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.
