How do you work out the size of a crowd?

The recent protests in Egypt were said to have been the biggest uprising in human history, with more than 30 million people taking to the streets to voice their opposition to President Morsi before he was ousted by the army.

But are those numbers right? Estimating the size of a crowd is a difficult business.

Hannah Fry, from the Centre for Advanced Spatial Analysis at University College London, explains why.