Photogrammetric 3D-Documentation of the Cathedral of Aachen 
141 
AACHEN 
Monument Protec- 
in the world by the 
:rformed considering 
f the damage assess- 
: results by maps and 
nnplete and outdated 
very old. This made 
survey the complete 
ty of Technology in 
ver modern methods 
tion by using digital 
. With the object re- 
ligital images and 65 
photogrammetric as- 
itzenswerten Kultur 
en Erhaltungs- und 
alpflegerische Erhal- 
aßstäbliches Aufmaß 
' schematisierte bzw. 
vermessen und gräf 
lich. 
nbauleitung für eine 
Institut der RWTH 
ntsprechender CAD- 
durch die Deutsche 
otogrammetrisch In- 
5 CAD-Zeichnungen 
lt. 
Dn. It needs no direct 
tape and tachymeter. 
le last decade due to 
coming into practice 
gh resolution. Meas- 
administration of re- 
-al‘s construction su- 
the new digital pho- 
i preferably for close 
range applications, which has been developed at GIA [Benning et.al. 1997]. As a special property its comprehensive integration into 
the standard CAD-system MicroStation has to be mentioned. MicroStation is a well known application developed by Bentley Sys 
tems Corp. for editing threedimensional drawings, which is well suited for photogrammetric purposes. The integration of PHIDIAS 
into the CAD environment enables the operator to combine the single processes „measuring“ and „editing“. Both are part of the pho 
togrammetric evaluation work and should be executed mutually. Many object elements, that are often not determinable by common 
photogrammetric techniques, may be measured instead by constructive, CAD-based tools. This statement is one of the most impor 
tant experiences, which were found out during the long time surveying of the cathedral using PHIDIAS. 
The documentation of the cathedral has been divided into two steps, the surveying of the outer face and the surveying of the interior 
area. The measurement of the outer face started in 1993 and ended at the beginning of the year 1997. 
2. SURVEYING OF THE OUTER FACE 
At the beginning the photographs covering the outer surface of the cathedral were taken. The cathedral of Aachen extends over a size 
of approximately 60 x 50 x 60 metres. It essentially consists of the central octagon, the choir hall, the west tower and five chapels 
altogether. Due to this geometric expansion the use of a crane was necessary, see fig. 1, so that is was possible to make suitable ex 
posures of the roofs and the upper part of the cathedral. Image acquisition concerning the outside took up two days only, how the 
moving of the crane vehicle was the most time consuming part of the process. 
Figure 1: Recording of exterior photographs using a crane vehicle and a gondola; beside it one of the exposures 
As camera a NIKON F-601 was used. The camera is equipped with a réseau glass and it works with normal analogous film in small 
image format (24 x 36 mm 2 ). The majority of the exposures were made by a calibrated lens with a focus length of 28 mm. Only a 
few photographs were recorded with 50 mm, 20 mm and 15 mm lenses, respectively. At that time we decided to use black-and-white 
film, because at the beginning of the project coloured photographs have been arising as a problem considering data processing. All 
images have mandatorily to be digitized before transmission to PHIDIAS and coloured images require approx, three times more data 
than black-and-white images. During photogrammetric evaluation PHIDIAS uses up to 8 images at the same time and this has 
caused a problem in computer performance at that time. Nowadays, computer performance, even if large images with a high geomet 
ric and radiometric resolution are processed, is a subordinated problem. Therefore we would prefer coloured images, because these 
encourage the recognition of details. 
All photographs have been taken by free hand, which was predominantly done from the gondola of the crane vehicle (fig. 1). The 
gondola moved vertical up and down, so that similar to the systematics in aerial photogrammetry the cathedral was recorded in 
strips. All images have got a lap of 75 % on average and the photo scale amounts to between approx. 1 : 200 and 1 : 300. Special 
areas of the building required additional photos, thus every detail of the cathedral was covered in two images at least and the follow 
ing object reconstruction process using two convergent image rays was possible. The outside of the cathedral amounts to approx. 
17.000 qm. This required nearly 1200 photographs in all. Digitization of the photos was done by use of Kodak‘s PHOTO-CD tech 
nique with a resolution of 3000 x 2000 image points, which turned out to an economical method. After digitization the images were 
stored onto 20 CD-ROMs. 
ember 18 - 21, 2001