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t h e b u i l d i n g c o n s e r v a t i o n d i r e c t o r y 2 0 1 2
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Professional services
Recording historic fabric
The use of laser scanning for recording and
archiving is now well established and English
Heritage (EH) has produced good guidelines
for the professional (see Recommended
Reading). Concern has been expressed in
the past over such methods because of the
stability of the electronic archive, but this
issue has been largely addressed by the
National Monuments Record. EH has recently
commissioned a comprehensive laser scan of
Stonehenge and work at Ironbridge is due to
commence shortly.
The choice of format in which the
archived data is kept remains an issue because
different manufacturers have different
formats. The common choice remains
the rather inefficient ASCII format which
generates very large files for storage. The
ASTM E57 format currently in development
may provide an effective alternative.
Monitoring movement
Scanning provides a highly accurate
contoured surface of buildings and structures.
By repeating a scan a few months or perhaps
a year later and overlaying one scan over
the other, three dimensional movement in
the surfaces can be detected. This provides
a powerful tool for monitoring structural
behaviour, given that it can be achieved
without physical access to the walls. Contours
(or ‘isopachytes’) can be produced using a
program such as Geomagic™ that shows the
difference in movement.
This technique can be used at the sub-
millimetre level to record the decay in surfaces
such as brickwork or even decay in objects,
using a more sensitive group of laser scanners
that operate on the principle of triangulation,
sited perhaps one metre from the object. A
tolerance of 0.5mm is currently achievable.
Recommended Reading
D Andrew (ed), Metric Survey Specifications
for Cultural Heritage, English Heritage,
Swindon, 2009
D Jones (ed), 3D Laser Scanning for Heritage,
English Heritage, Swindon, 2007
RICS, An Introduction to Terrestrial Laser
Scanning, London, 2006
Acknowledgements
The author would like to thank colleague
Daniel Niziolek, Andrew Dodson of Greenhatch
and Paul Bryan of English Heritage for their
assistance in the preparation of this article.
James Miller
MA CEng FICE FIStructE
Conservation Accredited Engineer is a
conservation engineer with 30 years’
experience in consultancy. His projects
include work at Westminster Hall, Wells
Cathedral and Chiswick House Gardens. He
is technical director of Historic Structures at
Ramboll (see page 54).
Figure 6 The final product: a working drawing showing a cross section of an archway through Chester city walls. Accurate, detailed drawings like this can be used for a wide variety of practical
applications, from preparing specifications to monitoring and recording. (Image: Russell Geomatics/Donald Insall)
is effectively 3D drafting. When existing
building fabric is to be repaired or modified,
laser scanning provides the key by which
survey data is pivoted into the model. By
surface modelling from the point cloud, a
historic building can reappear in reduced,
filtered, and rendered form in the 3D
drafting model, yielding very considerable
benefits in terms of fit and visualisation.
This process requires experience and it
is best to start simply. A number of survey
companies will build a surface model from
the point cloud, in the same way that they
produce 2D drawings. The tolerances on a 3D
surface model need to be carefully defined if
the model is to be reliable and the deviations
should be clearly understood (for example,
permitting a 5mm or 10mm maximum
deviation of the surface from the cloud).
The modelling process is much more
expensive than creating 2D drawings. It may
take a number of weeks to produce a model
of a complex building or a structure like
the Mary Rose (see Figure 5). Even so, some
spaces are likely to escape survey and so
cannot have surfaces fitted to them.
Drafting software such as Revit™ can
now accept point clouds directly into the
3D model. The manipulation of the cloud
by drafting platforms looks set to develop
rapidly over the next few years, as it becomes
easier to use scanned survey data for existing
and historic buildings on office PCs.