How do they “estimate” how many people were at events?

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For example, there were [an estimated 950.000 spectators](https://www.nnmarathonrotterdam.org/info/) at the Rotterdam Marathon of 2019.

The amount of spectators should be hard to estimate, because they don’t have to register anywhere and can travel by train, car, bike, or already live in the city…

In: Other

A photo is taken. Within the photo a fixed size is determines, like 5×5 feet. Within the square the number of people are counted. Then the total area being measured is divided into similar squares and the number of people in 1 square is multiplied by the total number of squares.