In Zagreb on July 16, 2018 at the Ban Jelačić Square there was a reception for the Croatian national football team. The reception turned out to be the largest assembly in the recent history of Zagreb and therefore it was practically impossible to cover all present persons in a single photography and count them. However, based on publicly available photographies, it is possible to calculate the crowd density if there are objects in the photography whose spatial distances are known. On the photography below the distances between columns are known and they can be measured by using e.g. the Google Maps service and then use three of them to calculate the area of the triangle which they encompass. After than the people in the triangle are simply counted and the crowd density is calculated. The triangle on the photography was chosen so that it contains a group of people without any banners that would make counting more difficult and there are 517 persons in it on an area of roughly 117 square meters giving a crowd density of roughly 4.42 persons per square meter in the triangle.
An impression about the relation between the triangle and the whole square can be obtained by taking a look at the photography below.
In order to check whether this crowd density is persistent on other seemingly highly crowded parts of the square, it is enough to take a look at the photography below.
The photography was taken at different time and it there are 1705 persons on it. Since there are not enough columns or similar reference object, the area of the shown surface cannot be directly calculated. However, based on parts in the lower left and right and upper left corner it is possible to assume at which part of the square is the surface located.
Since this part of the square is enclosed by columns whose distances are known, by applying perspective transformation it is possible to get approximate coordinates of the green quadrangle based on their relation to the column coordinates denoted by the red rectangle.
Based on the green quadrangle coordinates is is possible to calculate that its area is roughly 400 square meters, which would mean that the average crowd density in it is 4.26 persons per square meter. Although the calculation is only approximate, this result is very similar to the previous one. The reason why this number is lower is probably the occultation of several persons by the big flag and public lighting lamp in the photography.
For the sake of comparison, it makes sense to analyze the crowd density from the demonstration for the curricular reform held on the same square on June 1, 2016 since it was allegedly one of the biggest demonstrations in the recent history of Zagreb. If the photographies with reference objects for distance calculation are analyzed, it is possible for these same photographies to be perspectively transformed as shown below.
If the tram and the empty space are ignores, the crowd density on the shown photography is 3.38 persons per square meter, which is significantly lower than the crowd density at the reception of the Croatian national football team. A more detailed analysis is available in the video below.