After successfully modeling a CCTV system in a BIM-based virtual building space, this research conducted a case study to examine CCTV coverage. To make the BIM-based virtual environment correspond better to a real situation, a BIM design model of an MRT station under construction in Taipei city was adopted for the case study.
The application of CCTV systems in public spaces, especially mass transit facilities, for surveillance and protection purposes has grown at a considerable rate (Sanderson et al., 2007; March Networks News, 2008). Some research, focused on long-term experiments of advanced Intelligent CCTV (ICCTV) technologies in some sensitive public spaces, like major ports and railway stations, has also been published (Bigdeli et al., 2007). In Taipei city’s MRT system, the CCTV systems are mainly used at platforms and in the lobbies of an MRT station for the following functions (Sun, 2005):
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When the MRT train stops at a station, the train driver can monitor the movement of passengers getting on and off the train via CCTV systems. This helps to ensure the safety of passengers.
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Station staff can monitor the activities in the station through the CCTV systems so that proper reactions can be made promptly in the case of an emergency.
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The traffic control center manages the operations of the entire MRT system with the help of the CCTV systems in MRT stations. The operators can visualize all situations in MRT stations and respond correctly to ensure the safety of the trains in service.
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Station staff can monitor, through CCTV systems, the traffic flows of passengers at all entrances and exits of the station and ensure that entry/egress is smooth.
The purposes of application discussed above also define the design requirements of the CCTV systems in the MRT system. For example, CCTV cameras are needed in certain locations on the platform to ensure full coverage of the movement of passengers getting on and off the trains. They are also needed at all entrances and exits of stations for good coverage of passenger traffic flows.
Modeling of smoke curtains and ceiling signboards
In order to examine any design conflicts between CCTV systems and other systems, several architectural components in an MRT station that may block the views of the CCTV cameras need to be modeled as parametric elements in the BIM model. These include smoke curtains and ceiling signboards.
Traditionally, the placement of a CCTV camera is subject to the conditions of the on-site environment, such as irregular ceiling heights, suspended signboards under the ceiling, smoke curtains, and so on. For example, in the real MRT station shown in Figure 13, the CCTV cameras are located lower than the normal height due to the influence of the smoke curtain. A lower location for a CCTV camera can increase the possibility of being intentionally destroyed and decrease the range of the CCTV coverage. If a better location can be found in advance using the BIM model, a suitable height for the CCTV camera can also be determined. However, the consideration of these architectural components has relatively low priority during the life cycle of the construction project and may not be modeled in the BIM model. This is also the case for the BIM model the authors obtained from the design company.
Thus, this research models the smoke curtains and ceiling signboards as parametric elements in the Revit model. Among predefined system family types in Revit, the Wall element is most appropriate for modeling parametric smoke curtains and ceiling signboards. By means of modifying several Wall properties, such as base constraint, base offset, top constraint, top offset, and thickness, the appearance of Wall elements can resemble smoke curtains and ceiling signboards. The elements with different locations, heights and sizes can be easily and parametrically modeled in the BIM model.
In order to distinguish smoke curtains from ceiling signboards, the two kinds of elements were assigned different materials, colors, and appearances. Smoke curtains are set to be transparent meshed acrylic sheets, as shown in Figure 14, while ceiling signboards are set to be red opaque acrylic sheets, as shown in Figure 15. In addition, Figure 16 presents a bird’s eye view of both smoke curtains and ceiling signboards on the concourse level of the MRT station.
Process of modeling CCTV systems with CCTV parameter advisor
The first step of modeling CCTV systems is to correctly position Revit cameras according to the existing design drawings. Thanks to the assistance from the Department of Rapid Transit Systems, Taipei city government, 2D plan views of the CCTV system in the MRT station were provided in CAD format and therefore could be imported into the BIM model directly. With reference to the imported CCTV plan view, Revit cameras can be placed in accurate positions on the corresponding floor plan view of the BIM model (see Figure 17). For modeling CCTV cameras with different FOVs, the developed plug-in program, CCTV setup advisor, can be used for calculating the corresponding crop region size (see Figure 18).
The second step is to determine the other two necessary parameters for the Revit cameras within reasonable ranges: the eye elevation and the target elevation. As previously mentioned in the Configuring Revit cameras with parameter analysis section, the eye elevation depends on the camera height, while the target elevation depends on the target height. Normally, the reasonable range of the camera height may be between 2.3 and 2.5 meters in the environment of an MRT station. Also, it is appropriate to apply the average height of the Taiwanese people to the target elevation, that is, approximately 1.7 meters.
After creating all necessary Revit cameras in the first step, the CCTV setup advisor is able to read in all Revit camera instances as well as the elevations of all floor levels, as shown in Figure 19. Subsequently, users need to key in the correct eye elevation and target elevation values for the Revit cameras (see Figure 8), which can be advised by the CCTV setup advisor after the selection of the desired Revit camera, the floor level, and the subject height, and the input of desired camera height. As for determining the subject height, it should be chosen based on the monitoring requirements of users (see the discussions in the Configuring Revit cameras with parameter analysis section and Figure 6). It should be noted that the selection of the subject height does not affect the simulation of the CCTV view but the effective coverage of the simulated CCTV. As shown in Figure 20, a CCTV screen view can be simulated by the corresponding Revit camera view. The simulated views are accessible when users choose a certain Revit camera view from the list of all 3D views via the Revit project browser.
More importantly, the CCTV setup advisor can suggest an optimized real angle of depression for the specified CCTV camera (see Figure 19). This may help to reduce the time for installation and adjustment of CCTVs because the best angle of depression for a CCTV camera has been determined in advance under user-desired conditions (i.e. an optimized real angle of depression exactly corresponds to a virtual CCTV screen view simulated by the Revit camera).
Checking for design conflicts
Prior to the use of 3D technology, detecting interferences between systems was both difficult and time-consuming. Nowadays, 3D BIM technology helps detect clashes in advance through more sensible visual presentations and more seamless collaboration platforms (Eastman et al., 2011: 272–273).
One of the aims of this research is to take advantage of BIM technology for inspecting in the design phase whether a desired location for CCTV camera installation is suitable in the future construction phase. Once CCTV screen views can be simulated via Revit cameras, it is simple to check if anything blocks the view of the CCTV camera in the BIM model. Examples of these are smoke curtains and ceiling signboards, which can be modeled as parametric elements in the BIM model of the MRT station. Such design conflicts must be discovered in advance and eliminated to reduce the installation and adjustment time required for the setup of CCTV cameras.
However, it is difficult to give a precise definition of design conflicts due to the fact that how seriously something blocks the CCTV camera’s view may depend on subjective assessment by the operators. A more objective way to evaluate whether there is a clash is to use the optimized real angle of depression suggested by the CCTV setup advisor. Once an optimized real angle of depression is applied, most interference is supposed to be avoided. If there is still an obstacle blocking the CCTV camera’s view, this implies that the parameters of the CCTV camera should be reset, including the camera height, target height, FOV, or even its original location.
Figure 21 shows a simulated CCTV screen view where almost half of the view is blocked by smoke curtains and ceilings. After adjusting several parameters of the Revit camera with the assistance of the CCTV setup advisor, a clearer view can be obtained as shown in Figure 22. The setup of a CCTV camera must be based on the monitoring requirements of users. The simulation of CCTV screen views offers an approach for CCTV designers to conduct both detailed modifications and subtle adjustments prior to real construction.
Evaluation and visual representation of CCTV coverage
Even with the highly developed VR technology used today in the construction industry, CCTV systems are more often designed in a 2D environment. As CCTV systems have been introduced in 3D BIM models in this research, it is convenient to display the overall 3D environment and the CCTV coverage area through both 2D and 3D VR approaches.
In order to display visual representations of CCTV coverage, it is essential to define the effective area covered by CCTV cameras. According to Figure 23, the red trapezoidal area reveals the coverage area of a certain CCTV camera. For a known FOV, the area can be determined by the subject distance (D) and the length . Because the subject distance (D) and other parameters, such as h, H, and FOV, have been defined in the Configuring Revit cameras with parameter analysis section, the length of can be calculated using the following steps:
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Assuming that the coordinates of the point O and the point Q are (0, 0, H) and (D, 0, h), respectively.
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The length of .
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The length of , where the point Q is the midpoint of the line segment, .
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Based on the screen aspect ratio (i.e. 3/4) as mentioned in Simulating varifocal lenses of CCTV cameras section, the coordinate of the point R is .
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The coordinate of the point P is and the points of O, P and R are in the same straight line. Therefore, the length .
Although the red trapezoidal area can be calculated through the mathematical approach, it is more convenient and efficient to use the “Filled region” function in Revit. Once an area formed by a closed loop of lines is specified, the corresponding filled region can be shown on the floor plan view, and the measure of that area can be calculated automatically.
After all “filled regions” are created for all CCTV cameras in the MRT station, it is crucial to check if there are any overlapping areas among those filled regions. If so, the filled regions with overlapping areas must be redrawn as a single closed loop area to obtain accurate calculations of the areas. Although this may take a lot of time to do, especially when there are many CCTV cameras in an MRT station, it is a necessary step.
Finally, the coverage ratio can be determined by dividing the total area of all filled regions by the overall floor area in the MRT station. The overall effective coverage of the CCTV cameras in the MRT station can be displayed on both 2D floor plan views and 3D model views. Figure 24 and Figure 25 show the CCTV coverage in 2D floor plan views of the concourse level and the platform level, respectively. Figure 26 and Figure 27 show the effective CCTV coverage in 3D model views of the concourse level and the platform level, respectively. An additional movie file is provided to show the CCTV coverage in the MRT station in more detail (see Additional file 2).
Additional file 2: The CCTV coverage in the MRT station. (MP4 10 MB)