Some 7500 volunteers collected this info. They did this because at the last pro-choice march, the park police gave an estimate only half of what the crowd size actually was. As a matter of fact, the Million Man March organizers sued the park police because they drastically underestimated the numbers for that march. After that, the park police stopped doing estimates so the organizers of the April march decided to do their own count. Always before, the park department would take aerial photos from a helicopter and count bodies per square inch to come up with their estimates