used by Plowman Craven & Associates is a Cyrax model 2500, developed by US
company Cyra Technologies; now part of Leica Geosystems.
setting up a small machine on a tripod in the middle of a room. You switch it
on. It hums and buzzes for a few minutes as it scans and rotates. At the end of
a few minutes activity it has captured enough data to produce accurately dimensioned
drawings of all the room’s features – window reveals, mullions, transoms and soffits,
intricate cornices, perhaps the balustrade of a minstrel’s gallery or even the
complex mouldings of Grecian columns. Welcome to laser scanning.
the data the laser scanner has captured during its brief period of activity, not
only sections and plans can be generated, but also a three-dimensional model for
use within a CAD system or perhaps for incorporating in a virtual reality scene
or other computer modelling system. The potential is vast and the results are
of these advanced machines is already being used in the UK by Plowman Craven &
Associates as an alternative to conventional survey techniques. To date the survey
company has applied the scanner to a range of projects, which include one of London’s
historic bridges over the River Thames; an art deco department store in the West
End; a barrel-vaulted ceiling to a railway terminus; and the complex iron and
steelwork of a Tilbury fort in the Thames estuary.
PROFESSIONAL SKILLS NEEDED
well as the substantial investment in cash and learning time required, the effective
use of a laser scanner also needs the skills of a professional surveyor, initially
to evaluate its suitability for a particular project, but also to process the
data. These machines capture vast amounts of data – a point cloud – very quickly,
and it takes a surveyor with knowledge of building construction as well as laser
scanning to know what can be discarded and what data is necessary for the final
drawing or digital CAD model.
how can a laser scanner help in building conservation? Building renovation and
conservation projects invariably need accurate plans and drawings of the existing
layouts and features before the specialists can begin their work. But historic
buildings are notoriously poorly documented and accurately dimensioned drawings
either don’t exist or have been lost with the passage of time. Even structures
built in the 1960s and 70s often lack any detailed record of how they were built
and in some cases are beginning to pose a hazard for those who have to alter or
surveyors have been called in to produce accurate, as-built, drawings for buildings
using the tools they’ve always used: steel tapes, theodolites and, for the intricate
and otherwise untouchable or unreachable features, photogrammetry – a painstaking
process of measuring accurate dimensions of complex shapes from photographs. Some
of this work has become easier in recent years with the arrival of reflectorless
distance measurers. These small EDM units, often incorporated within the surveyor’s
total station, can accurately measure distances of up to 200 metres without the
need for reflective targets. They usually incorporate a visible red dot laser
so that the user can see exactly what point is being measured to. The distances
captured, when fed into suitable software, can help the surveyor slowly build
up an accurate 3D model of both the exterior and interior of a building’s features.
But this technique can be time consuming and expensive for clients.
technology has now evolved a stage further. Instead of a point-and-press EDM collecting
measurements one at a time, a laser scanner automatically and rapidly captures
a vast swath of points, either horizontally or vertically, to build up a 3D image.
The scanning process used by the Cyrax machine is controlled by a laptop computer
which also ensures that each scan taken overlaps sufficiently to create a complete
and accurate picture of the scene.
scanner is capable of capturing detailed data externally such as this oriel
as well as larger scenes, such as the Privy Gardens at Hampton Court.
laser scanner works on similar principles to the EDM but much more rapidly. It
also has the advantage of being able to capture all of its data from ground level;
no scaffolding, hoist or crane is necessary as the scanner can do its work at
ranges of up to 200 metres from the surfaces being measured. The accurate measurement
of the scanner’s laser pulse (which is emitted at a rate of up to 1,000 per second)
is critical and can be a major source of errors in the hands of an inexperienced
operator. Experienced surveyors can then carefully analyse and process the point
cloud data so that the most accurate measurements possible are captured for drawings,
as well as ensuring that digital files are not overloaded with surplus data. The
density of the point cloud captured will depend on the range of the scanner from
the surfaces being measured: the closer the range the denser the point cloud.
SOFTWARE AND MODELLING
quality and accuracy of data collected will depend on processing software, the
skill of the operator and the surfaces being measured to. White masonry and other
lightly coloured surfaces produce better results/are easier to measure to than
darker ones, with black rubber being the most difficult surface to work with.
The software is a key component of the system. It is important to appreciate that
much of the point cloud data captured will be superfluous and therefore the software’s
ability to recognise distinctive features such as lines and curves is essential.
If it doesn’t do this, unnecessarily large amounts of data will be processed,
slowing the computer system and producing cumbersome files which are slow to load
or copy. The processing stage, which is carried out back at the office, is often
easier if a video camera has been incorporated with the scanner to provide the
operator with an accurate visual record of the features which have been scanned.
The video record also provides a useful archive source for building owners.
scanning systems first became available in the US in the mid 1990s and quickly
found use amongst oil refinery and chemical plant operators who had seen their
plants evolve and change over time but without accurate as-built drawings being
kept. Within such plants the cost of shut down while changes or renovations are
carried out can be enormous and therefore owners want to limit downtime to the
absolute minimum. The use of a laser scanner to create an accurate as-built record
of an area perhaps hazardous for humans to access has been of great advantage
Complex Mesh: At close range, complex shape
application of laser scanning to the survey and recording of buildings has been
pioneered by Plowman Craven & Associates in the UK. The company is one of a small
number of larger survey companies which have evolved into managers and processors
of data by using a range of software and modern system-based technologies such
as laser scanners. The term ‘geomatics’ is increasingly applied to these activities
and the Royal Institution of Chartered Surveyors, which now operates through 16
faculties where each member can choose to belong to up to four faculties, has
a dedicated geomatics faculty.
arrival of laser scanners for building surveys heralds a new era – not just for
surveyors but also for building owners and managers who need accurate plans and
drawings quickly, without the inconvenience of providing special access equipment
or of fitting targets and other measurement markers which may damage surfaces.
Laser scanning also offers the potential to move into 3D modelling with the confidence
that the data captured is an accurate representation of every detail of a building
or structure, however complex or intricate the detailing. There is little doubt
that this exciting new technology will be used increasingly as an alternative
to existing methods.
Engineering Surveying Showcase:
Scanner provides As-Built 3D data for Refinery Upgrade',
- 'Scanner opens new doors', April 2001
World (formerly Surveying World ):
- 'Redefining the three R’s - Reflectance,
Resolution and Reference: Important considerations for Laser Mapping Systems',
- 'Close-range laser scanning case studies', November/December
electromagnetic distance measurement. A term widely used by surveyors and engineers
to describe a range of devices from handheld measurers such as the Leica Disto,
now widely used in construction, to very precise long-range measurers for control
surveys. Many EDMs today will be found incorporated with a theodolite and data
recorder within a total station.
article is reproduced from The Building Conservation Directory, 2002
BOOTH is editor of the RICS’s specialist
bi-monthly journal Geomatics World which reaches over 4,000 surveyors in both
the UK and abroad. A quantity surveyor by training himself, he has had over 40
years experience working in construction and the built environment, over 20 of
them as a journalist and author.
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