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Fig. 3 Megalith scan assembled from three view points 
inch 100Hz scan monitor run MENSI’s proprietary 3DIPSOS 
software which is used primarily for laser scanning restitution. 
Two other Compaq workstations, also with 1.6Ghz processor 
and 1 Gb RAM but equipped with MATROX graphics cards 
driving two Sony 24 inch monitors apiece run the Microstation 
and DIAP family of Products from ISM which permits all the 
digital photogrammetry post-processing required by 3D1D. 
3 The Development of the hybrid system: mar 
rying laser scanning and photogrammetry to 
overcome the defects of either system 
The hybrid system has come about through our experiences 
with the 3D imaging systems described above: Table 1 summa 
rizes the strengths and weaknesses of these 2 systems which we 
gathered from 3DID’s experiences with the three systems. This 
section will further expand on this table with the aid of illustra 
tions from projects carried out by3DID. We have taken as our 
primary case study, a sample survey of 3 apses and part of the 
façade from Ggantija, a megalithic temple structure in Gozo. 
Ggantija in fact provided the data both for laser scanning and 
digital photogrammetry that was successfully integrated in 
March 2002. (Stage 6 above) . 
Ggantija is a large temple complex at the ridge of one of the 
largest plateaus on the island of Gozo. As we can see in figure 
a, it consists of 10 apses. It is made up of great slabs of Cor- 
raline stone which span several feet in length and height. For a 
sample survey using by laser scanning, our concern was to es 
tablish a suitable grid to achieve a good representation of the 
surface of the stone, thus establishing the level of data the scan 
ner could provide for monitoring the erosion of the stone. Sec 
ondly, we needed to establish whether the data provided by the 
scanner would be sufficiently precise to monitor the movement 
of the stone, thus acting as a reference if any stones were to 
collapse. 
3.1 Speed & detail obtained by laser scan 
Scans of Ggantija were taken from different viewpoints. The 
temple proved to be relatively difficult to scan as it has many 
cavities that are not easily accessible to a laser scanner. Entry to 
many parts was only possible through additional scans and 
some openings. On the other hand, the level of detail provided 
by the SOISIC LG proved to be highly impressive. Figure 3 is 
a megalith from the temple that was scanned. The different 
colours represent the three different viewpoints from which it 
was scanned and highlight how a surface is mosaiced together 
so as to create the whole visible form.. It is planned that the 
entire temple structure would later be scanned from different 
viewpoints and consolidated later using 3D Ipsos and/or Diap 
software. 
Total number of points collected in this first stage were: 
2,496,832 over two nights, with an average grid of 2mm at an 
average distance of 3.0 metres. 10 
In terms of 3D data collection, the laser scanner is impressive in 
the amount of surface data which is collected in such a short 
period of time. This means that the human hours involved are a 
fraction of what it would take any other system in processing 
collected data in order to display a 3D surface at such accuracy. 
Figure 4 below demonstrates the level of detail obtained. 
10 There were areas where the grid was reduced to 0.3 mm thanks to 
the overlap between one scan and th eother