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Terrestrial Laser
Scanning Provides Major Benefits to the Construction Industry
 In the past decade, laser scanning has experienced
steadily increasing use in reverse engineering and inspection of
reasonably sized products ranging in size from small parts that would
fit in the palm of ones hand, to automobiles and small airplanes. These
laser scanners operate by projecting laser light onto the surface of the
part and using a camera to triangulate the laser line as it sweeps
along, enabling the object to be accurately replicated. This approach
requires, of course, that the laser and camera be quite close to the
surface of the object being scanned, making it impractical to scan very
large objects. More recently, a new generation of laser scanners has
emerged that operate on very different principles and can scan much,
much larger objects such as building sites, the interior and exterior of
existing buildings, petrochemical plants, and offshore oil rigs.
How Terrestrial Laser
Scanners Work
The new laser scanners often called terrestrial
laser scanners or “large scale” laser scanners, work by projecting a
laser beam onto an angled rotating mirror that reflects the beam to the
object being scanned, while the entire unit rotates around a vertical
axis. Thus the unit scans 360° in a horizontal plane and 320° in a
vertical plane. The net effect is that everything the scanner can “see”
within a sphere of 76 meter radius can be digitized. When the beam hits
the object being measured, which can be at a considerable distance, the
beam is reflected back to the scanner.
The distance of the object being measured can be
determined by either of two different methods. One way is to measure the
time of flight of the laser beam and multiply it by the speed of light.
The other method involves projecting constant waves of varying length
and measuring the phase shift of the reflected beam in relation to a
reference signal kept at the laser scanner. For example, the Faro LS
laser scanner splits the laser beam into three component parts operating
on three different modulation lengths, 76 meters, 9.6 meters and 1.2
meters. The distance of the reflecting object from the scanner is
determined by identifying the location of the reflection on the 1.2
meter cycle. The cycle on which the reflection has occurred is
identified through its registration on the longer wavelength cycles.
 Terrestrial laser scanners build a 360° data set,
or “point cloud”, that records all objects within the range and sight
of the laser scanner. Gray scale values are applied to the X, Y, and Z
data points based on the strength of the returned laser signal. While
this gives the data a first-glance appearance of a black and white
photograph, data registration spheres can be placed within the area to
be scanned and the device repositioned to capture objects out of the
line of sight or out of the range of the scanner. The individual scans
can then be linked together to complete the 3D point cloud. The software
offers a variety of additional options such as overlaying digital images
on the top of the point cloud to provide a color image for enhanced
realism.
Applications in the
Construction Industry
Terrestrial laser scanning has many applications in
the construction industry at every stage of the construction lifecycle.
3D models can assist the design process by providing more complete site
surveys. They can help during construction by providing more complete
and accurate dimensional measurements. 3D laser scans can be used to
model the interior and exteriors of existing buildings. Most existing
buildings were constructed without 3D models ever being created and even
in the case where 3D models were created the current building often
differs from the original design intent. 3D models can substantially
reduce the time required to plan future additions and alterations and
often can deliver maintenance savings.
A detailed topographic survey is required for
beginning design work on many different projects. Laser scanning is much
faster than traditional total station instruments that combine
electronic theodilites and electronic distance measuring devices and can
provide complete 3D models as opposed to discrete measurements. For road
measurements, laser scanning is also safer, since the need for lane
closures is eliminated and no one has to stand in the road. Digital
photos may be tied to scans to provide better visualization and enable
designers to identify specific attributes.
Survey Work in Office
Tower Construction
 A key terrestrial laser scanning application
involves performing survey work required to support construction of new
office towers. For example, the verticality of concrete elevator cores
needs to be monitored on a floor by floor basis after the floors are
poured and set. The core thickness and position is also critical to
avoid fit problems with horizontal structural steel. The traditional
method of performing these measurements is with total stations. But this
method cannot detect variations in concrete geometry over its full
surface and it can be time-consuming and hazardous. Laser scanning
provides far more complete measurements by providing the ability to
register scans together and tie them to the building grid. The outside
core geometry can be compared to internal geometry to determine core
thickness. Thickness variations along the core surface can easily be
identified in the model.
Many chemical process plants lack up to date
documentation. Compliance with the latest Occupational Safety and Health
Administration (OSHA) requirements makes such documentation necessary
for most plants. Terrestrial laser scanning provides a fast and
relatively inexpensive method to produce accurate drawings. Laser
scanning can be used to document both the indoor and outdoor areas of
existing plants. The resulting 3D models can be used for many purposes
such as measuring piping in process plants as is required for compliance
with safety regulations. In a typical example, a small plant was scanned
in 4 days compared to an estimated month that would have been required
using a manual measurement methods.
 Offshore oil production facilities provide
tremendous dimensional control challenges throughput their entire
lifecycle. During construction, tolerances must be closely managed
between topsides and hull structures even though they are often built in
different facilities. Laser scanning can be used to validate the entire
jobsite geometry including locating tie-ins and anchor bolt locations
and verify footing dimensions and positions. In the case of damage from
a storm, the structured must be rapidly documented to aid in repair or
decommissioning. Laser scanning can be used to provide measurements on
land prior to subsea installation which will, when combined with
acoustically acquired position data about the installation point,
eliminate the need for much deepwater measurement while providing an
accurate first-time fit.
Terrestrial laser scanning can improve every phase
of the construction lifecycle by having the complete existing geometry
of the construction site, building or existing landscape description in
3D. Building modifications, upgrades, add-ons, or reconstruction of the
assembly at a different location can now be accomplished against an as
built condition that is completely documented in 3D. This technology
holds information you don’t know you need until long after you have been
to the site. The 3D data file of the scan of the site or building allows
you to get dimensions after the fact and without going back to get more
information. This greatly speeds up the project and most importantly,
provides greater reliability that the project will be completed
correctly with no surprises when materials and pre fabricated components
arrive to the job site.
Most customers do not need to do this sort of
measuring very often so they will rely on the fast availability of the
services of expert users that can be outsourced to provide this valuable
documentation and information. GKS Inspection Services Inc. specializes
in being able to measure anything – large or small, anywhere in the
world. For more information about how GKS
Inspection Services can improve your manufactured product, save you
money and decrease your development time, send an
email to
measure@gks.com.
 
All the above photos and data used w/ permission, and with thanks to
St. Paul’s LCMS, New Boston, MI.
Photos above courtesy of
©
FARO Technologies Inc. |
