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The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B5. Beijing 2008 
• Production of true-orthophoto mosaic at a scale of 1:50 (pixel 
size 5mm) of the north, west, and east circuit wall façades. 
• Production of true-orthophoto mosaic at a scale of 1:25 (pixel 
size 2mm) of the wall areas where built-in significant 
architectural members exist (areas of special interest). 
• Production of true-orthophoto mosaic at a scale of 1:100 
(pixel size 10mm) of the top view of the hill. 
• Production of full 3D surface models (and photo-textured 
surface models) of the rock, the walls and Erechtheion derived 
from 3D laser scanning at a resolution of 1cm for the areas of 
special interest, 5 cm for the rest of the walls, including its 
interior façades and 0.5 cm (or even better where it is needed) 
for the Erechtheion. 
• Development of a GIS based on the above mentioned 
orthomosaics and the existing architectural plans, aiming at 
constituting a valuable documentation and restoration 
management tool. 
• Connection of the GIS with the upgraded documentation 
database and publication on the Internet. 
Major problems and difficulties 
Currently, the archaeological site of Acropolis (see Figure 1) is 
both an enormous worksite and the main tourist attraction of 
Athens. Several problems exist for this project, which can be 
summarized as follows: 
1. The outstanding importance of the site, which determines the 
high level of accuracy required and the platforms of image 
acquisition to be used. It is worth mentioning that all flights 
over Acropolis monument should be performed above 5000 feet 
for propeller aircraft or helicopter, prohibiting their use for the 
specifications of this project, and also that no UAV, or any 
other remotely-controlled platform is allowed. Thus, the only 
accepted platforms were balloons and/or cranes. 
2. The fact that most of the days are extremely windy on the hill 
can cause serious problems in handling of the balloon platform 
but also the tripods for laser scanning and geodetic 
measurements. 
3. The constant presence of hundreds of visitors. 
4. The large object size and the difficult morphology. 
5. The abrupt changes in relief and the monument height, 
compared to the short data acquisition distance. 
6. The obstructions caused by vegetation, scaffoldings, cranes, 
and scattered architectural members, pose severe problems to 
automated tie point measurement in aerial triangulation, 
matching for DSM generation, true orthoimaging and laser 
scanning. 
Therefore, it is evident that the supervision, the organizing, 
management and provision of efficient, functional and realistic 
solutions is a challenge for the researchers involved. 
More detailed investigations of this project are reported in 
Tsingas et al. (2008), El-Hakim et al. (2008) and Remondino et 
al. (2008). The Acropolis of Athens has been an object of many 
studies. Within the context of the previously mentioned detailed 
investigations, other related studies are mentioned in Blomerus 
and Lesk (2007), Debevec (2005), Lundgren (2004) and 
Stumpfel et al. (2003). 
2. DATA ACQUISITION AND PROCESSING 
The major included technologies involved were: 
- Digital imaging using different medium format digital 
cameras and different lenses and sensor platforms, incl. a 
balloon. 
- Geodetic instrumentation and GPS for the various geodetic 
tasks and establishing the various networks and measuring 
control and check points. 
- Various laser scanning technologies including Time-Of- 
Flight (TOF) and phase-shift with three laser scanners of 
varying specifications regarding minimum and maximum range, 
object size, point density and accuracy. 
The balloon and the associated platform with GPS, WEB 
camera, Bluetooth for wireless data transmission and camera are 
shown in Figure 2. The camera was a Mamiya ZD (22 MPixels). 
Two Canon 5D (12 MPixels) were used for texturing 
Erechtheion and its image-based modeling, while Mamiya was 
also used for imaging some selected sites of the Erechtheion for 
scientific purposes. A Canon 400D was used together with the 
terrestrial laser scanner Trimble GX for scanning of the walls 
and the rock. Two more scanners were used for the Erechtheion, 
the Surphaser 25HSX phase-shift based laser scanner and the 
Leica HDS 3000 TOF scanner. The latter was used to scan the 
higher parts of Erechtheion but also for scientific investigations, 
involving penetration of laser beams into marble by comparing 
these point clouds of these two laser scanners but also to digital 
surface models derived from digital images by matching (see 
El-Hakim et al. 2008; Remondino et al., 2008). 
The data processing has been performed both with commercial 
software packages and own developed software, the latter being 
critical for critical processing steps and higher performance. 
Figure 1. The hill of Acropolis (aerial photo). In the center the 
Parthenon, left the Propylaia, at the top Erechtheion, on the 
right the old Acropolis museum (Tsingas et al., 2008). 
Figure 2. The balloon system (left) and the base with GPS, WEB 
camera, Bluetooth and the Mamiya camera (right) (Tsingas et al., 
2008).