  For a group of pedestrians without any spatial boundaries, the methods of
density estimation is a wide area of research. Besides, there is a specific
difficulty when the density along one given pedestrian trajectory is needed in
order to plot an `individual-based' fundamental diagram. We illustrate why
several methods become ill-defined in this case. We then turn to the widely
used Voronoi-cell based density estimate. We show that for a typical situation
of crossing flows of pedestrians, Voronoi method has to be adapted to the small
sample size. We conclude with general remarks about the meaning of density
measurements in such context.
