Charlie, you mentioned that the park service considers a “dense crowd” to be about 10 sq. feet per person, but the article you link to about the Inauguration crowd has a quote from someone named Barna (horrible editing they took out the reference of who this person is) who said that an average density is 1 person for 5 sq. feet and tightly packed is 2.5 sq. feet. Using these numbers (and your area measurement) since we at least have a name attached for the source, the crowd was more likely between 400,000 and 800,000. Thus the people estimating 500,000 to 600,000 should be right in the ballpark. These higher density numbers make sense to me for a standing crowd since 10 sq. feet per person is approximately 3 ft. X 3 ft. which is a lot of room between people.
