To See or Not to See – THAT is the View shed

With any type of security incident, it is imperative that the situation be assessed quickly and accurately so the necessary response can be taken.  In many cases, this involves the use of cameras via a video management system (VMS).  However, this is no small task as today’s systems can consist of tens, hundreds or even thousands of cameras.  With this many cameras, selecting the most appropriate camera with the best view of the situation can prove difficult.

Viewshed is an advanced display feature for Map Based Video Management Systems

Viewsheds can be used when trying to locate the best location and placement of a camera to maximize the views.

Automated VMS systems can be very helpful in facilitating this selection.  However, outside of selecting the camera where the alarm occurred, the next logical decision is to select the camera which is closest to the desired viewing location.  This approach often works, but there are many situations where the “closest” camera may be blocked by a building, a tree or a hill.  This can result in a loss of valuable time while attempting to find a more appropriate camera.  A more ideal solution would be to allow the VMS to help select the camera that is known to have an unobstructed view of the area of interest.  This concept of knowing what areas are visible to a specific camera is referred to as viewshed. 

What is ViewShed?

The idea of viewshed has been around for a long while.  In its most general definition, a viewshed is an area of land, water, or other environmental element that is visible to the human eye from a fixed vantage point. The term is used widely in such areas as urban planning, archaeology, and military science.   An architect uses viewsheds when trying to locate the best location and placement to maximize the views of a new home or building (Figure 1).  If the intent is to have a view of the river, viewshed will help determine the best location to place the home to ensure the million dollar view

The Forest Service uses viewsheds to determine tower locations to provide the best vantage point for forest fire detection and cellular companies use them to estimate cell tower coverage from a particular location.  From a military standpoint, viewsheds may be used to scout the best vantage points for observing the enemy or be used to postulate probable sniper locations, as viewshed will indicate where the sniper has a clear view to a specific location.

Use within the Security Sector

From a physical security standpoint, a more relative definition of viewshed is the area visible to cameras from their fixed locations.  If a point is within the viewshed, the camera can see that location without obstruction.  It is typically shown using an overhead map view of the region of interest.

Figure 2 further describes the concept.  The bottom portion of the image is a profile view, showing the fixed location of the camera.  The area shaded in grey represents parts of the camera view that are blocked due to changes in elevation.  As the view becomes obstructed that specific “view ray” from the camera is terminated and anything past that point is no longer viewable.

Profile and Overhead Viewshed for Fixed Camera

Figure 2 – The alignment of profile and overhead views showing how viewsheds are created for a fixed camera.

The top portion of Figure 2 represents the same scene as viewed from above.  The viewshed, representing areas the camera can view, is shaded in green.  You can see that the obstructions shown in the profile view result in large areas where the camera view is occluded.  These areas are not shaded, because they cannot be seen, and by definition, are not in the viewshed. The red lines further illustrate the relationship between the two views, by indicating the beginning and the end of the viewshed in both the profile and overhead views. 

Camera Considerations – Fixed vs PTZ

When thinking about viewshed, it is important to note that the definition refers to a fixed vantage point, but it does not restrict the ability to look right,  left, up and down.  The same is true when we refer to camera viewsheds.  Viewsheds for fixed cameras will result in a pie-like region, representing the portion of the camera’s frustum that is in view.  All fixed cameras will have some type of blind zone at their base, which increases with their mounting height.  This blind zone manifests itself in the viewshed as the clipped end on the pie shape (See Figure 2).

A PTZ creates a 360 degree viewshed

Figure 3 – Viewsheds are based on a specific vantage point, but it accounts for the ability for a PTZ camera to pan and tilt, resulting in a 360 degree pattern.

PTZ viewsheds are a slightly different animal.  Their view is not restricted.  From their mounting location, they have the ability to pan left or right and tilt up or down.  As a result, a PTZ’s viewshed is circular in nature, showing that it can view an entire 360° area (Figure 3).  Since a PTZ can typically look straight down, it also has little to no blind zone at its base.

Reverse ViewShed

At this point there are probably some human factors and user experience experts asking themselves, why would I take my beautiful high resolution map and shade the viewshed area, covering up the map details that may be important to the operator?  Indeed that seems fairly unintuitive.  Viewshed was not originally defined with the idea that it would be utilized as part of a user interface.  As such, a new definition was in order, and so the concept of “reverse viewshed” was introduced.

A reverse viewshed align with user experience and human factors

Figure 4 – A typical user interface will shade the reverse viewshed, indicating which areas the camera cannot see.

Reverse viewshed means exactly what you think it means.  It is the reverse of viewshed.  In terms of user interface, instead of shading viewshed (the areas that can be seen), the reverse viewshed is shaded (the areas that cannot be seen).  Using this method, a shaded area is a quick indication to the user that it is not viewable from the camera.  This results in a more intuitive user interface (Figure 4).

Enabling Viewshed Capability

So what does it take to allow your VMS to provide viewshed information (Figure 5)?  First off, viewshed is most advantageous when it is utilized within a VMS that includes some type of map-based user interface.  If you want to take it to the next level and utilize some of the dynamic aspects of viewshed, PTZ cameras should include absolute positioning and your VMS should have geospatial capabilities.

PureActiv VMS User Interface showing viewshed

Figure 5 – Use of reverse viewshed within the user interface of a video management system (VMS).

However, the main item which is essential to utilizing viewshed is a terrain or digital elevation model.  In addition to latitude and longitude, these models also provide elevation.  This data, combined with the location, height and angle of the camera, provides the necessary information to allow an understanding of what is viewable and what is not.  It is also important to note that most terrain models do not typically include man-made objects, such as buildings, water towers or billboards.  Nor do they include vegetation, like large trees or bushes.  To get the most accurate viewshed, these types of obstructions need to be added to the terrain model. 

Use in Planning

We’ve focused on viewsheds from the aspect of utilizing it within the user interface of VMS systems, however, much like an architect might utilize them for determine the best placement to maximize a home’s views, they may also be used by savvy security designers to understand the best placement locations for surveillance cameras.  This upfront cost may be well worth it, avoiding any future surprises where key surveillance areas do not fall within the cameras’ viewsheds.

Final Thoughts

From a user’s perspective, understanding what a camera can see and what it cannot see, is valuable information.  This is especially true during a quickly developing situation that requires immediate visual verification.  The concept of viewshed, although somewhat new to the physical security sector, has been used in other disciplines for many years.  Enabling this capability within video management and C2 systems is yet another way security technology is moving forward and providing added value and efficiencies for security operators

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