Figure 6 The new ITC.
Analytical and digital photogrammetry can deliver 3D
topographic data. Although measuring by photogrammetry
is traditionally done in 3D, the transition from 2D to 3D
models considerably increases the effort needed for data
collection and data structuring. It also drastically expands
data volumes. Manual extraction of houses does not pose
fundamental problems, only practical ones. Until further
progress is made in semi-automatic extraction, strict
digitizing rules should be followed to minimize measuring
866
time and facilitate automatic 3D model construction,
especially in cases of complex buildings and when a high
level of detail is required. We have not yet accomplished
fully automatic model construction, since building of
topology is significantly more demanding in 3D than it is in
2D. Stereosuperimposition of 3D wireframes is useless if
there is no hidden line removal, since it becomes
indecipherable. It is also useless for interactive correction if
the digitizing software does not support 3D topology.
Conventional stereoplotting software can only keep the roof
polygon 'alive' when we move a corner point, but cannot
modify the walls accordingly. Attempting to change a corner
point of a footprint would be an even more insane
undertaking since the coding system only recognizes roofs,
thus even off-line rebuilding of topology is not possible.
Current DPs offer a higher level of computer support than
APs, but this does not concern feature extraction for
topographic mapping. Our comparative tests using the C120
and the T10 showed no significant differences in time
efficiency, which conforms with the results reported by
(Colomer et Colomina, 1994). This implies that the
additional costs of working all-digitally cannot be recovered
yet, unless a hybrid analytical-digital production line is set up
for 3D data collection or a significant productivity boost is
attained through automating feature extraction.
5. FURTHER DEVELOPMENT
Digital photogrammetry offers the prospect of automated
information and texture extraction from photographs; in
both it is distinct from analytical photogrammetry. Semi-
automatic, local image matching on digital plotters seems
feasible on short term, thus speeding up reconstruction of
certain objects such as roads and buildings. Once the
geometry of the topographic objects is known, the
candidate areas for texture mapping can automatically be
extracted form the digital images.
Apart from (semi-)Jautomating object recognition and feature
extraction, system enhancements are also needed,
especially for efficient quality control and updating
operations. Such desires require very powerful graphic
subsystems with large image memory for effective viewing
of object reconstructions. Because of the possibility of
occlusions in built up areas and the countless number of
possible object shapes and compositions, only a direct
visual comparison of object reconstruction with the original
images can give a final validation. Efficient verification would
be possible if an object could be viewed--almost
instantaneously--in more than one stereomodel. Adequate
3D viewing must be combined with interactive digitizing
software that supports 3D topology. With fully automatic
extraction of topographic information from aerial
photographs still miles away, extended image access and
computer graphics (integrating digital photogrammetry and
CAD) will increase productivity of 3D vector data acquisition
and thus make a broad introduction of 3D-GIS more
feasible.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
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