Dealing with Gaps in Surveillance Coverage

Gaps, or holes, in one's surveillance coverage is a fact of life for most organizations. Unfortunately, it can become a painful fact when incidents occur in areas where surveillance coverage is inadequate or missing. Often, users, management and outside observers cannot understand why this 'mistake' happened. In this note, we examine the key tradeoffs and options involved in dealing and minimizing gaps in surveillance coverage. We conclude with a video screencast to show these principles in action.

A February 2010 article criticizing surveillance in Mumbai rail stations is a good example of the frustration and concern gaps create:

• "Close-circuit cameras at most stations are fitted at improper angles and do not cover the entire area though the police often bank on CCTV images to solve crimes."
• "The CCTVs do not move 360 degrees and we get blurry images of offenders. The cameras should move at least 180 degrees. We have complained to the railways many times"

While each site is different and we do not know the details of this case, we can easily examine some general principles that impact surveillance usability:

• Very Narrow Areas For Clear Face Shots: If you want to capture a clear image of someone's face that can be used to identify a person, even with a 1080p HD camera, the maximum width of your cameras' FoV can be no more than about 30 feet or 10 meters. Unless you are in a very small confined environment everywhere (e.g., a bank branch), delivering such clarity/detail is extremely difficult / expensive. Inevitability, there are going to be areas captured by the camera but too blurry to be capable of identifying an unknown suspect.
• Restrictions on Camera Placements: Often, you know a certain area needs surveillance but have major logistical barriers placing a camera. This frequently happens when ceilings are very high or there are no walls or poles close by to mount the camera. In those cases, cameras are frequently mounted in areas where it is cheaper / less complex. Additionally, sometimes there are ideal places for mounting cameras but other stakeholders block camera placement for aesthetic reasons.
• Use of Choke Points: Because it is difficult to capture high quality video everywhere, most organizations focus their coverage on choke points (typically doorways/entranceways) that are relatively narrow and funnel people. Choke points are always a good tool. However, there are 2 common issues:
• Open Area Challenges: Some facilities, by design, are open areas. Common examples include parks, city centers and smaller rail stations. In these scenarios, there are vey few 'natural' choke points and a high probability that people will enter from many directions. 
• Tracking Suspect Back to Choke Point: Even if you have a choke point, incidents often occur outside of those choke points. An example is a person entering a stadium and then causing a fight near their seats. While the camera may capture a detailed image of the suspect entering, the video of the fight may be blurry. Tracking that person with a blurry image back to their entrance can be quite difficult.

For all of those reasons, depending on fixed surveillance cameras to saturate your grounds is a difficult proposition. Indeed, this explains the motivation and the frequent use of PTZ (controllable) cameras.

On the one hand, someone could argue that a PTZ has no gaps because when controlled it can capture in any direction over great distances (see our note on how far a PTZ can see). On the other hand, the PTZ is essentially blind in whatever direction it is not currently pointed. This can lead to a false sense of surveillance/security.  See our Should You Use PTZ Cameras? review for an in-depth analysis of PTZ tradeoffs.

In Action

In the video below, we examine a public park camera layout demonstrating the tradeoffs and gaps a surveillance designer faces:

In the video, we referred to our pixels reference chart and used the Pelco camera design tool [link no longer available].

Recommendations on Reducing and Dealing with Gaps

• Increased resolution is the most powerful new tool to reduce gaps. However, as we demonstrated in the above video, even multi-megapixel video will struggle to eliminate blurry or unclear video over larger areas.
• Since gaps in coverage are essentially inevitable, this must be communicated up front to key stakeholders so they have appropriate expectations
• Making clear the risks of potential gaps up front may also help the user to justify spending more money or getting approvals to place cameras in more advantageous locations