Geopath Viewsheds

A “viewshed” is the geographical area that includes all points from which an advertisement may be seen by traffic as it moves in various directions around an OOH media unit (display). Utilizing proper viewshed methodology is essential in order to responsibly convert circulation to opportunity to see impressions (OTS).
Failure to identify a unique and methodologically accurate viewshed for every individual display can result in undercounting, overcounting, inconsistent counting of OTS and/or a misidentification of exposed and unexposed audiences for OOH attribution measurement.

This document discusses:

The components that are considered in the development of each individual viewshed
The foundational geometric calculations that are used to create the viewshed
Best practices for curating viewsheds
Unique circumstances to consider
Examples and a glossary

Viewshed Components

Every media unit, regardless of format, has a unique viewshed.
The shape and magnitude of a viewshed is a function of:

The size of the display
The elevation of the display in relation to the audience
Location and orientation of the display
Place Type and Area type, i.e., the urbanicity of where the display exists
Visual obstructions
Direction of all modes of traffic

These attributes define where a display is observable at specific points within space around it. Outside of this viewshed boundary, the advertisement will be unconsumable.

Best Practices

Selecting an arbitrary noting radius or distance is not enough to assess the audience reached by an OOH ad unit.

As you can see in this example, while this car was indeed captured within the radius of this ad unit, it had no opportunity to see the ad, only the back of the structure.

Each OOH display has its own unique viewshed and links from which it can be seen.

Directionality

Even once an accurate viewshed is constructed, incremental assessments must be made to properly determine exposure.

Depending upon the approach of the audience, an ad unit may not be visible from all points within a viewshed. As a result, simply capturing devices “seen” in a viewshed is an invalid approach and will lead misclassification of exposed audiences.

Even though the car below was captured within the viewshed of this ad unit, it did not have an opportunity to see it since it was traveling in the wrong direction.

Field of View

The viewshed is intersected with the moving field of view of the passers by. This is a function of:

The orientation of the billboard
The direction of traffic passing the billboard
The distance of the billboard from the road

There is a point during the approach to an ad unit where the audience stops being able to see the display. The field of view of a driver is 120 degrees. Once the unit is past that point in the driver’s vision, the driver is no longer able to see the ad unit but is still within the viewshed.

Take this example:

At the bottom right of the image is a billboard, identified with a purple square. The viewshed for this display is shaded in yellow. A vehicle is approaching from the Northeast, circled in red. Once that vehicle crosses the blue line in its path, the billboard falls outside of the 120-degree field of view of the driver and is no longer visible for the remainder of its trip through the viewshed.

This is important when calculating the dwell time of the vehicle and especially vital for determining the number of ad plays this individual has the opportunity to see on a digital screen. The time required for the vehicle to pass entirely through the viewshed is more than twice as long as the actual dwell time for this in this example.

Visual Obstructions

Assessing the visibility of a display from different viewpoints is essential to determine the links and approaches from which the OOH media unit can actually be seen. In the example viewshed below, it appears from the aerial view that the inventory can be seen from multiple approaches, all circled in red.

In reality, there is only one approach from which this unit can be viewed, circled below. The unit is on the side of a train trestle. The roadway running Northeast to Southwest is an elevated overpass and none of the traffic has the opportunity to view the display. Additionally, the display is obscured from the view of the audience passing under that overpass until their vehicle is very close to the overpass itself.

Utilizing a simulation to assess the visibility of the display from the vantage point of each approach rather than a simple aerial viewpoint is essential to ensure that the exposed audience is not inflated.

Accurately Identifying Exposed Audiences Outside of the Viewshed

It is not enough to consider only the devices that are captured within a viewshed as exposed to an OOH advertisement.

Delivery of location data from mobile devices varies in frequency. In some cases, a member of an exposed OOH audience may deliver a “ping” while within the viewshed.

In other cases, however, the device may deliver a ping before entering a viewshed, deliver nothing while in the viewshed, but then ping again on the other side of the viewshed.

The device had to have passed through that viewshed and therefore is included as having been exposed to the ad unit. In the example below, the blue car sent a location ping prior to the viewshed and then again after passing through it. Both the orange car and the blue car were exposed.

Pedestrian Traffic

Unlike vehicular traffic, pedestrians and people traveling on bikes, scooters, etc. may be moving in any direction and on any side of a given link. In fact, during their occupancy within the viewshed, individuals may move in multiple directions. They may be wandering around while taking a phone call, buying lunch from a food truck, or chatting with a friend, and not moving in a single linear direction. As a result, the majority of pedestrians that cross into the viewshed have an opportunity to see a display.

Geometric Calculations

Effective Size

Whether or not an advertisement is able to be consumed by a viewer depends on how large it appears within one's field of view, or its effective size.

A person’s field of view can be described as a round three-dimensional shell centered on where an observer’s gaze is focused. When measured in degrees (like a protractor) a person will not be able to see something in their peripheral vision more than 60-degrees to the side measured from the focus point (in total 120-degrees in front of a person). Consider standing directly within a few feet of a large mural, it will take up a person’s entire field of view. Half-block away it may still take up a large portion, maybe 20-degrees. And several blocks away would be tiny relative to the observer’s field of view, maybe 4-5 degrees.

The calculation of the effective size of a display is highly dependent on how close the observer is to the display and the size of the display. However, the calculation of the effective size also must consider the angle at which the observer relative to the orientation of the display (direction the display is facing). Consider the figure below that shows a profile of eyesight looking at a display with an angle of alpha horizontally. The effective width, as seen by the viewer, is smaller than the actual width of the display if it is looked at straight ahead. A similar logic applies when considering the vertical angle for the effective height of the display. The elevation of the display above the ground comes into play as it directly correlates with the vertical angle the display is being seen. Effective width and effective height govern the overall effective size.

Maximum Noting Distance

If the effective size of a display within an observer’s gaze is larger than a minimum noting threshold, it can be considered an opportunity to see by that individual. As an observer moves farther away from a display, the effective size shrinks and past the threshold the display cannot be noticed. The distance at which the effective size is equal to the minimum noting threshold, is defined as the maximum noting distance.

Using eye tracking studies, Geopath has developed a set of thresholds for different environments. Within a store, or outside in a cluttered urban environment, a display needs to take up a larger proportion of an individual’s field of view to have an opportunity to be noticed. Along an interstate freeway in a rural area, a display will be noticed at a much farther distance as there are fewer visual distractions. We use Place Type and Area type (urbanicity) of the display to select the appropriate threshold defining when an observer has an opportunity to see a display.

As described above, the Effective size of a unit in an observer’s field of view can be calculated using the size of the display, it’s orientation and the relative location of the observer (to calculate the distance and angle to the display). Given the minimum noting threshold for a display, it is then possible to calculate ALL locations where the Effective Size is equal to the Maximum Noting Threshold. These points define the boundary of the Viewshed. The following figure shows an example of viewsheds in Peachtree Street, Atlanta, GA.

Glossary

Circulation – The total number of individuals passing a display within a specific time period regardless of whether or not they had an opportunity to see an advertisement.

Dwell Time – The amount of time an individual spends within the maximum noting distance of an OOH display.

Effective Size – The size that display appears within an individual’s field of view. It is dependent upon the angle the advertisement is in relation to the individual’s view as well as the distance from the viewer.

Display – The surface area on an OOH or DOOH unit where advertising copy is displayed. A structure may have more than one display.

Field of View – The area an individual can comfortably see at any given moment in time.

Likelihood to See Impressions -- The portion of the OTS (Opportunity to See) audience who are likely to see an ad. This is determined by applying a visibility adjustment to the OTS audience utilizing methodologies including eye tracking studies and facial detection technologies.

Link – A distinct segment of road or pedestrian pathway.

Maximum Noting Distance – The point at which a display become large enough within the field of view of an individual for it to reach a specific threshold making it able to be consumed.

Opportunity to See Impressions – The measure of the audience that can see an display based upon their proximity, approach, and dwell time. This measure does not take into account whether or not the individual made eye contact with the display.

Orientation - The cardinal direction that a display faces.

Traffic – The count of vehicles or pedestrians, passing a specific point. This value, be definition, is less than or equal to the circulation, a count of persons.

Viewshed – The geographic area from which an individual has an opportunity to see a display.