GPS Systems

32 • Interface March 2004
By now, most people are quite familiar with GPS (Global
Positioning Systems). GPS are turning up in more
places every day, from rental cars to emergency vehicles,
from ocean liners to the weekend hiker. Not only have
they become more economical, but also the accuracy has
improved. These changes have opened up even more possibilities
for the systems.
My company’s first experience with the systems was quite
a few years ago. A large, complex facility established a need
Left: Two GPS units entering utility
line data. Multiple GPS files can be
joined seamlessly to create a single
drawing.
Above: GPS unit collecting data to
generate a roof plan.
March 2004 Interface • 33
for a better way to generate roof and site plans. At that time,
GPS systems were expensive and the accuracy was not suitable
for most applications. Developed by the Department of Defense,
civilian usage was not a major priority and was in ways discouraged
through the scrambling of signals, reducing accuracy.
Economically, they were not what was required for generating an
accurate roof or site plan. Our definition of “accurate” was less
than twelve inches off for a given length. For some, this may not
seem very precise. The alternative would be to measure utilizing
a nylon tape. Granted, on small, simple buildings, pulling a 300-
foot nylon tape and measuring to the nearest inch will probably
be more accurate. However, on larger facilities with multiple levels
or complex shapes, during inclement weather, and with penetration
obstruction, errors can add up. Even on calm days under
perfect conditions, the stretch in the tape will vary by inches.
Methods are available to achieve more precise readings, but most
require a larger investment in equipment, more time to collect
the data, or both.
As with every new venture, the time comes when payback
outweighs the investment. Our first GPS system drew some speculation
even within our own organization. Was this a tool or a
toy? Being reasonably priced did not mean that we were running
out to buy each consultant his or her own GPS system. It meant
that the company could venture into the idea with a single unit
and see where it took us.
Two grades of systems are available, mapping and surveying.
Mapping – being less accurate and much more economical –
seemed the best fit for the company’s needs. There were no hard
data to prove that the system would work effectively. The manufacturer
of the system the company purchased had never sold
one for measuring roofs and was of minimal help. We were pretty
much on our own.
After a few initial projects, some distinct advantages and disadvantages
were obvious. Most projects showed great potential.
Others taught us a valuable lesson, and one showed that return
trips to re-measure a building could be quite expensive.









 





 

 !”#


$
 
“”


 

  
! 
Roof Consultants Institute
1500 Sunday Drive, Suite 204
Raleigh, NC 27607
800-828-1902 (p)
919-859-1328 (f)
www.rci-online.org

 
  
 
   
!”#
%
&





$
“”

’
‘(

)


$%&
&
 ’

*”

)”


()*+
’,  
-***
Raw data retrieved from a GPS showing a building ideally suited
for GPS survey.
34 • Interface March 2004
Advantages:
We discovered the accuracy was not only within our desired
parameters, but was exceeding our expectations. Periodically, we
would tape roof sections taking readings
to the nearest inch to compare
to the GPS data. Typical readings
were different by less than 6 inches
from the taped dimensions.
The time to measure a building
was cut by half in some cases. The
more unusual the building shape or
the more scattered the penetrations,
the more time was saved.
While a building simple in size or
shape does not necessarily merit
using a GPS, the advantage in doing
so is that we now have it located on
the world. The readings can now
generate other useful information as
to the distance between particular
buildings, which is essential information
in forming a site plan.
Since the data are stored digitally,
extracting a plan is generally
quite easy. Normally, within minutes
of collecting data, a rough plan can
be printed. A final plan requires
drafting time to clean up intersections
and corners and to check alignment
and placement of penetrations.
Accumulated errors are limited.
In measuring a multiple level or a
long building that requires moving
a tape multiple times, errors are
going to accumulate. In using the
GPS, each corner or junction is
located with the same accuracy.
Inclement weather can slow or
stop a normal survey. The GPS
does not care if the sun is shining
or it is the middle of a downpour.
There is only minor degradation of
the signal sent by the satellites
through poor atmospheric conditions.
Of course, this could also be
seen as a disadvantage if you are
the person operating the system.
Disadvantages:
Although one of our goals was
to have a system that was simple
enough for almost anyone to operate,
certain characteristics complicated
data collection. A good
knowledge of the systems is not
required but is very helpful in
avoiding data collection problems.
One of the hardest traits of the
system to get used to is the lack of
a physical drawing. There are certain features that tend to be
more difficult to enter into the system. Understanding these problems
helps but can be more time consuming. The mapping feature
Numerous pipes would present a problem to either tape or GPS. Capabilities were added to collect
pipe features by recording the centerline and adding the number of pipes.
Data collector screen in map view showing portion of a facility ideal for GPS use.
March 2004 Interface • 35
of the data collector provides the user with a graphic
image of the data. It must be utilized and trusted.
Potential problems must be corrected before leaving the
project.
Since the system requires a signal from above, areas
with high walls, overhangs, adjacent trees, or other
objects could block the signal or reflect the signal, creating
what are known as multipath errors, rendering the GPS
almost useless. There are ways to work around these problems
by using offsets or collecting the feature from a higher location
where the signal is stronger. There are times when it is just
more efficient to measure the feature with a tape.

 

  
 
 


 




 

 

  


 
Partially finished AutoCAD file showing completed pipe
network.
Finished AutoCAD drawing file.
Conclusion:
Individual applications determines whether a GPS is beneficial.
Although the systems can generate very accurate readings
– in the 2-centimeter range – a lengthy time is required to collect
sufficient data to do so. If extremely accurate readings are
required in a timely manner or in crowed locations, GPS may
not be a feasible tool. For those in need of very accurate data,
look to the surveying grade, but keep in mind that disadvantages
still apply. From our experience, the mapping grade systems,
under ideal circumstances, will provide readings less
than 6 inches different from actual.
Benchmark, Inc. has been utilizing GPS for over three years
and currently owns three GPS systems used not only to create
roof plans, but also paving and site plans. ■
36 • Interface March 2004
Michael Bassett, CET, is the general
manager for Benchmark, Inc., a
national roof and paving consulting
firm, where he has been employed
for the past 15 years. Bassett’s varied
duties include maintenance and
development of the GPS. He has an
associate of applied science degree
in civil engineering technology and
has attained certification from
NICET.
ABOUT THE AUTHOR
MICHAEL BASSETT, CET


 


 













  
  !




The Cool Roof Rating Council (CRRC) announces the initiation
of the aged testing component of its product rating program.
The completed aged testing protocol complements the initial testing
protocol implemented in Fall 2002. It provides third party
verification of weathered performance claims to aid in the selection
of cool roofs that save energy and reduce a building’s impact
on the local environment.
The CRRC was established in 1998 to focus on implementing
a fair, accurate, and credible radiative energy performance rating
system for roof surfaces and to disseminate the information to all
interested parties. The Product Rating program, the core element
of CRRC, was launched in September 2002 with the purpose of
reporting initial and three-year aged values for solar reflectance
and thermal emittance. The CRRC board of directors pledged to
launch this final element of the rating program by January 2004,
and in fact, weathering of products for CRRC rated products for
aged testing has already begun, according to CRRC Chairman
Peter Turnbull.
The performance of a cool roof over time is greatly influenced
by environmental exposure. Several variables, such as different
climates and slopes of roofs, contribute to the amount of dirt,
staining, and fungal growth that accumulate on roofs over time.
This weathering can impact the reflectance and emittance of a
roof, pointing to the importance of aged testing in evaluating cool
roof performance.
Over 130 roofing products are currently listed with their initial
ratings in the CRRC Rated Products Directory (found at
www.coolroofs.org). Both initial and aged performance ratings
(as they become available) are displayed on product labels and
the online directory.
As the multiple benefits of cool roofs are increasingly being
recognized, code bodies are starting to require increased roof
radiative performance. In many locations, cool roofs not only
save energy and money by reducing air-conditioning needs for
individual buildings; their implementation on a broader scale
also promises to reduce the urban heat island phenomenon by
lowering a community’s ambient temperatures by several
degrees.
Current revisions (adopted in November 2003) to California’s
energy efficiency code, Title 24, require a cool roof, defined by
the state as having an initial solar reflectance of 0.70 and thermal
emittance of 0.75 for low-sloped roofs of commercial buildings,
as rated and documented under the CRRC system.
CRRC’s Rating Program complements other programs, such
as EPA’s EnergyStar® program, that set minimum performance
thresholds. The growing cool roofing market relies on clear and
accurate information about product performance. The CRRC
hopes that the development of the aged testing procedure will
provide up-to-date information for the industry. All roofing materials
manufacturers and sellers are encouraged to participate in
the CRRC Product Rating Program.
For more information on the CRRC, the new aged testing protocol,
program participation, and membership, visit www.coolroofs.
org, or contact the CRRC directly at (866) 465-2523.
CRRC BEGINS
AGED TESTING PROGRAM