Barack Obama became the 44th President of the United States last Tuesday, January 20th at precisely 12:00 PM ET. Many converged on the National Mall for this historic occasion, but exactly how many made this historic trek? Thanks to satellite imagery, provided by GeoEye, an accurate estimate of attendance, or at least area, can be ascertained.

I downloaded the high-resolution satellite image and traced the population areas in Adobe Acrobat. Using the resulting polygons, the area can be calculated:

(Click the image for a larger version)

The effective scale is acquired by measuring the diameter of the Washington Monument's base (which we know to be 250 feet) on the image, and converting to real world measurements. In this case, each inch represents 500 feet. A table of each polygon's area is listed below:

Label Area (sq ft) Label Area (sq ft) 0 132,500 19 15,000 1 20,000 20 25,000 2 27,500 21 105,000 3 302,500 22 10,000 4 5,000 23 52,500 5 287,500 24 100,000 6 12,500 25 27,500 7 72,500 26 125,000 8 117,500 27 65,000 9 12,500 28 75,00 10 25,000 29 75,00 11 155,000 30 7,500 12 40,000 31 57,500 13 172,500 32 27,500 14 27,500 33 17,500 15 27,500 34 7,500 16 45,000 35 32,500 17 32,500 36 20,000 18 140,000 Total 2,365,000

Despite their appearance, these figures are not exact. The area calculations are quantized based upon the capabilities of Adobe Acrobat and the resolution of the image. It would have been better to use a tool like AutoCad to trace the regions, but even so, the resolution issue would still persist. The resolution of the image makes it impossible to accurately trace each and every crowd region. Based upon these assertions, and the knowledge that low-density crowds are not visible to a satellite, we know that some spectators were not and cannot be identified; their exact number will forever remain unknown. It then follows that our 2,365,000 sq. ft. figure excludes these spectators.

While we have a baseline measurement for the size of the crowd, we still do not know the quantity of the crowd. Fear not, our Clinton Era tax dollars were hard at work:

In the model, all people are modeled in circles individually to consider human spatial requirement. The diameter of a circle is determined as 0.4 meter with consideration of the size of body. Using the distance between evacuees and spatial requirements, interactions between people are assessed. It is an important factor in predicting congestion or contra-flows of a crowd. Source: Building and Fire Research Laboratory via NIST.gov

The area within a circle of diameter .4 m is 0.125 sq. meters; this then converts to a crowd density of one person per 1.35 square feet. If this density is uniformly applied to our baseline area of 2,365,000 ft. sq, we can obtain a minimum boundary for the number of inauguration onlookers; at least 1,748,445 people attended Barack Obama's Inauguration.