The U.S. Park Police estimate of 175,000 people on the Mall for the papal mass yesterday was based on an analysis of a series of aerial videotapes made from a police helicopter during the day.

Police technicians placed a gridded overlay on the tapes and computed the number of people in each square. For densely packed areas near the papal altar, for example, police estimated that each person occupied 3.5 square feet of space. Under the formula, seated persons in less crowded areas were assumed to occupy about 8 square feet each.

The Park Police analysis of aerial photos from dawn to 4 p.m. showed this pattern of crowd growth: dawn, 3,000; 9 a.m., 10,000; 10 a.m., 20,000; 11:30 a.m., 45,000; 12:30 p.m., 65,000; 1:30 p.m., 85,000; 3 p.m., 100,000; 4 p.m., 175,000, the maximum for the day.

Police said the jump from 100,000 to 175,000 between 3 and 4 p.m. occurred after the pope's parade motorcade passed Constitution Avenue and many of the thousands of people along the parade route went to the Mall to hear the mass.