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Sensors 2015, 15, 23341-23360; doi:10.3390/s150923341
sensors
ISSN 1424-8220
www.mdpi.com/journal/sensors
Article
CCTV Coverage Index Based on Surveillance Resolution and
Its Evaluation Using 3D Spatial Analysis
Kyoungah Choi and Impyeong Lee *
Lab. for Sensor & Modeling, Department of Geoinformatics, University of Seoul, Seoulsiripdaero 163,
Dongdaemun-gu, Seoul 02504, Korea; E-Mail: shale@uos.ac.kr
* Author to whom correspondence should be addressed; E-Mail: iplee@uos.ac.kr;
Tel.: +82-6490-2888; Fax: +82-6490-2884.
Academic Editor: Gonzalo Pajares Martinsanz
Received: 9 June 2015 / Accepted: 10 September 2015 / Published: 16 September 2015
Abstract: We propose a novel approach to evaluating how effectively a closed circuit
television (CCTV) system can monitor a targeted area. With 3D models of the target area
and the camera parameters of the CCTV system, the approach produces surveillance
coverage index, which is newly defined in this study as a quantitative measure for
surveillance performance. This index indicates the proportion of the space being monitored
with a sufficient resolution to the entire space of the target area. It is determined by
computing surveillance resolution at every position and orientation, which indicates how
closely a specific object can be monitored with a CCTV system. We present full
mathematical derivation for the resolution, which depends on the location and orientation
of the object as well as the geometric model of a camera. With the proposed approach, we
quantitatively evaluated the surveillance coverage of a CCTV system in an underground
parking area. Our evaluation process provided various quantitative-analysis results,
compelling us to examine the design of the CCTV system prior to its installation and
understand the surveillance capability of an existing CCTV system.
Keywords: closed circuit television (CCTV); surveillance performance; surveillance
coverage; surveillance resolution
OPEN ACCESS
Sensors 2015, 15 23342
1. Introduction
CCTV surveillance operations have rapidly expanded due to the technology’s important role in
crime prevention, traffic monitoring, and security [1]; although controversies regarding privacy and the
effectiveness of CCTV installation have continually arisen [2,3]. Currently, many municipal
governments throughout the world independently operate integrated CCTV control centers, whereby
CCTV images are used to arrest criminals; additionally, corresponding news items can easily be
encountered [4–6]. Furthermore, the use of CCTV in public locations such as shopping malls,
apartments, and underground parking lots has reduced the possibility of crime, including theft, assault,
and/or fraud [7–10]. The use of CCTV images has expanded beyond crime prevention; for example, to
ensure the safety of people on a train-station platform; to observe public-transport passengers for
unexpected behaviors; and to monitor patients at hospitals [11–14].
To solve problems regarding social welfare, transport safety, crime prevention, and other social
issues, the establishment of CCTV systems in public and residential areas has been proposed [15].
Further CCTV systems were installed and existing systems were upgraded to decrease the blind spots,
thereby improving the surveillance quality [16]. In addition to enhancing hardware specifications, the
performance of CCTV systems can be improved by incorporating new software and technologies; for
example, some researchers attempted to enhance the overall performance by optimizing the camera
configuration in a CCTV system [17,18]. In addition, using GIS, they determine the optimal locations
of CCTV following an analysis of CCTV images [19,20]. For example, a GIS tool, Isovist Analyst is
used to identify a minimal number of CCTV for complete coverage of a target area based on a greedy
search [21]. In reality, however, when analyzing CCTV performance in terms of quantifiable
indicators, we usually calculate the ratio of the blind spots depending on whether or not the target areas
are observable by a CCTV. The observable area is determined using the location and field of view of
each camera on a two-dimensional (2D) ground plan of the target space. Importantly, this analysis may
not provide sufficient accuracy because it does not consider three-dimensional (3D) locations and the
distributions of the cameras, objects, and targets in the 3D space. For example, the coverage of a
camera significantly differs according to the height of the camera and the target plane. Such a
conventional 2D analysis may cause unnecessary overlapping coverage or over-estimated coverage.
Further research is therefore required to provide a quantitative evaluation of surveillance performance
including surveillance resolution or blind-spot calculations with the height in 3D space.
With the improved Building Information Modeling (BIM) technology, both the physical and
functional characteristics of a building can now be generated in a digital format [22]. As a result, the
creation, visualization, and simulation of a 3D virtual model of a building can be performed more
conveniently [23,24]. The BIM models allow for the manipulation of surveillance locations and
viewpoints; therefore, the idea of using the BIM as the basis for simulating CCTV coverage has been
proposed and verified to determine surveillance performance [25,26]. By referring to this idea, the
redundant overlapping coverage of CCTV could be effectively prevented with the generation of a 3D
model during the design phase of a CCTV system; however, it will still not be possible to determine
the quality of the surveillance performance using the resolution at which an object can be identified in
a given area. For example, the use of CCTV images to trace the movements of a suspect at a crime
scene may not provide an image of the suspect’s face; or even if it does, the resolution is insufficient
Sensors 2015, 15 23343
for facial recognition. This may occur because the surface direction of the target object was not
considered and the achievable resolution was not computed through a simulation process. Although the
target area appears covered at the ground level, the coverage of each camera narrows as the height
increases from the ground. As most targets are off the ground, the surveillance performance for the
target is lower than the results from the existing evaluation method, which calculates coverage at the
ground level. Additionally, the existing evaluation method assumes that the target is facing toward the
camera; however, targets are usually looking in a horizontal direction so that the surveillance
performance is significantly reduced when compared with the existing method.
In Figure 1, the weak points of the existing surveillance-performance evaluation method are shown,
whereby there are four people in the 2D coverage of a CCTV camera; therefore, complete surveillance
is achieved for the four people based on the existing method. The faces of the people, however, cannot
be recognized. Three-dimensional coverage of the face of the male in the black T-shirt is not
observable from the camera image, while the entire face of the female in the green jacket is hidden by
the male in the red polo shirt. The male in the red shirt is positioned with his back to the camera so that
his face cannot be detected from the camera image. The child is looking at the TV in a different
direction from the optical axis of the camera and is sitting in a lower position away from the optical
axis of the camera; consequently, the resolution of the camera image is insufficient to recognize his
face even though his face is in the 3D coverage. With commercial software such as VideoCAD, we can
check 3D coverage and simulate images on virtual avatars captured by a certain camera in a site
interactively through a 3D graphic interface. However, because the software does not provide any
comprehensive quantitative indicator about surveillance performance of the system, we still cannot
understand how completely a CCTV system monitors the target area in a certain level of detail [27].
Figure 1. It is not possible to observe the faces of the four people from the image even
though they exist in the 2D coverage of a CCTV camera.
In this study, we therefore propose new comprehensive indicators to quantify CCTV-surveillance
performance, which quantitatively represents how completely a target space can be monitored by a
CCTV system with a degree of detail enough for given surveillance requirements. Here, it is carefully
considered that the details of objects appearing in a CCTV image depend on types and specifications
of the sensors in the system and the orientation as well as location of the objects in the target area.
We also develop a performance evaluation method using the proposed indicators and applied it to a
real case to verify its feasibility. The remaining part of this paper is organized as follows: proposed
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concepts and methods are explained in Section 2; examples of surveillance coverage evaluation are
shown in Section 3; and conclusions are presented in Section 4.
2. Surveillance Coverage Evaluation
Surveillance performance indicates how effectively a target area is being monitored by a CCTV
system. As quantitative measures of surveillance performance, we propose the following two
indicators: surveillance resolution and surveillance coverage index. Surveillance resolution indicates
how closely a specific object can be monitored with a CCTV system, depending on the location and
orientation of the object as well as the cameras of the CCTV system. Surveillance coverage index
focuses on a specific region rather than an object, indicating how completely a region of interest can be
monitored with more than a specified surveillance resolution. The region can also be a path, an area, or
a 3D space as a subset of the object space; for example, to what extent a pedestrian path in a parking
lot, a crowded area in a mall, or the entire inner space of a building can be completely monitored may
be of interest. The resolution threshold can be established according to its own application; for
example, it can be two px/cm for facial recognition.
The evaluation process to derive the proposed performance indicators requires two kinds of inputs.
The first group includes those with almost constant properties—at least during the evaluation
process—which are a 3D geometric model of the object space, and the intrinsic and extrinsic
parameters of all of the cameras of the CCTV system. The second group includes the changeable
parameters, which are the resolution threshold and the regions of interest that are specified within the
object space.
In this section, we first describe the definition and derivation of the proposed indicators, the
surveillance resolution of an object with a specified location and orientation, and the surveillance
coverage index for a specified region of interest. We then explain the proposed evaluation process to
derive these indicators in an actual practical situation.
2.1. Surveillance Resolution and Coverage Index
The following four types of resolutions are used to define the quality of an image: geometric,
radiometric, spectral, and temporal. Among these resolutions, the geometric resolution is the most
effective when describing an object’s geometric properties such as position and shape. The geometric
resolution is typically expressed in terms of Ground Sampling Distance (GSD), which refers to the
distance of an object surface in a single pixel of an image. In this context, the surveillance performance
of a CCTV can be evaluated by observing the minimum GSD required to monitor an object. An
arbitrary length can therefore be set within the target area in the object space and the actual length
projected on the CCTV image can be calculated; we define the latter value as the surveillance
resolution and apply it when assessing the surveillance performance of a CCTV. The surveillance
resolution depends on sensor’s physical characteristics, such as the focal length, the principle point, the
pixel size, the projection type. It also varies according to the relative geometric relationship between
the camera and the object. Even with an object at the same position, the resolution can be different in
accordance with the orientation of the object surface. By considering these diverse factors affecting the
resolution, we derive a formula to derive the resolution as follows.
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When an object locating at a location with its surface normal is monitored by a CCTV camera, it is
projected to the image with a resolution. The defined surveillance resolution is represented as the ratio
between the actual length of an object and its projected length on the image, which is
defined as Equation (1). It consists of four terms that accurately model four steps of the projection
process from the object space to the image space. The steps are computing (1) the projected length
′ of on the surface where the object can be observed at a maximum resolution;
(2) the incident angles when the object is projected through the perspective center; (3) the
projected length of on the image according to a lens formula without any distortion; and
(4) the projected length of on the image considering distortions, respectively. Figure 2
illustrates the geometric meanings of the main parameters associated with the derivation of the defined
surveillance resolution.
(1)
Figure 2. Definition of surveillance resolution.
The first term considers the orientation of the object surface. In spite of the same object position, the
resolution of its projection can be different from the object’s orientation. The highest resolution can be
achieved when the surface normal corresponds to the direction toward the perspective center. The
length projected to the surface and directed to the perspective center can be derived as the following:
(2)
where is the angle between the actual orientation and the orientation resulting in the highest
surveillance resolution.
The second term transforms the projected length ′ into the range of the incident angle ,
which can be derived based on the arc length formula, as follows:
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(3)
where is the distance from the object to the perspective center; and is its projected length to the
optical axis, which is represented as .
The third term reflects a projection model. In a narrow angle system, it is usually assumed that the
central projection model is available, whereas different models apply for wide angle systems, which
are mostly utilized for CCTV systems. Some useful models are presented as Equation (4). Through a
lens of a focal length (, an object point in the direction is projected to a position distant from
the principal point. The first of the models in Equation (4) signifies the central projection model, while
the last term models the distortion correction. If we consider only the radial distortion of the lens, the
correction model can be expressed as Equation (5).
(4)
(5)
As shown in Figure 2, when an object locating at location with its surface normal is
monitored by a CCTV camera , it is projected to a CCTV image with a resolution . The
defined surveillance resolution is represented as the ratio between the actual length of an object
and its projected length , which is formulated as Equation (6). Here, it is assumed that the
camera follows the central projection without any distortion:
(6)
The vertical distance between the object and center of projection and the offset angle from the
optical axis are determined based on ’s location , and the camera’s position and attitude.
According to the orientation , the angle between the actual orientation of and the
orientation resulting in the highest surveillance resolution
is decided. The and
represent the normal vector of on the object surface and the normal vector of on the image plane at
the maximum surveillance resolution possible, respectively. In addition, focal length is computed
from the camera modeling process.
Although CCTV cameras will acquire images at a maximum resolution of if the object orientation
is facing
, the resulting image resolution will be lower when the object is placed on the surface
tilted by . When the angle of increases to 90°, the object will not be identifiable in the images.
Moreover, the surveillance resolution will have a negative value when is larger than 90°, and this
is the case when the opposite side of the object is projected to the image; for example, only the back of
a suspect is captured in the CCTV image when the intention was to observe the facial features of the
suspect. The negative surveillance-resolution value is not useful information in this case and it is
therefore replaced by 0.
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The camera parameters describe its projection characteristics. The intrinsic parameters are focal
length, principal point, and distortion coefficients, whereas the extrinsic parameters are the position
and orientation of the camera in an object coordinate system. As the camera parameters can be
estimated through a camera modeling process, such as self-calibration using the acquired CCTV
images and reference data, the intrinsic and extrinsic parameters can be assumed as known. In addition,
the same camera parameters can be applied in cases where the coverage originates from an identical
camera. The surveillance resolution of a camera is therefore derived from an object in a specific
location and orientation in a target space.
Referring back to Equation (6), and must be known to determine the surveillance
resolution at a certain location and orientation. As it is assumed that the CCTV camera’s intrinsic and
extrinsic parameters and the object’s location and orientation are known, the following equations can
be used to compute and . First, the distance from the object location to
the center of the projection and the unit vector of the optical axis in a 3D coordinate
system defined by the camera’s extrinsic parameters are calculated. Then,
the normal vector of the surface where the CCTV camera can monitor at maximum resolution
can be calculated using Equation (7), as follows:
(7)
Next, the off-axis angle from the CCTV camera’s optical axis is determined by Equation (8)
and Equation (9) is used to compute . In addition, the angle created by the actual surface where the
object exists, and the surface at which the CCTV camera can observe the object at maximum
resolution can be calculated with the relation shown in Equation (10), as follows:
(8)
(9)
(10)
The values of vary with respect to the local reference frame defined by the CCTV
camera and the object as expressed in Equations (8)–(10), and these values will determine the
surveillance resolution defined by Equation (6). Nonetheless, in general, multiple cameras are installed
over the target area and one object may appear in many camera images. As the relative position and
orientation differ between each CCTV camera and the particular object, each image will attain
different surveillance resolutions. Although there may be different surveillance resolutions for a given
object, the largest value will be assigned as the object’s surveillance resolution. Consequently, the
surveillance resolution of an object in a CCTV system with multiple cameras can be expressed as
the following, where is the total number of cameras in a CCTV system:
(11)
The surveillance resolution is calculated with , which are obtained with respect to
the object’s position and orientation in a space. To assess the surveillance coverage , the
surveillance resolution at every possible position and orientation in a given space is produced. Then,
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the percentage of the surveillance resolution that exceeds over a pre-defined threshold can be computed by
Equation (12), as follows:
(12)
where is the total number of positions sampled; is the total number of orientations sampled at each
position; and is the number of samples that meet the requirement in the bracket.
2.2. Coverage Evaluation Procedure
By applying the surveillance resolution and coverage index described in Section 2.1, the evaluation
of surveillance coverage can be conducted using the steps shown in Figure 3. First, we need to
generate 3D spatial models of the target surveillance area and determine the CCTV camera’s extrinsic
and intrinsic parameters. Next, samples are selected at each location, whereby the object is visible
from different orientations. Then, the surveillance resolutions at each of the sampled locations and
orientations are derived from Equation (1). Finally, the completeness of the surveillance coverage is
evaluated by computing the surveillance coverage index based on Equation (12).
Figure 3. Surveillance Coverage Evaluation Procedure.
In the first stage, we need to generate polyhedral models of the area from existing 2D architectural
floor plans or newly acquired sensory data. If we use the floor plans, rather than using other sensory
data, it is easier to create the corresponding 3D model of the building; however, floor plans contain the
detailed architectural design of a building, and inconsistencies between the designed model and the
actual building construction may exist. To accurately model the physical building, we need to acquire
the sensory data of the area, such as images and laser-scanning data; then, the polyhedral model can be
generated manually from stereo images or semi-automatically from point clouds. With the 3D
polyhedral model, we need to know the camera’s extrinsic and intrinsic parameters. For extrinsic
parameters, we have the location of the camera’s perspective center, expressed by three coordinates,
and its attitude, expressed by three independent rotation angles. To obtain the parameter values, we
distribute GCPs (Ground Control Point) in the area and perform bundle adjustment with images
including the GCPs. The intrinsic parameters describe the metric characteristics of a camera, and can
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be determined through a camera-calibration process using a specially designed calibration target. The
position of principal point, focal length, distortion amount, and pixel size are the main intrinsic
parameters. In this case, unlike most, we cannot change the camera’s position and attitude, so we must
position the calibration target on the floor in various directions and acquire images of the target.
In the second stage, we determine the amount of sampling desired from all of the possible 3D
locations of the object, along with their corresponding orientations, in the entire target space. For the
determination of sampling locations, we first define the target space in an arbitrary 3D Cartesian
coordinate system. Then, each of the axes is divided to form a 3D grid. It is possible to define the
surveillance resolution at each of the 3D grid points by using ; for instance, when
the three axes are split every 10 cm in a space, there would be a total of 1000 locations
to sample. On the other hand, the orientations that an object can face range from 0° to 360°
horizontally, 0° to 180° vertically, and 0 to for their solid angle. To include all of the possible
orientations, we divide the angles both horizontally and vertically using an arbitrary location as the
center. Using the same idea that applied to the 3D grid points, the surveillance resolution of all of the
orientations at a given location can be found by using ; for example, if the
sampling was conducted at an interval of 1°, the total orientation samples for a given location will be
. When the interval is increased to 10° or 45° to reduce the number of
orientation samples, there would be 648 and 32 orientations, respectively. This means that even when
orientations are observed at each location, the number of calculations for finding the
surveillance resolution reach 32,000 in a 1 m3 with 1000 locations.
The advantage of using such a method to sample at every given interval for both location and
orientation seems logical; however, the disadvantage is that each of the defined orientations do not
cover the same solid angle, whereby the solid angle covered by each orientation decreases as the
vertical angle increases. The vertices of the regular polyhedrons inscribed in a sphere or the center of
their faces therefore provide the same solid angle across the orientations for a location. The five
regular polyhedrons are the tetrahedron, hexahedron, octahedron, dodecahedron, and icosahedron.
For example, assuming a particular location as the center of an icosahedron, orientation sampling can
be made facing each of the vertices, which will provide the same solid angle throughout and the
number of samples will decrease to 12.
In the third stage, when deriving surveillance resolutions at each of the sampled locations and
orientations, we must analyze the visibility of the position from a camera by applying a ray-tracing
algorithm. With this algorithm, we define a ray from the position to the perspective center of the
camera and determine whether this line is intersected with other obstacles. If an intersection is
determined, the position is within the occluded area of the camera and we cannot define the
surveillance resolutions at the position. Before applying ray-tracing for each position, we need to
compute the horizontal coverage of every camera and the 2D Minimum Bounding Rectangle (MBR) of
all of the obstacles in the target area. Although the obstacles are defined in a 3D sense, most of them
are extended to the ceiling starting from the ground, with the same horizontal outline such as a pillar.
In this case, by examining the 2D overlap, we can determine whether it is overlapped in a 3D sense
without a complicated 3D process.
Ray-tracing for the calculation of the surveillance resolution at a position with an orientation by a
camera is performed as follows: (1) determine whether the position is within the horizontal coverage of
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the camera; (2) determine the 2D MBRs of the line between an object point (a sampled position) and
the perspective center; (3) check whether there is a 2D overlap between the MBRs of a line and an
obstacle; if there is no 2D overlap, stop at this step with no 3D overlap; (4) check whether the line
segment intersects with the 2D boundary polygon of the obstacle; if it does, stop at this step because
the obstacle has an identical horizontal outline according to the height, signifying that it also intersects
with the obstacle in a 3D sense; (5) determine if the line segment intersect with a 3D polyhedron
model of the obstacle; and (6) if there is an overlap, the position cannot be identified in the camera
image and the surveillance resolution is set to 0.
In the final stage, we compute the surveillance coverage index by comparing the achievable
resolution with the desired one. Here, we determine the proportion of the instances that reach above
the desired resolution among the resolution values computed at all the sampled locations and
orientations in the previous stage. The desired resolution can be established or derived from the
specified surveillance requirements for recognition and tracking. For example, one may want to
monitor every location in the target space with a resolution of 0.5 px/cm required for a meaningful
recognition process. Using the resolution values computed in the previous stage, we can easily
compute the proportion of the locations monitored with at least the desired resolution and present it as
the overall coverage index of the target space. In addition, we can visualize the computed surveillance
resolution at each location with different object orientations in the 2D/3D space to visually inspect the
weak and strong surveillance areas. Furthermore, we can check the coverage index for a special
surveillance sector such as a doorway or moving path in a parking area. In a doorway, one may want to
recognize even the faces of the people going in and out through an exit; and for the face recognition,
the required resolution of at least 2 px/cm may be assumed. We can also determine how well such a
requirement is satisfied in the space of interests in a quantitative way by checking the computed
resolution values at all the locations with different orientations within the target space. In addition, we
can identify the weak area and propose the location and orientation for additional camera installation to
fulfill surveillance requirements. Adding cameras or constructing a CCTV system involves a major
expense; therefore, we need an elaborate design for achieving the surveillance purpose before the
installation. In addition, there are available so many different cameras with different prices and
performance, and thus we can check the surveillance performance by changing the camera models or
the camera parameters to derive more optimal camera specifications and configurations.
3. Application Example and Analysis
3.1. Experimental Data
The underground parking lots of buildings such as apartments are the places where CCTV systems
are encountered in daily life. We therefore produced a simulation of a typical configuration and size of
a parking lot, like that in Figure 4, based on the concept of the proposed surveillance resolution for the
evaluation of the surveillance coverage of the target area. The surface area of the generated parking lot
is 5079.47 m2 with a height of 3 m. Additionally, the CCTV cameras were positioned to reflect the real
world, whereby the cameras were installed in pairs, facing the opposite direction. In addition, CCTV
cameras are usually installed on the ceiling of the path that the cars and people mostly use. Here, we
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assumed that each pair of cameras is rotated by ±12° in the -axis, and the approximate distances
between them are in the -axis and in the -axis. We also assumed that the focal length,
pixel size, and detector dimension of each camera are 10 mm, 5 µm, and 4000 by 3000 pixels,
respectively. In this case, the coverage of each camera is about 6 m by 4.5 m at a place of 3 m distance.
Figure 4. Arrangement of CCTV cameras in an underground parking lot.
3.2. Evaluation Results and Analysis
3.2.1. Overall Analysis
The possible locations of the object were sampled at each of the 3D grid points with an interval of
and the possible orientations of the object were sampled every in the horizontal and vertical
planes. Then, the observed surveillance resolutions from the CCTV cameras were determined for all of
the samples. In Figure 5, the horizontal position of the object monitored above a surveillance
resolution of 0 is displayed, when the object is located on the ground and is facing the
ceiling. Out of the total 20,541 ground locations, 13,186 locations can be observed in CCTV images,
and 64.2% of the ground surface is computed as the surveillance area. This evaluation of surveillance
coverage is similar to an existing method for determining blind spots, whereby the coverage of CCTV
cameras in 2D space is derived [17,28].
For example, to identify a suspect who has trespassed through a parking lot, the suspect should
appear in a CCTV image and his face should be recognizable from the image; however, it is difficult to
estimate the success of such an objective when the surveillance coverage is determined with the
existing 2D blind-spot-analysis method. Although slight parameter differences exist between CCTV
cameras, the resolution that is typically required for monitoring an object in detail is about 2 px/cm,
whereas it is about 0.7 px/cm for general surveillance (Theia Technologies, 2009). Accordingly, the
surveillance resolution must be at least 2 px/cm to distinguish the appearance of a suspect without a
criminal record. The surveillance resolution from the ground surface when the orientation of an object
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is upper vertical is illustrated in Figure 6; accordingly, a surveillance resolution exceeding over
2 px/cm is displayed in Figure 7. The resulting outcome produced 4930 locations, from the total of
20,541, as the positions that exceeded over 2 px/cm, signifying that, at this height, the suspect’s facial
features can be identified approximately of the time. With a CCTV-surveillance-coverage
evaluation method like that which is previously explained, the percentage of surveillance resolutions
that reach the required resolution to successfully fulfill the CCTV system’s purpose can be determined.
Figure 5. Areas with a surveillance resolution greater than 0 when objects are on the
bottom and their orientations are upper vertical (white: resolution > 0 ; black:
resolution = 0 ).
Figure 6. Surveillance resolutions when objects are on the ground and their orientations
are upper vertical (blue: resolution = 0 px/cm; red: resolution = 7 px/cm).
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Figure 7. Areas with a surveillance resolution greater than or equal to 2 px/cm when objects
are on the bottom and their orientations are upper vertical (white: resolution ≥ 2 px/cm;
black: resolution < 2 px/cm).
As the height of the object increases, the surveillance coverage significantly decreases, as shown in
Figure 8; this represents a surveillance coverage index with a threshold of 1 px/cm according to the
elevation when objects have an upper-vertical orientation. Figure 8c shows that the subject’s face
could be observed in the area with a probability of 19.7% if the individual’s face is at a 1.5 height
and looking in an upper-vertical direction.
Figure 8. Surveillance coverage index with a threshold of 1 px/cmaccording to the
elevation when objects have an upper-vertical orientation; (a) the index at the elevation of
0.5 m; (b) the index at the elevation of 1 m; (c) the index at the elevation of 1.5 m; (d) the
index at the elevation of 2 m.
Furthermore, the surveillance coverage index changes according to the object’s orientation. Figure 9
shows the surveillance coverage index with a threshold of 1 px/cm according to the orientation when
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objects are at an elevation of 0.5 m; 0.5 m is the general height of a car’s license plate. Figure 9f shows
that if an object is moving in the area looking downward, it is not possible to recognize the
object’s surface.
Figure 9. Surveillance coverage index with a threshold of 1 px/cm according to the
orientation when objects are at an elevation of 0.5 m; (a) the index with the eastward
orientation; (b) the index with the northward orientation; (c) the index with the upward
orientation; (d) the index with the westward orientation; (e) the index with the southward
orientation; (f) the index with the downward orientation.
3.2.2. Areal Analysis
The suggested methodology incorporates the geometric properties and movement trend of an object
to allow for a detailed evaluation of the surveillance coverage in a 3D space; for example, the red
rectangular area in Figure 4 represents the entrance from a parking lot into a building, such as an
escalator or elevator, where more careful monitoring is required. Surveillance coverage in such areas
can therefore be determined to analyze the vulnerability of the CCTV system when it comes to crime
prevention and reaction. For this analysis, we limited the vertical range—from 1.2 to 1.8 —in
consideration of the average height of a human face, and sampled the target space of this vertical range
with locations of 20 cm intervals, which is the space denoted with the blue line in Figure 4. At each
location, we considered four horizontal orientations with a 90° interval. The number of sampling of
locations and orientations in the area totaled 100(x) × 100(y) × 4(z) × 4 (horizontal angle) × 1 (vertical
angle) = 160,000, where all of their surveillance resolutions were determined. The head of a person
would roughly be at a height of 1.6 m, when the average height of adults is taken into account.
Illustrations of the surveillance resolutions at the 1.6 height when the orientation is in the direction
in which the maximum resolution is achieved are shown in Figure 10. The corresponding surveillance
coverage index is 23.7%, and it can be concluded that it is very difficult to verify the identity of an
individual in the target area using the CCTV system.
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Figure 10. (a) Surveillance resolutions when objects are at a 1.6 m height and their
orientation is in the direction in which the maximum resolution is achieved;
(b) Surveillance coverage index with the threshold of 2 px/cm.
To improve the existing monitoring quality, the installation of two additional CCTV cameras is
planned. We examined the difference of the target-area resolution after the additional cameras are
installed, with a possible location as the center and a rotation of ±12° in the -axis.
Figure 11 shows the surveillance resolutions at the height of 1.6 when the orientation is in the
direction in which the maximum resolution is achieved, after the cameras are added. In this case, we
can conclude that, by adding the additional cameras at the possible location, the surveillance coverage
for the target area was enhanced, as the surveillance coverage index increased from 23.7% to 38.7%.
Figure 11. (a) Surveillance resolutions when objects are at 1.6 m height and their
orientations are in the direction in which the maximum resolution is achieved; (b) Surveillance
coverage index with a threshold of 2 px/cm (after adding two cameras).
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Table 1 shows the surveillance coverage index at different elevations when an object is facing the
direction that provides the maximum resolution, before and after adding the cameras. From this, we
can check that the surveillance coverage for the target area is improved by approximately 60% after the
two cameras are added; however, as the height of the object increases, the surveillance coverage
significantly decreases.
Table 1. Surveillance coverage index before and after adding cameras (unit: %).
Z = 1.2 m
Z = 1.4 m
Z = 1.6 m
Z = 1.8 m
Total
Before
35.0
28.9
23.7
18.4
26.5
After
59.9
48.5
38.7
29.9
44.3
Finally, Figure 12 displays the surveillance resolutions at the most probable orientations at the
height of . From this, we can observe that, even though the location of the object is identical, the
surveillance resolution significantly differs between the orientations. In the case where the object is
facing in the direction, as illustrated in Figure 12f, the object does not appear in the CCTV image,
even with the additional camera. This implies that, when a person intentionally faces the ground as
they travel, their facial features cannot be observed from the CCTV system. To solve this problem,
additional cameras facing in an upward direction should be installed at a lower height range of 0 cm to
50 cm, as needed. After installing the additional cameras, we can enhance the surveillance coverage in
the area three dimensionally and omni-directionally.
Figure 12. Surveillance resolutions according to the orientation (after adding a camera); (a) the
resolutions with the +x direction; (b) the resolutions with the −x direction; (c) the resolutions
with the +y direction; (d) the resolutions with the –y direction; the resolutions with the −x
direction; (e) the resolutions with the +z direction; (f) the resolutions with the +z direction.
Sensors 2015, 15 23357
3.2.3. Path Analysis
The red line in Figure 4 represents the moving paths for vehicles in the parking lot, where careful
surveillance is required. We sampled the paths with an interval of 0.1 m and 0.5 m in horizontal and
vertical directions, respectively. At each location, we selected six orientations with a 90° interval. We
then computed the surveillance resolution at each sampled location and orientation. We presented the
surveillance resolutions of the sampled locations, when the orientation is the one direction where the
maximum resolution is attained, using vertical bars, as shown in Figure 13, where the length of a vertical
blue bar indicates the magnitude of the surveillance resolution. As shown in Table 2, the surveillance
coverage index with the thresholds of 0 px/cm and 1 px/cm are 87% and 83%, respectively.
Figure 13. Surveillance resolution at the sampled locations, when the orientation is the one
direction where the maximum resolution is attained.
Table 2. Surveillance Coverage Index (SCI) along a certain path.
Threshold ()
All Directions (E/W/N/S)
Maximum Direction (E/W/N/S)
Total
>Threshold
SCI
Total
>Threshold
SCI
0
13864
6908
50%
3466
3016
87%
1
13864
3192
23%
3466
2864
83%
4. Conclusions
Although the efficient design and installation of CCTV systems is recognized as important, there is
a lack of comprehensive indicators, which are useful to understand quantitatively how well a CCTV
system covers a target area while satisfying specific surveillance requirements. In this study, we thus
have proposed new indicators, surveillance resolution and coverage index, which allow us to evaluate
quantitatively the effectiveness of CCTV systems on the task-specific surveillance. The surveillance
resolution, indicating how closely an object can be observed by cameras, is derived from a rigorous
projection model from objects to cameras. The derivation reflects the 3D orientation as well as the
location of objects and cameras; and it can also be applicable to various kinds of cameras with
Sensors 2015, 15 23358
different projection and distortion characteristics. Based on the surveillance resolution, we defined the
surveillance coverage index representing how completely an area is monitored with a certain level of
detail. Using these two indicators and the presented derivation associated with them, we established an
evaluation process to enable versatile, practical and visual analysis on the CCTV’s surveillance
coverage. For example, one can derive the overall surveillance coverage of the entire target area, check
whether a specific interesting area (or path) is monitored with a required resolution, and provide
various alternatives to improve the current coverage. During these processes, one can easily
incorporate the dynamic and static attributes of objects and cameras, for example, the movement of
persons or vehicles.
In the near future, we will adapt the proposed evaluation approach to field problems related to the
regions of more strict specific surveillance requirements, for example, crime-ridden districts, subway
stations, complex malls, casinos, and other places. With the adapted approach, we can assess the
current status and provide appropriate solutions. In addition, using the proposed surveillance index as
the target values to be optimized, we can provide the optimal position and orientation of the cameras to
maximize the performance of a CCTV system according to its surveillance requirements.
Acknowledgments
This research was supported by a grant (14SCIP-B065985-02) from Smart Civil Infrastructure
Research Program funded by Ministry of Land, Infrastructure and Transport (MOLIT) of Korea
government and Korea Agency for Infrastructure Technology Advancement (KAIA).
Author Contributions
Kyoungah Choi and Impyeong Lee collaborated to perform the study presented in this paper.
Kyoungah Choi and Impyeong Lee defined the concepts and derived the mathematical formula related
to the concepts. Kyoungah Choi performed the experiments and analyzed the results. Kyoungah Choi
wrote the paper. Impyeong Lee revised the manuscript. All authors read and approved the manuscript.
Conflicts of Interest
The authors declare no conflict of interest.
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