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much the same way that image based techniques are presently
combined with hand recording. Integration will take the form
of combined results, where products are augmented with
different techniques, or in the integration of methodologies to
produce new and improved end products.
5.2. Integration of products
The basic integration of imagery with scan data includes the
draping of scan data with imagery to improve interpretation and
detail in complex areas. Sequeira et al., (1999) and Bemardini
et al., (2001), are two examples that have used images to
augment 3D geometry captured using laser scanners. Much of
this work is aimed towards the visualisation, rather than the
metric survey of objects and structures.
5.3. Integration of flow lines
A more complicated merging of workflows can be envisaged
through the integration of observations to produce better
estimates for object coordinates or to produce quality control
information on the data captured by providing redundancy in
observations. This would have to deal with occlusions in scan
data (real occlusions caused by objects and false occlusions
caused by lack of data) and the problems of matching
corresponding high resolution features in scan and image data.
Photogrammetric networks typically require high density target
networks and automated photogrammetric surface measurement
often requires manual editing, especially across surface
discontinuities. Laser scanning on the other hand deals well
with surface discontinuities and has a much lower requirement
on the density of target networks, but it does not provide as
much information on surface texture or data as appropriate for
use as an archive data source as image based methods. It would
seem, therefore, that a complementary use of the techniques
would provide a more efficient, better value product. Future
survey flow-lines will use each technique to its strength - using
laser scanning for object models and augmenting the main
object models with stereophotography, especially in areas of
high detail or areas at particularly risk.
6. SUMMARY AND CONCLUSION
This paper has identified the reasons and the methods for
cultural-heritage recording and detailed the use of flow-lines to
provide survey data of good value and high quality. In
particular it has focused on how laser scanning could be used
for cultural-heritage recording based on the experience gained
during the survey of two typical heritage subjects using laser
scanning and photogrammetry.
The paper noted that Laser scanning was able to capture data in
areas where traditional photogrammetric techniques could not,
such as in areas of shadow, and create profiles comparable to
those produced by photogrammetry. It highlighted the reduced
density of control points required for laser scanning work in
comparison to photogrammetry and showed how use of a 3D
meshed model with directed lighting can highlight different
features to a standard photographic image.
It is acknowledged that image based survey techniques have an
important role in cultural-heritage recording, providing a
recognised archive product in their own right and established
final products, however, survey workflows could be adapted to
include laser scanning as a complementary technique aiming to
improve the overall value and efficiency of survey work. The
next stage in the acceptance of laser scanning for use in
cultural-heritage recording applications would be the
introduction of specifications to govern the use of laser
scanning and define useful final deliverables for the end user.
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8. ACKNOWLEDGEMENTS
The authors would like to acknowledge 3D Laser mapping Ltd.
of Nottingham, UK; Riegl laser measurement systems GmbH,
Austria; Cyra Technologies Inc; LH systems LLC and the
Engineering and Physical Sciences Research Council for their
support during this research programme.
