Falls Church image analysis and urban planning firm Digital Design & Imaging Service calculated the total crowd of Saturday’s Women’s March on Washington at 440,000 people, give or take 50,000.

DDIS used a photo taken by a specialized tethered surveillance balloon using a proprietary camera at 525 feet at 2:46 p.m. on Jan. 21.

According to DDIS, this elevated perspective is used to help calculate the relative density, coverage area and location of the event’s crowds as they crossed the National Mall to Pennsylvania Ave.

This is how they calculated the number: on the image above, there are two separate overlays. The yellow dots represent an individual person, which has been hand counted and sampled for density. The red, yellow, and orange polygons are a general indicator of density by zone. The DDIS team calculated out an average density of 1 person per 9 square feet for the red zones, 1 person per 25 square feet for orange, and 1 person per 200 square feet for yellow.

DDIS stated that “due to our limited viewshed, it is fair to assume that this total could fluxuate by +/- 50,000.”