CIP A 2003 XIX th International Symposium, 30 September - 04 October, 2003, Antalya, Turkey
■ Scanning artefacts - These are the result of
selections made during the scanning process. For
example, by choosing to use a higher point density
in the horizontal axis than in the vertical axis a
point cloud can wrongly emphasise vertical linear
features. A second artefact is the cropping of a
scan scene so that important parts of the
subject/scene are not visible. This may be a
particular problem with tall buildings. In worst
cases a viewer looking at only the point cloud
could misinterpret such cropping as actual
geometric features, Figure 4 shows an example of
the problem.
Figure 4 Example of cropping a scan scene
5. SUMMARY AND CONCLUSIONS
The definition of a specification for the collection of point
clouds using TLS is a major step in the adoption of laser
scanning as a standard survey technique for cultural heritage.
The use of a standard specification has advantages for both
client and contractor in addition to focusing the needs of a
particular technology.
The specification has been divided into three segments: data
capture, data use and data storage; however each segment is
clearly interrelated and has been considered in relation to
each other. Standard point densities have been defined in
addition to the required accuracy for point measurements.
Definitions for terms used in scanning surveys have also
been outlined.
A major theme of the discussion has been ensuring the
impartiality of the document from any one particular
scanning system. Three different systems, each representing
a particular type of laser scanner, were used. The use of
multiple systems ensured the workflows employed by each
were considered. A notable difference was seen between
systems that used a known station location and those that
relied on a resection calculation.
The specification defined as a result of this project does not
claim to be the definitive standard but does provide a basis
for future discussion and revision. The paper presented here
has provided an outline of the issues covered by the
specification, illustrated with examples of some of the
problems faced. As the technology continues to develop it is
anticipated that the specification will evolve. The results
reported in this paper represent the initial experiences of the
project. Readers are recommended to consult the final
addendum once the project has been completed. This will
report the requirements of the specification in full.
6. REFERENCES
ANSI, 2000. Safe Use of Lasers (Pub 106) American
National Standards Institute Z136.1.
Barber D. M., Mills, J. P. and Bryan, P. G., 2002.
Experiences of laser scanning for close range structural
recording. Proceedings, International Workshop on Scanning
for Cultural Heritage Recording, Corfu, Greece. 160 pages:
121-126.
Boehler, M. and Marbs, A., 2002. 3D scanning instruments.
Proceedings, International Workshop on Scanning for
Cultural Heritage Recording, Corfu, Greece. 160 pages: 9-
12.
Bryan, P. G. and Blake, B., 2000. Metric survey
specification for English Heritage. English Heritage,
Swindon. 111 pages.
BS, 2001. Safety of Laser Products Part 1 Equipment
classification. Edition 1.2. British Standard, BS EN 60825-1.
Clark, K., 2001. Informed conservation - understanding
historic buildings and their landscapes for conservation.
English Heritage. 123 pages.
English Heritage, 2003. An addendum to the metric survey
specification for English Heritage - Draft June 2003-
Terrestrial laser scanning. (Unpublished) 18 pages.
HADCOCK, R. N., 1991. Tynemouth Priory and Castle. Fourth
Edition, English Heritage, London. 32 pages.
Mills, J. P., Peirson, G. C., Newton, I. and Bryan, P. G.,
2000. Photogrammetric investigation into the suitability of
desktop image measurement software for architectural
recording. International Archives of Photogrammetry and
Remote Sensing, 33(B5): 525-532.
IEC, 2001. Safety of laser products - Part I: Equipment
classification, requirements and users guide. Edition 1.2.
International Standard, IEC 60825-1.
LAS Specification., 2002. LAS Format Definition.
http://www.ziimaging.com/lasformat/_notes/LAS_format.pdf
Last accessed June 2003.
7. ACKNOWLEDGMENTS
The authors wish to thank Clive Boardman, Bill Blake, Tom
Cromwell, Tony Davies Graham Hunter, Faraz Ravi and
Tony Rodgers, in addition to acknowledging the assistance of
Leica Geosystems, Z+F-UK and Riegl UK. The authors also
wish to thanks the project sponsors the English Heritage
Archaeology Commissions Team (Project No. 3378).
