R THE 
hole complex of 
he renovation a 
ented it - at the 
| of the fortress. 
her maintenance 
enovation of the 
To record and 
ompare it to the 
ic recording was 
bove downtown 
) meters. 
in the sense of 
for scientific 
ation in tourism 
  
The Institute of Geodesy at University of Innsbruck, Austria 
used some of the existing data and complemented it for 
scientific interest with own measurements to extend the task 
to 
- combine different kind of basic data (metric analogue 
and non-metric digital photos) 
: combine and compare conventional restitution with 
analytical plotter and digital restitution with low-cost 
software 
= extend the task to create a complete 3D-model of the 
fortress 
show the power of digital products to the public. 
Beside the primary task to make a documentation of the as is 
status of the buildings a second goal had to be taken into 
account. The fortress is planned to be used as an arena for 
cultural events such as operas, musicals and other 
performances where the model can be used to plan the 
seating for the audience and the lights and scenery for the 
stage. 
2. DATA SCOURCES 
On the one hand a private surveying office in Austria - well 
experienced in the task of documentation of cultural heritage 
- has overtaken the job of fagade documentation by means of 
analytical plotter leading to conventional facade plans. The 
photos have been taken using a helicopter (see figure 2) and a 
metric camera Zeiss UMK 10/13/18 to record the outer part 
of the monument and the rocky hill on which it is based. 
  
Fig.2: Zeiss UMK mounted in helicopter 
—831- 
About 180 metric images have been take from this helicopter 
platform with an average overlap of 80%. 
On the other hand a modern digital camera Olympus E 10 
with a resolution of over 4 Million Pixel was calibrated and 
used to record the interior and hidden parts. In addition to the 
regular lens, which is build in the camera, a wide angle 
adapter was used to achieve an all together focal length of 
7mm (equals 28 mm with a small format camera). 
This was necessary because from the helicopter it was not 
possible to reach the interior parts as yards and paths within 
the fortress (see figure 3) 
  
Fig 3: digital photo of interior yard 
Over 350 digital photos have been used to set up the 3D 
model of the medieval fortress and to get good overlap even 
in the “problematic parts” of the medieval fortress. 
3. WORKFLOW OF THE RECONSTRUCTION 
The process of object reconstruction for a virtual computer 
model happens in 3 individual steps (for a detailed 
description see Hanke, Ebrahim, 1999). 
In the first step the camera's inner geometry, its position and 
orientation during exposure as well as the objects’ 
characteristic wireframe and faces have to be reconstructed. 
To get a consistent solution of the entire object these 
elements are computed during a photogrammetric bundle 
adjustment where a probable and homogenous result of the 
whole measurement system is guaranteed. There is a large 
number of photogrammetric systems available for this task, 
ranging from very advanced to rather low-cost hard- and 
software. 
In the next step within a common CAD environment the 
object's wireframe is reviewed and if necessary completed. 
There also additional measurements (tape, theodolit, etc.) can 
be introduced. After that the 3D model will be closed 
defining faces between the structure lines. Subsequent it will 
be investigated for leakage performing a first rendering 
process. The surface model that arises during that step will be 
used as a kind of "projection screen" and may have very 
different level of details. 
The third step is the actual retro projection of the photos (also 
see Hanke, Ebrahim, 1996). Similar to a slide projector 
selected photos are projected to the surface model using 
raytracing techniques. The selection of photos is based on 
visibility and projection direction. The "Digital Projectors"