EuroSDR Projects - Host Ireland

5 Discussion and Conclusions
It can be concluded that, presently, photogrammetric techniques and hybrid techniques provide the
highest accuracy and level of detail in 3D city reconstruction. Despite the high amount of research
during the last decade the level of automation is still relatively low.
Improvement in automation can be achieved most significantly by utilising the synergy of laser and
photogrammetry. The photogrammetric techniques are powerful for visual interpretation of the area,
measurement of the building outlines and of small details (e.g. chimneys), whereas laser scanning
gives height, roof planes and ridge information at its best. When the advantages of both methods are
implemented in a single system well, high accuracy and a relatively high level of automation can be
achieved. In this test, there was only one hybrid system.
The quality of the laser scanning data has a significant impact on the accuracy of the fully laser based
models. Since pulse density is increasing substantially (even though the novel European Toposys
system already had an 83 kHz system in 1997, the PRF of ALS produced by today’s leading
manufacturer Optech has almost followed Moore’s Law. In 1995, the PRF was just a few kHz, while
today it is 100 kHz) and there are new possibilities to use pulse information (multiple pulse,
waveforms), it is expected that the accuracy of the laser based techniques will follow this technical
development. The rapid rate of implementation development of laser scanning data can also be seen
from the improvement of FOI results during the test. On the other hand, when the study was initiated,
the implementation level of the majority of laser-based techniques was low. It can be seen from Table
3-3 (time spent), that thanks to the good implementation of photogrammetric methods, the time spent
on these models is not much higher that those of laser scanning.
We see, based on the results and development of the last ten years, that
- There is a need to integrate laser scanning and photogrammetric techniques in a more
advanced way. The accuracy of single height measurements with the laser should be used in
the photogrammetric process to increase the automation. There are currently some
developments in this field.
- The quality improvement of digital camera data will increase the quality of photogrammetric
techniques and it will also allow higher automation by using remote sensing methods in the
image processing.
- The rapid technological development in laser scanning will allow higher accuracies to be
obtained by techniques relying only on laser scanning.
- Laser scanning allows higher automation needed in the building extraction process.
- There is a huge need to implement laser-based techniques into practical working processes.
- New algorithmic innovations will emerge in all areas, thanks to digital camera data and laser
scanning developments that will increase automation.
In order to be able to separate methodological development from development due to improvement of
the data, it is recommended to use the same test sites, with old and new aerial data, for future
verifications.
In practise, the availability of the material (aerial image or laser point clouds) determines significantly
what kind of techniques can be used. The costs of laser scanning are significantly higher, although in
many cities the material has already been collected. In many cities, building outline information exists
as well as some kind of existing 3D models. In future, the challenge is to find the answers to: how
existing models are to be updated, how changes are verified and how existing information can be used
optimally in a more automatic process. In the project, first pulse data was used for the extraction. It is
expected that the use of last pulse and combined use of first and last pulse would lead to
improvements.
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