On the 5th of september 2021, an anti Covid-19 protest march was held in Amsterdam, The Netherlands. Part of the discussion afterwards was how many demonstrators came to this protest march. This raises the question of how to calculate how many protesters are present at a specific location and a specific moment in time. In this article we explain a method that, according to the dutch news channel NOS, would have been used by the Dutch Police to estimate the number of people during the protest march.
Calculating the number of square meters
In order to estimate how many people are in a specific location, the first step is to calculate the size of an area. Normally, the area can be calculated by mutiplying the length of an area by its width. That would mean, that a football field with a length of 100 meters and a width of 64 meters has an area size of 6,400 square meters (m²). Using a website such as Mapchecking.com, the number of square meters on a map can be calculated by applying geographic parameters with simple mouse clicks. In the image below you can see that the number of square meters of “De Dam” in Amsterdam is around 14,000, depending on where exactly the geographical parameters are applied.
Please note: in practice the size of a demonstration area is much more difficult to calculate. Not only the length and the width of the location may be unclear (partly because of the shape of the location), also the location of a demonstration is often dynamic. In other words, demonstrators regularly mix up and move to other locations. In addition, it should also be taken into account that the maps and/or satellite images that are used may be dated.
Calculating the density of a crowd
If the size a location is known, it can be calculated how many people are at the location. This is done via the Mapchecking.com website by entering the density of the number of people per square meter, on the basis of which the total number of people on the total surface is calculated. In other words, suppose that there is exactly one person per square meter on a 6,400 square meter football field, that would mean that there are 6,400 people on the football field.
However, the question in reality is how many people are standing on a square meter during a demonstration. While this question may seem simple, the answer to this question is a lot more complex. During a demonstration, the number of people per square meter is not evenly distributed over every square meter, and it is also often the case that people move to different spots. It can be useful to take an average to calculate the density. This means that the average number of people per square meter is taken into account.
But how do you arrive at an average of the number of people per square meter during a demonstration? That can be quite complex too. First of all we recommend to use multiple data sources. In addition to camera images, think of drone imagery, imagery from social media and estimates from colleagues “on the ground”. So if there are an average of two people per square meter on a football field, it could be an estimation that there are a total of 12,800 people on the field. And at 1.5 people per square meter, the geographically marked area could hold well over 20,000 people, according to Mapchecking.com.
A few side notes
A few side notes
This article describes only one way to estimate the number of people on a demonstration. In reality, you will have to use multiple methods and different sources to arrive at the most accurate estimate possible. You should also take into account obstacles that reduce the possible area of a location that can be occupied. Finally, crowds are very dynamic: people move around on different areas, which can make it difficult to estimate the density of groups of people and thus the number of people at a location.