mmi 
Figure 3. Interface for relative and absolute orientation 
2.3.3 Parallax calculation 
The parallax calculation in the stereopair is done fully 
automatic by image matching. The Least Squares Matching 
algorithm (LSM) is based on the minimazation of the squared 
differences of the grey values of the image patches [Schenk, 
1996]. The image patches consist of 9 by 9 pixels or larger. If 
for example a large part of a wall is taken by a window or other 
cavities, larger image patches gave a better result. 
2.3.4 DEM and orthophotogeneration 
With the parameters of the exterior orientation and the results 
from the matching algorithm, the Digital Elevation Model and 
the orthophoto can be created. This happens automatically, and 
requires no action from the user other than giving the DEM 
spacing (10cm) and the orthophotoresolution (1cm). 
2.4 Orthophotoplan 
photogrammetric reference system 
The X, Y and Z axis show the local reference system, while the 
X’, Y’ and Z’ show the reference system for the stereopair of 
one particular object. 
In the final stage the orthophotoplan is created. This is the 
result of putting one or more orthophotos together in a layout 
with some additional information which is useful and needed 
for an efficient and reliable interpretation of the result. Unlike a 
map or a line drawing, the orthophotoplan gives an non- 
generalised image of the object. Two scales are used : 1/25 and 
1/50. 
2.3 Stereoprocessing of the photographs 
2.3.1 Scanning the photographs 
All of the 130 photographs were scanned at a 21 micron 
resolution. A 6 cm by 6 cm diapositive scanned at 1200 dots per 
inch (dpi) results in a 24 Mb file in tiff format. The stereopairs 
were processed without compression in VirtuoZo 3.1. This is a 
digital photogrammetric software pack. 
The following items are displayed on the plan : 
• Orthophoto 
• Scale : graphical and numerical 
• Ground plan showing what part of the site is on the 
orthophoto 
• The number of stereopairs the orthophoto is made of 
2.3.2 Orientation 
The interior orientation was possible because the calibration 
certificate of the camera and the lense was available. The cursor 
must be put manually on the fiducial marks. 
In the relative orientation the PC finds between 60 and 200 
tiepoints in a fully automatic way. After the manual absolute 
orientation, an area is defined for which epipolar images are 
resampled and the stereomodel is built up for.Once the 
orientation is done, all parameters of the exterior orientation are 
known and automatically calculated. 
An important issue is that it is possible to mosaic several 
orthophotos of one wall to one consecutive orthophotplan, 
without a visible edge between two orthophotos, other than 
differences in color or illumination.