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Disclaimer—This paper partially fulfills a writing requirement for first year (freshman) engineering students at the University
of Pittsburgh Swanson School of Engineering. This paper is a student, not a professional, paper. This paper is based on publicly
available information and may not provide complete analyses of all relevant data. If this paper is used for any purpose other
than these authors’ partial fulfillment of a writing requirement for first year (freshman) engineering students at the University
of Pittsburgh Swanson School of Engineering, the user does so at his or her own risk.
UNMANNED AERIAL VEHICLES AND THEIR USE OF VISUAL IMAGING TO
ASSESS CIVIL STRUCTURES
Matthew Gabrin, [email protected], 10:00 Budny, Laura Fritz, [email protected], 3:00 Mena
Abstract- Unmanned aerial vehicle (UAV) imaging is an
innovative technology that allows humans to view objects,
structures, and landscapes that they otherwise could not see.
This UAV imaging innovation is applicable to many fields,
including public safety, environmental mapping, and
journalism. The main focus of this paper is the use of this UAV
imaging technology to monitor the health of civil structures,
such as buildings and bridges. Like any man-made object,
civil structures wear down over time and are susceptible to
damage. This damage must be monitored to maintain safety
standards. UAVs provide the means to access hard-to-reach
inspection places on structures and increase the speed and
efficiency of the monitoring of these structures. This
technology will offer long term sustainability in social,
economic, and environmental aspects of its use. This paper
will explore the technical details of UAV imaging technology,
as well as the direct application of this technology in the
health monitoring of civil structures. It will also discuss
ethical problems related to UAVs and the public perception
of drone use in a civil capacity.
make the assessment of civil structures a more sustainable
practice. Unmanned aerial vehicles combined with imaging
technology and analysis provide an effective alternative to the
structural health monitoring of civil structures.
UAVS: BEHIND THE BOLTS
UAVs come in a variety of shapes and sizes, but the main
components of each individual system are similar. This paper
will specifically focus on the use of small drones, weighing
less than 150 kg [1]. A UAV must have a body, electrical
source, propulsion system, and flight controls [2].
Considerations when building UAVs include size, weight,
cost, and durability. Efficiency in all of these areas is
prioritized in the description in order to maximize the
economic sustainability of the drone. The body of the drone
varies based on the model of the unit, and does not affect the
application of UAV imaging as long as it can support a
camera. Lightweight and compact materials, such as carbon
fiber-reinforced polymers, are preferable to keep the drone
cost efficient and maneuverable. The body must be able to
hold the drone components, including the motor and
propellers, power source, computer, and any additional
attachments, such as cameras for the application of UAV
imaging.
For an efficient power system, batteries are the most
common choice. Nickel-cadmium or zinc-silver batteries
provide enough power for operation while still being
affordable. In addition, small drones have limited distances
they can travel, making the requirements less strenuous than
other battery powered vehicles. The battery powers the
propulsion system as well as any other attachments, such as
the imaging technology. It is mounted to the body of the UAV
to access the rest of the structure.
The most common propulsion system is a network of
propellers rotated by a motor or motors. As seen in figure 3*
below, the quadcopter, consisting of four rotors facing
upwards, is an extremely common drone system accessible to
the public. Variations in speed between the rotors allow the
UAV to move in all three dimensions. Another popular
system is the helicopter, where a single propeller facing
upwards keeps the body in the air while a rotor on the back
perpendicular to the ground turns the vehicle. Other options
include an eight propeller system or a plane system, although
Key words- Civil infrastructure condition assessment, Crack
detection, Drone demilitarization, Grey scale and HSV
thresholding, Remote Visual Imaging, Structural health
monitoring, Unmanned Aerial Vehicles (UAVs).
EXPLORING UAV TECHNOLOGY AND ITS
APPLICATIONS
Unmanned aerial vehicles are a unique innovation that are
quickly becoming a part of everyday life. From military use
to mail delivery, UAVs are being implemented in many
different ways as solutions to modern problems. One such
solution is for the health monitoring of civil structures.
Currently, to ensure the safety of structures such as buildings
or bridges, crews of professionals need to overcome
dangerous conditions to go to hard-to-reach inspection sites.
Not only is it unsafe, but the process is time consuming,
expensive, and inefficient. Unlike humans, UAVs can reach
otherwise inaccessible places, giving them a unique and
useful advantage. If a UAV, consisting of a motorized body
with a mounted camera, is controlled to fly around structures
and capture images, the monitoring process becomes both
faster and safer. These benefits along with others will help to
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3.31.2017
Matthew Gabrin
Laura Fritz
light for a clear image. Finally, ISO is the camera’s sensitivity
to light. This setting compensates for any faults in aperture or
shutter speed. The higher the ISO, the brighter the images
become, so lower ISO is preferable. An ISO of less than 400
will take high quality images in daylight, making this a good
range. All of these settings need to be optimized based upon
the individual conditions of the survey [3].
One example of camera specifications is from the
University of Pisa’s test of a UAV monitoring the health of
civil structures. They used a Nikon D600 SLR camera with
an aperture of f11 and a shutter speed of 1/800-1/1000 [4].
These all fit within desired specifications, allowing them to
capture images with a high enough quality to process later.
Lastly, the quantity of images needs to be taken into
consideration in addition to quality. One approach to
collecting images is to collect a massive amount of data to get
as clear of an image of the structure as possible. The volume
of image data will be helpful for analysis, such as 3D
reconstruction, or to get a variety of angles for manual
inspection. After the images are collected, they are then sent
to the computer to be analyzed.
they are not as popular or as effective for structural imaging.
The plane system moves at a velocity that is too great to
capture high quality images. An eight propeller system is
effective but more difficult to manufacture. Both the
quadcopter and propeller networks are considered viable and
have equal effectiveness.
For the flight controls, a trained remote operator can
control the direction and altitude of a UAV with a remote
control. For the use of drones to monitor civil structure health,
the operator is usually within eyesight of the drone, allowing
for a real-time video of the drone’s view to be optional. A
computer on the drone is able to read the frequency from the
remote control, and built-in algorithms power the motors to
rotate at a certain speed, changing the velocity of the drone.
This is not the only algorithm used by the drone, however.
Each UAV has built-in flight control systems in addition to
being controlled by an operator. This is to ensure the integrity
of the drone, performing tasks like keeping the UAV in a
stable position or protecting the drone with safety nets such
as parachutes, should a system fail. Mechanisms such as
gyroscopes are in place, allowing the onboard computer to use
correction algorithms [2] to keep the drone in homeostasis.
The algorithm has three steps: collecting flight data and
calculating corrections, updating altitude data, and calculating
what forces need to change for the velocity and position to be
where the operator directs the drone. This algorithm plays a
key role in ensuring the cooperation and responsiveness of the
UAV.
While the drone is important, its ability to accommodate
additional attachments makes it functional. For civil structure
health monitoring, drones will have cameras mounted on
them to capture images.
IMAGE ANALYSIS
After the images have been collected with the UAV,
computer programs are used to sort through the data to make
error detection as easy and accurate as possible. Manual
inspection of captured images is viable, but only for small and
specific civil structure inspections. The number of pictures
needed to be sorted through for crack detection or surface
degradation would soon become overwhelming, making the
process more time consuming than the original technique. As
an alternative, images can be processed by computer
algorithms to make inspecting the images easier, faster and
more sustainable for practical use. 3D reconstruction and
grayscale thresholding are two promising techniques that can
process the images to make them much easier to read.
3D reconstruction consists of using a program to splice
together many images. This is an extremely challenging
technique, but recent innovations have led to the creation of
viable models. The algorithm consists of many steps to create
these models. First, the images need to be analyzed for key
points. These are features of the image with a distinct
structure or feature unlike anything else on the image, usually
an edge. This will help the algorithm obtain a sense of how
the images overlap. This is done on each image by analyzing
the pixel colors in the image and choosing distinct groupings.
Next, the key points of the images are matched up. The
computer sorts through the chosen key points of each image
and lines them up with matching images. Error analyses are
performed for each match to ensure the points are the same,
and any incorrect matches are eliminated [5]. This effectively
matches all of the pictures together; however, the result is still
in 2D. Then, the images are preliminarily analyzed for depth
based on focal length. Images will have different levels of
clarity or focus based on the settings of the camera. By
IMAGING TECHNIQUES
Mounted cameras offer a unique innovation when attached
to drones. With the ability to fly around virtually any obstacle,
attached cameras can capture images that would be extremely
difficult for any person to take on their own. The most
common attachment is a digital camera.
The most useful type of camera is the single-lens reflex
camera (SLR) for its ability to allow the user to see the picture
before it is taken. This is useful in ensuring the pictures taken
are the correct ones and that the subject is surveyed correctly.
Since pictures will be taken mostly outdoors, settings on the
camera need to be adjusted to optimize image quality for
processing later on. Important aspects of the camera include
aperture, shutter speed, and International Standards
Organization (ISO). Aperture affects how much light is let
into the camera. Since pictures will be taken in the daytime, a
higher aperture setting of f/11 to f/16 would provide better
quality. Additionally, the larger the aperture, the smaller the
depth of field, giving more focus to objects closer to the
camera. Shutter speed is how long the lens is opened for light
to enter the camera. Since the sun provides ample amounts of
light, a low time value for shutter speed will provide sufficient
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Matthew Gabrin
Laura Fritz
analyzing the clarity of each image, a relative depth can be
given to each location on the image. When done to the spliced
images, this creates a rough layout of the depth across the
whole surface. The layout is refined in a later step. The
relative depth of the images compared to each other is then
calculated horizontally. This is called the disparity [5]. This
calculation creates a more accurate model of the depth of
images relative to the structure as a whole, rather than the
adjacent images. Finally, the last step is triangulation. By
analyzing the distances from each camera location to each key
point, a calculation can be made for its exact depth:
𝑢
𝑣
𝑓
𝑥=𝑏∗ ,
𝑦=𝑏∗ ,
𝑧=𝑏∗
𝑑
𝑑
𝑑
X, y, and z are the positions, b is the baseline, or lowest point
of the image, f is the focal length, and u and v are the 2D
dimensions [5]. This photogrammetric calculation can give
extremely accurate depths for each key point, and therefore
the entire image. After triangulation has been performed on
each key point and image file has been created, the finished
model is complete. This model can then be analyzed by an
inspector to look for disturbances in the structure, cutting the
time and difficulty of analyzing an entire structure down by a
significant amount.
Grayscale thresholding is another promising technique to
make image analysis faster and easier. The basic concept of
this process is to take an image and extract the brightness
levels of the picture. Cracks appear darker in images than a
normal surface, so their brightness level would be
significantly lower, allowing a computer to detect this
difference. The algorithm for this technique includes first
converting the image to black and white, otherwise known as
grayscale, using hue, saturation, and value (HSV)
thresholding. This accentuates the brightness levels of the
whole image, clearly defining bright and dark areas. The
program then analyzes the color pixels composing the image.
If the difference between adjacent points is large enough, the
algorithm recognizes this as a crack and alerts the user.
Naturally, there will be many different levels of brightness on
a surface, especially on surfaces that have faced various
weather conditions over many years. This leads to grayscale
thresholding detecting more cracks than are actually present.
To counterbalance this, the HSV thresholding is performed
twice. Once by extracting all of the bright sections and
analyzing for cracks, and then once more by extracting all of
the dark sections and analyzing for cracks. As shown in figure
1*, the dark parts of the image have been extracted,
showcasing a crack along the wall. This provides a more
accurate representation of both surface degradation and
cracks, making them more defined and easier to spot. These
algorithms can connect the computer engineering aspects of
UAV technology to the civil discipline with their application
of structural health monitoring.
UAV imaging can be applied to many fields relating to the
public. Security and police authorities can use them to
observe large events to maintain safety. Fire authorities can
use them to view large scale fires, such as forest fires, to gauge
their size and determine a solution. The imaging technology
detailed in this paper is most effectively applied to the
structural health monitoring of large scale civil structures.
The structural health monitoring of civil structures is
crucial to the function and longevity of structures used by the
public. Buildings are constructed with sustainability in mind
for the reduction of safety hazards or costs in the future.
However, nothing can stop the forces of entropy as civil
structures decay over time. Civil Structure monitoring
involves many techniques and procedures to monitor and
assess the structural conditions. Routine inspections occur a
set amount of times per year and may also occur when
conditions deem it necessary. While traditional methods of
assessment work, UAV imaging technology provides a fast
and efficient alternative to monitor and assess civil structures,
particularly buildings. UAVs, when paired with a measuring
instrument to collect visual data, are capable of transmitting
information in real time to allow for inspection of structures
from the ground. The aforementioned image processing
systems can greatly accelerate the assessment process,
leading to longer lasting structures.
The identification of cracks is one important part of
structural health monitoring. Cracks can be caused by
expansion from temperature and moisture changes. Materials
initially shrink and then expand, causing cracks to form.
Cracks can also be caused by subsidence, which results when
the foundations of a building move. Cracks on concrete
surfaces can be detected from their two main properties: they
have a thin structural shape and a low luminance [6] . UAVs
can be used to collect images of structures that can then be
analyzed for cracks or other structural problems. Most
methods of crack detection involve grayscale images, as
mentioned above, because dark spots indicating a crack can
be detected on the lighter background of the concrete. While
grayscale imaging reveals most cracks, it also detects other
structural edges as cracks as well, resulting in over
detection. Newer technologies like HSV thresholding allow
for minimal detection with grayscale imaging while
maintaining accuracy. In the figure 1*, combinations of
imaging techniques on a portion of the Karnak Temple in
Egypt are shown. It can be seen that HSV thresholding gives
optimal results for crack detection.
Surface degradation is another cause of safety issues in
structures. Degradation is caused by prolonged exposure to
outdoor conditions and human interactions, which causes
erosion and the change in surface color texture, or
efflorescence [6]. Grayscale thresholding, the combination of
grayscale images with a computer algorithm for analysis, is
the most effective method for detecting surface degradation.
Figure 2* shows the result of this combination when applied
to images of a brick wall with possible surface degradation.
CIVIL STRUCTURE HEALTH
MONITORING
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Matthew Gabrin
Laura Fritz
The image taken by a UAV is transformed to highlight the
surface degradation, allowing the user to easily see the
problem areas on the wall.
In addition to the grayscale thresholding, 3D models are
extremely efficient when analyzing structures. After the
models have been created, trained experts can study the
models and determine if surface degradation or cracks have
occurred. Unlike grayscale thresholding, only one model
needs to be studied, decreasing the study time significantly.
The entire building or bridge can be contained within one
model, and every surface on the structure can be reviewed for
damage. One potential application is to use both 3D
reconstruction to identify potential defects and then use
grayscale thresholding at the specific location to obtain better
evidence of these structural problems. While just one
possibility, this shows that these techniques can be combined
to further improve the accuracy of structural monitoring.
Overall, the UAV is found to be extremely effective when
paired with imaging technology to detect defects in civil
structures.
Improvements by UAVs
UAV monitoring and imaging can be beneficial in many
aspects, but also has its drawbacks. When compared to current
methods of inspection, UAV imaging provides a faster and
safer way to obtain data on building defects. However, it
requires training and new safety measures, and also comes
with some use restrictions depending on environmental
effects.
The use of UAVs for inspection of civil structures provides
many improvements when compared to conventional
methods. Firstly, UAVs only require a ground operator
knowledgeable in the control of the UAV, while conventional
methods require multiple individuals skilled in accessing the
hard-to-reach construction locations and in assessing
structural problems. Additionally, UAVs can be used in high
risk areas without risk to human life, making them a safer
method of inspection. They also allow for easy access to high
surfaces or areas inaccessible by cranes and elevated
platforms. While UAVs provide high quality assessment of
structures through the imaging technology, this technology is
designed to be cost effective, and reduces overall cost of
inspection by eliminating expensive equipment usually
needed [7]. UAVs also allow for increased speed of
monitoring as they can transmit collected data in real time to
computers on the ground. In addition to this, the data collected
can be stored off site for further scrutiny if needed, and can
be assessed by multiple individuals. Conventional methods
only allow for individual observation and inspection of
structures, while UAV imaging allows multiple professionals
to view images to make their assessments. This also
eliminates error in the assessment of the structures, which
improves accuracy and helps to detect problems early on.
Overall, UAV inspection methods are faster, safer, more cost
effective, and more accurate when compared to conventional
inspection methods.
Current Monitoring Techniques
Drawbacks of UAVs
Inspection of large, hard to access structures such as
bridges or building usually requires elaborate equipment and
highly trained workers. Cranes and elevating platforms are
conventionally used to access the inspection locations. This
process generally creates disturbances in the surrounding
environment, such as the need to change traffic patterns in
order to complete inspection. The cost of this equipment, in
addition to the cost of specially trained workers can be
burdensome. The need for added safety methods makes
conventional inspection less efficient. In addition to this,
results of visual inspection can vary greatly depending on the
individual that accessed the inspection site [7]. According to
an Autonomous Systems researcher at Vel Tech University of
India, “The conventional [structural health monitoring]
procedures tend to be laborious, time consuming and capital
intensive” [6]. As companies and public works employees
strive to maintain safety standards, it is also important to
employ cost efficient methods to monitor structures.
Improvement upon these current methods of inspection would
not only cut cost and length of time for monitoring, but would
also benefit the public by eliminating outside disturbances
caused by current inspection methods.
While the use of UAV imaging technology for the
inspection of civil structures has many advantages, there are
also some drawbacks to the technology. GPS automated flight
control is still in development and is currently not viable for
implementation, requiring the UAV to have a trained pilot to
control the system. Such operators require more skill and
must be trained appropriately, which can cost time and
money. Also, because of the low weight of the UAV itself, it
can be greatly affected by changing weather conditions. This
includes conditions such as heavy winds or rain. Heavy winds
could dangerously affect flight patterns and possibly damage
the UAV body. Both could negatively impact the inspection
by rocking the UAV or by blocking the camera lens,
respectively. Both could therefore decrease image quality and
may produce images not fit for computer analysis. In addition
to environmental effects, the use of UAVs is regulated heavily
by law, and these rules must be observed in order to use this
technology. Specifically, flight permits are required and
autonomous flight is generally illegal [7]. However, with
conventional methods of inspection, other permits are
required to use the needed equipment, so this requirement
does not necessarily create any additional work for the
inspectors. In addition to these drawbacks, the public
EFFECTIVENESS OF UAV IMAGING
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Matthew Gabrin
Laura Fritz
perception of UAV use also affects the implementation of this
technology. Developers must ensure that UAVs are perceived
as useful and efficient to the public in order to fully access the
capabilities of the technology.
a new drone certification process which requires drone pilots
to go through a multi-step process in order to operate UAVs
commercially. Some of the steps in this process include
passing an aeronautical knowledge test, being vetted by the
TSA, and preflight inspection of the UAV. Pilots are also
required to pass a recurrent aeronautical knowledge test every
24 months afterward [10]. These measures implemented by
the FAA soothe public worry over the dangers of UAV use by
ensuring that pilots are more than qualified. These regulations
make the use of UAVs a socially sustainable technology by
eliminating possible doubts in the public about safety
concerns.
Besides the precautions taken for the societal impact on
safety, this technology significantly improves upon
environmental safety. There are approximately 600 accidents
each year from road work related incidents [11]. This type of
accident is exactly what occurs during a civil inspection using
current methods, as previously described. By eliminating the
dangerous and time consuming road delays with a UAV,
inspecting civil structures becomes much safer for workers
and the general public. In addition, the purpose of the health
monitoring is to ensure the safety of any civil structure. Basic
infrastructure is used universally by all people, making
damage to these structures potentially catastrophic. By
improving the quality and efficiency of these tests, the
environment created by the many civil structures across the
country becomes safer for everyone.
Finally, the biggest improvement UAV health monitoring
offers is its cost effectiveness. The only costs associated with
the drone are the UAV itself, UAV licenses, and maintenance
costs. When compared to the money sunk into road closures
for teams of workers to access difficult inspection sites, the
drone is much more cost effective. This has large implications
for health monitoring in general. More inspections can be held
on a regular basis because of the low cost, increasing structure
safety. This also increases the number of civil structures that
can be monitored, without a change in budget from the local
government or an increase in taxes on the local population.
Finally, the increased and improved health monitoring will
make damage detection much more cost effective. Accidents
can happen when damage is not caught in time. Long term
damage also results in extremely expensive repairs. By
detecting cracks or surface degradation early on, repairs can
be made swiftly and inexpensively. This technological
solution creates a waterfall effect in favor of the sustainability
of environmental safety and financial costs associated with
civil structure health monitoring, while keeping the public
happy.
ETHICAL IMPLICATIONS
UAVs, colloquially known as drones, are often
immediately associated with military applications. While they
have a plethora of uses, UAVs were first created as military
weaponry, and this is how many in the public still perceive
them. There is still an associated fear among the public for the
safety of the use of UAVs in civil applications. It is important
to address these fears and ensure the safety of this technology
before the implementation of it in the American public [8].
In addition to the military association of UAVs, the use of
UAVs equipped with camera technology creates a concern for
the privacy of the public. A camera on a flying UAV moves
the line of site from the ground among people to the air, which
shifts the boundary of what is considered “public” [1]. This
can cause questions about the unregulated use of UAVs, as
current security and privacy policies only address the invasion
of privacy by individuals on the ground, not flying objects in
the air that can record and stream images in real time. Public
policymakers are faced with the task of regulating the use of
UAV imaging, and operators are ethically obligated to use
them for their original purpose of inspection and data
collection. However with proper use and regulation, UAV
imaging can be extremely beneficial to the public by ensuring
the efficiency of inspection of the buildings and bridges that
the public uses every day.
SUSTAINABILITY FOR THE FUTURE
Sustainability was defined by the United Nations
Conference on Sustainable Development as the emphasis on
a “holistic, equitable and far-sighted approach to decisionmaking at all levels. It emphasizes not just strong economic
performance but intragenerational and intergenerational
equity. It rests on integration and a balanced consideration of
social, economic and environmental goals and objectives in
both public and private decision-making” [9]. By this
definition, sustainable development is beneficial for current
and future generations and considers all fields affected by this
development, including social, economic, and environmental
spheres.
For any new technology, societal impact must be taken
into consideration before implementation of the technology.
As mentioned before, UAVs can be seen as dangerous to the
public because of their original military use. In order for this
technology to be sustainable, or something that is efficient
and long lasting while having minimal negative effects, this
issue must be addressed. In June of 2016, the Federal Aviation
Administration (FAA) finalized a new regulatory system for
the commercial use of small UAVs. These regulations created
FUTURE APPLICATIONS
In addition to UAV imaging technology, new processes
are currently being developed that utilize unmanned aerial
vehicles for structural health monitoring without using a
camera. Visual inspection through the use of cameras
efficiently detects cracks and other outer defects, but cannot
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Matthew Gabrin
Laura Fritz
pick up internal problems on a structure. To overcome this
problem, researchers are looking into using UAVs combined
with an impedance based vibration inspection method. This
method uses a UAV to attach a “piezoelectric transducer onto
a specific region where excitation and data acquisition occurs
simultaneously” [12]. This could allow the user to identify
internal damage to the structure, as well as detect any defects
early on, reducing the overall maintenance cost. This shows
that the use of UAVs to inspect structures is not only viable
for the present, but also has the capability of being improved
upon in the future with better features and properties to solve
more problems.
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LOOKING FORWARD
Unmanned aerial vehicles combined with imaging
technology give an efficient alternative to conventional
processes of civil structure inspection. A variety of imaging
techniques can be applied to the outer surfaces of civil
structures to collect data, which can be analyzed for defects
on the surfaces. UAVs provide many advantages when used
in this application, such as their ability to stream data in real
time, reach high places, and provide a cost effective approach
to maintaining safety standards. While there are some
obstacles to the complete implementation of this technology,
such as public policies and public perceptions, the technology
is the best solution for more efficient monitoring. This
efficiency, along with implemented regulations, help
contribute to the sustainability of this technology. Continued
development will ensure safety measures are met and that the
UAVs are as efficient as possible. Future improvements upon
the technology itself will increase efficiency of inspection
even more. If this technology continues to advance at this rate,
unmanned aerial vehicles may become a common part of
everyday life.
SOURCES
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Accessed
1.9.2017.
http://www.int-arch-photogramm-remote-sens-spatial-infsci.net/XL-1-W4/103/2015/isprsarchives-XL-1-W4-1032015.pdf
[5] A. Zingoni. “Real-Time 3D Reconstruction From Images
Taken From a UAV.” International Archives of
We would like to thank our parents for their continued
support in our engineering journey and Bella Sedor for
providing use with support and caffeine in the final hours of
writing. We would also like to thank George Lucas for
creating the life-changing concept and universe of Star Wars.
Without it we would not be here today.
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Matthew Gabrin
Laura Fritz
*Images have been placed at the end of this document for maximum clarity of the visual content.
FIGURE 1 [6]
This image shows the results of grayscale thresholding when detecting cracks on a surface.
FIGURE 2 [6]
This image shows the result of greyscale thresholding when detecting surface degradation.
7
Matthew Gabrin
Laura Fritz
FIGURE 3 [4]
This image shows an example of a quadcopter drone with a mounted camera surveying a civil structure.
8
Matthew Gabrin
Laura Fritz
9