christian tradition of the life of St. George and the saint is depicted without a face, as if to remind .The flying dragon is a symbol of immortality, because its function is to transport the immortals, the character who stabs him is a mysterious creature who takes possession of such immortality by defeating the fair.
St. George and the Dragon can be seen that the color of the bronze changes in the various plans to scale it up, as if the lower level lacked any glimmer of light that, instead, shines in the

SURVEYING AND DATA PROCESSING
In order to represent a large and complex site like the M.A.B., several models, having different Level Of Details must be created.For this aim different methodologies has been used, based on classical surveying (total stations and GNSS), image data (aerial and close range photogrammetry), range data (aerial LIDAR, Time of Flight based Terrestrial Laser Scanner, Triangulation Laser Scanner).The building facades, the paving and the whole open-air museum have been surveyed with a Time of Flight Laser Scanner, in order to optimize the time needed for the data acquisition and processing.To obtain the surfaces of the statues, shielded during the acquisitions, two different types of laser scanners have been used: the Leica Scan Station C10, based on time of flight, and the Minolta VIVID 300 triangulation scanner (see Table 1).For every statue, the 3D models have been obtained by using three scans of C10, while 60 scans have been necessary to obtain the model using the VIVID 300.The comparison is summarized in table 2. It is possible to observe that the performances of the triangulation scanner are better and almost independent of the surface, while the T.O.F.
shows increasing accuracies passing from the dark surface (drake wings) to the shiny surface (head of St. George).
The average of the deviations has negligible values.

The Wolf of Sila
For the statue The Wolf of Sila, the comparison of the results obtained by the T.O.F. and triangulation scanner has been performed for the zone of the head.Figure 13 shows the merge of 5 VIVID scans; in figure 14 we can observe the textured model positioned in the 3D city model.3. It has to be underlined that the statue has a peculiar form, composed by several plane surfaces and with sharp edges.The material is green marble: this implies some systematic errors due to apparent penetration of laser beam, above all for TOF scan.
Another remark regards the model obtained in semiautomatic way by using Geomagic TM .The edges are too much smoothed, so maximum negative deviation (-17.8 mm) is very different from the positive one ( 9.8 mm).
The triangulation scanner shows a somewhat similar behaviour.Also in this case the maximum deviations are distributed along the edges, but the smoothing performed by software is less influent.The presence of border effects causes higher deviation values respect to the other statues: a manual refinement of the model is requested.obtained by using a T.O.F. and a triangulation laser scanner has been carried out.The data processing has been performed by using commercial programs.The use of automated techniques allows to rapidly obtain models with just three T.O.F.scans.This rapidity is paid with greater deviations, above all if a priori known geometry (e.g.sharp edges) is not imposed.The obtained models are really effective for representation aims, above all when they are textured.The use of triangulation scanner, which implies many scans and long processing work, is still mandatory for documentation or restoration aims.
Figure 1.The 3D model of the M.A.B. Consagra), are a donation of the patrons Carlo Bilotti and Enzo Bilotti.The 3D model has been used for documentatio representation and animation.An urban open-air museum is a peculiar site, where the works of art are surrounded by buildings; the size, shape and textures of buildings contributes to create the museum, which fusion of the works of art and of the surrounding architecture.For this reason, the surveying cannot be limited to the works of art, but it must regard a portion of the city, the streets and the squares, the single buildings and finally the statues.

Figure 2 .
Figure 2. The Wolf of Sila

Figure 3 .
Figure 3.St. George and the Dragon

Figure 4 .
Figure 4.The positioning of the Vivid 300 before the shadowing of the statue The data processing has been performed by using commercial programs (Rapidform TM , Geomagic Studio 12 TM ).In order to compare the performances of T.O.F. and Triangulation scanners, the same operations have been made, and several nurbs interpolating the point clouds have been obtained.The comparison has been performed taking into account the statistics generated by the used programs.VIVID 300 C10 Sampled points 200 x 200 points/shot Up to 50,000 points/sec Accuracy 0.45 to 1 mm 4 mm Spacing between points da 0.95 a 1.91 mm < 1 mm Min / Max range 55 to 120 cm 0.1 to 300 m Min /Max Field of View 185 / 395 mm Max 360° Horizontal 270° Vertical Table 1.Vivid 300 Minolta and Leica C10 characteristics 4. TEST RESULTS 4.1 St. George and the Drake For the statuary group of St. George and the Dragon, the comparison of the results obtained by the T.O.F. and triangulation scanner has been performed for three different zones, showing different surface characteristics.Figure 5 shows

Figure 5 .
Figure 5.The point cloud of St. George and the Drake obtained with three scans of T.O.F.laser scanner

Figure 7 .
Figure 7.The deviations points-model (from the C10 scans) for the wing of the dragon

Figure 13 .
Figure 13.The merge of 5 VIVID 300 scans for the Wolf of Sila Deviations for the statuary group St. George and the Drake 5. CONCLUSIONS Two statues sited in the MAB open air museum of Cosenza, Italy, has been surveyed.The comparison between the results