used to ensure reliability. 
Post-processing includes check, add, transfer, delete and 
refinement of tie points and semi-automatic measurement of 
control points. All of the image windows (two to eight) related to 
a tie point will displayed on screen after the tie point is picked by 
the mouse on any image. These can be selected randomly and are 
displayed on the screen. All of the interactions can be semi- 
automatic or manual, similar to the relative orientation. Control 
points can be automatically transferred to the relative images after 
having been identified on the digital image. Refinement is the 
same as for tie points. 
2.5 Relative Rectification 
Once the relative orientation is complete, an automatic process 
resamples the left and right portions of the stereo model to 
produce left and right epipolar images. In essence, y-parallax is 
removed from the original images, so that the subsequent image 
matching routine can be implemented in one dimension. Also, the 
visual stereo model can be displayed for checking. A fast epipolar 
line rearrangement method has been developed for use with aerial 
images, close range images and SPOT images. The epipolar line 
parameters are computed on the basis of the coplanar condition. 
2.6 Image Matching (Automatic Measurement) 
The matching algorithm within VirtuoZo is an area, feature and 
bridge mode based global image matching procedure using 
probability relaxation and neural network techniques. To ensure 
the reliability of the matching results, as well as fast processing, 
an image pyramid is used in the image matching procedure. The 
image pyramid is dynamically generated according to the rule of 
minimum workload. At the top of the image pyramid (with lowest 
resolution), a coarse grid is established in the left image and 
matched to the right image. Matching results on higher levels are 
approximations of a finer grid on subsequent levels with higher 
resolution. The geometry distortion caused by ground slope is 
rectified approximately by a bridge mode method. The 
compatibility coefficients used in the relaxation matching consist 
of matching quality and geometry condition restraints. The 
reliability factor for each conjugate pair describes the matching 
reliability and helps the operator to edit unreliable results. 
2.7 Editing of the Match Results 
The editing of the match results is a computer aided procedure to 
correct errors resulting from the image match. The major portion 
of the screen (centre-right) is a stereo window of the image pair, 
which can be zoomed in and out, while the entire epipolar image 
pair is reduced and displayed in the lower-left for a global stereo 
view. The buttons initiating the editing functions are arranged in 
the upper-left portion of the screen, and a popped menu includes 
other editing tools. Any single point, profile, polyline, rectangle 
or polygon selected by the user can be edited with measurement, 
smoothing, interpolation, surface fitting and rematching under the 
guidance of the operator. The stereo visualisation includes match 
iso-lines (effectively parallax contours), true contours, “pegs” 
(match points colour coded to confidence levels ) and profiles. At 
any time the user can undo modifications iteratively, back to the 
start of the session. 
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2.8 DTM Creation 
The DTM created by VirtuoZo is a regular grid. The grid interval 
and direction of the DTM can be input by the user or computed by 
the VirtuoZo software. Before interpolating the DTM, a random 
retrieval table of reference points is established for the fast 
generation of the DTM, and a moving curved surface fitting 
technique with medium value filtering is used to create the DTM. 
The features extracted from editing and digitising are applied in 
the procedure and break-lines can be processed. 
2.9 Orthoimage Creation 
The orthoimage is created based on the DTM and the 
corresponding original image. The user nominates parameters as 
to whether the left or the right image is to be orthorectified, what 
is to be the scale of the output image and what is the resolution of 
the plotting device (if any). According to the theory of digital 
orthorectification, the coordinates of the point projected on the 
original image are computed with collinear equations based on its 
ground coordinates. Height is interpolated from the DTM. To 
reduce the processing time, suitable algorithms are used, so that 
the orthoimage can be acquired quickly. 
2.10 Contour Creation 
The parameters for contour creation and their annotation can 
default to be system generated or, more realistically, may be 
nominated by the user. Contour vector data are produced from 
tracing the intersections of contours on the DEM grid. A contour 
image is created by rasterising the contour vector data, and a cubic 
spline function is used to smooth the raster contours. If required, 
the vector data can be converted to a number of proprietary vector 
formats. DXF, MOSS and Civilcad are some that are currently 
supported. 
2.11 3-D Digitising of Objects 
The digitising of objects has the same functionality as that within 
the analytical stereo plotter. However, in VirtuoZo, stereo 
measurement is based on the digital epipolar image pair instead of 
the photographic pair, and the height of the digital float mark is 
automatically controlled by the results of the image match, or by 
the operator, manually. An attribute code is input before 
measuring objects for eventual input to a database or GIS or 
drawing to a map through some vector plotter at a later time. All 
of the digitising operations can be accomplished in photo space or 
ground space. Semi-automatic methods are available for some 
kinds of objects, such as buildings, roads and rivers. The editing 
functions are suitable for correcting measurement errors. A 
symbol library can be conveniently created and edited. The 
graphics, symbolised from the vector data of objects, can be 
superimposed on the stereo model or displayed as a map sheet. In 
essence, the digitising module replaces the analytical plotter, 
without the optical-mechanical units. 
2.12 Mosaic 
It is difficult to seamlessly mosaic orthoimages in analog and 
analytical photogrammetric instruments. This is a routine 
operation within the VirtuoZo Digital Photogrammetric System. 
After the stereo models, namely the DTMs, have been created and 
the areas of interest have been nominated by the user in a graphic 
window in which the DTM area of all the relevant models are 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B2. Vienna 1996 
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