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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B5. Istanbul 2004
linear surface features like cracks is based on the analysis of the
images, whereas information on object geometry is provided
from the laser data. Additionally, areas, which are not accessible
in the laser scanner data due to occlusions are added based on
semiautomatic evaluation of the imagery. By these means, a
complete 3D features for the scene can be generated with
sufficient and clear details.
Within the paper the presented approaches are demonstrated ir
the framework of a project aiming at the generation of a 3D vir-
tual model of the Al-Khasneh, a well-known monument in
Petra, Jordan. In section 2 the collection and pre-processing of
the relevant image and LIDAR data is discussed. This pre-
processing is mainly required in order to coregister laser and
image data for further processing. Section 3 exemplarily pre-
sents our feature extraction approach using the hybrid system
for the left door of Al-Khasneh.
2. DATA COLLECTION AND PREPROCESSING
The collection of the data, which has been used for our
investigations, was performed in cooperation with the
Hashemite University of Jordan. One of the project goals is the
generation of a 3D documentation of the Al-Khasneh
monument in Petra city, Jordan, which is depicted in Figure |.
Figure 1. Al-Khasneh facade, Petra
2.1 Al-Khasneh Monument
The ancient Nabataean city of Petra has often been called the
eighth wonder of the ancient world. Petra city in southwestern
Jordan prospered as the capital of the Nabataean empire from
400 B.C. to A.D. 106. Petra's temples, tombs, theaters and other
buildings are scattered over 400 square miles, these
architectures are carved into rose-colored sandstone cliffs. After
a visitor enters Petra via Al-Siq, a two-kilometer impressive
crack in the mountain, the first facade to be seen is Al-Khasneh,
which is considered as the best-known monuments in Petra city.
The Al-Khasneh facade is 40m high and remarkably well
preserved, probably because the confined space in which it was
built has protected it from the effects of erosion. The name Al-
Khasneh, as the Arabs call it, means treasury or tax house for
passing camel caravans, while others have proposed that the AI-
Khasneh Monument was a tomb. Behind the impressive facade
of Al-Khasneh, large square rooms have been carved out of the
rock [Sedlaczek, 2000]. :
2.2 Sensors Applied
For point collection, the 3D laser scanning system GS100,
manufactured by Mensi S.A., France was applied. The scanner
features a field of view of 360° in the horizontal and 60° in the
vertical direction, enabling the collection of full panoramic
views. The distance measurement is realized by the time of
flight measurement principle based on a green laser at 532 nm.
The scanning range of the system allows distance measurements
between 2 and 100 meters. The scanner's spot size is 3 mm at a
distance of 50 meters; the standard deviation of the distance
measurement is 6 mm for a single shot. The system is able to
measure 5000 points per second. During data collection a
calibrated video snapshot of 768x576 pixel resolution is
additionally captured, which is automatically mapped to the
corresponding point measurements.
In addition to the laser data, digital images were captured for
photogrammetric processing using a Fuji S1 Pro camera, which
provides a resolution of 1536x2034 pixel with a focal length of
20 mm.
2.3 Measurement Configuration
Because it is not possible to have a complete 3D coverage for
the Al-Khasneh facade based on data collected from a single
station, three different viewpoints with five scans were done to
resolve the occlusions. The problem to choose the viewpoint
positions represents an important phase of the survey for such a
monument since potential sensor stations are restricted by the
mountainous environment surrounding Al-Khasneh. Three
positions were selected, from the entrance area of the
monument, from the left of the monument, and one scan was
collected from an elevated viewpoint. Since the vertical field of
view of the laser scanner from these positions could not cover
all the facade from one scan, the left and top scanning were
done using 2 scans from the same position, taking into
consideration sufficient overlapping regions to allow for a
subsequent integration. In total, the five scans resulted in almost
5 million collected points.
All the acquired 3D models have been processed using
Innovmetric Software, PolyWorks. The model of Al-Khasneh
facade resulted from merging the five scans in an independent
coordinate system into an absolute coordinate system. After
registration of the scans using corresponding points, the
software constructs a non-redundant surface representation,
where each part of the measured object is only described once.
The result of the combination of the five laser scans is given in
