Center of U.S. population shifts again, and Census is on the trail

Path to the west and south continues into Texas County, Missouri

Over the past 220 years, the population center of the United States has shifted westward and southward as the country has grown.

When it was first computed the center was in Kent County on Maryland’s Eastern Shore, about 23 miles east of Baltimore, said David Doyle, of the National Geodetic Survey. “From 1790 to 1930 there is almost a straight line going due west. Then it starts to shift, turning to the southwest.”

As of 2010, the geographic center of the U.S. population shifted 23 miles, to 37 degrees 31 minutes north, 92 degrees 10 minutes west, in the northwest corner of Texas County, Missouri. On May 9, the Census Bureau and the National Geodetic Survey will mark this shift — figuratively and literally — by placing a survey marker that will become part of the NGS National Spatial Reference System.


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The marker will be part of a network of 1.5 million marks making up the system used to map and chart geographic position and height in the United States.

“We are trying to showcase the positioning technologies” available today, Doyle said. “The public today is vastly more spatially aware than they have ever been,” because of the growing availability of Global Positioning System data in consumer devices, from driving direction systems to smart phones.

The job of computing the center of population has become more complicated over the years as the population has grown, said Paul Donlin, programmer with the Census Bureau’s Geography Division.

“The formula is not complicated,” Donlin said, but the large amount of data needed to perform the calculation makes it complex.

The data comes from 12 million census blocks of varying sizes, from a city block to a sizable portion of a rural county. The center of population for each block is calculated and this data is used to find the weighted center point for the entire continental U.S. population.

“We need a computer to do that,” said Census geographer Ted Sickley.

The job was not always done with a computer. The first such calculation was done by hand in 1880, when the center was placed about one mile south of the Ohio River, across from Cincinnati in Kentucky. Eighty years later, the calculation was done by machine, according to a 1970 Census report.

“The population centers and the population counts for each of these areas were recorded on punched cards and then transferred to magnetic tape for processing through an electronic compute,” the report said. “The 'program' introduced into the computer controlled the mathematical processes which the computer executed.”

Today the data is stored in an Oracle spatial database and the computation is done using software-as-a-service, Donlin said.

Why go to the trouble of figuring this theoretical point? Each point is a visual expression of what people were doing at that time.

“It gives us a way of characterizing the population of the U.S.,” Sickley said. “It becomes useful when you look at it over time.”

From 1850 to 1860, there was a big jump westward from West Virginia to Ohio, reflecting the westward expansion of the country as new states were added and settled. In 1870, a slight shift to the north illustrated the migration to northern cities following the Civil War. From 1890 to 1940, movement slowed and the center remained in southern Indiana, reflecting the large European immigration into eastern cities. A southwesterly arc since then reflects the growth of the sunbelt states.

The National Spatial Reference System consists of about 1.5 million passive markers installed by the NGS over the last 200 years, as well as about 1,700 Continuously Operating Reference Stations that provide streams of real-time GPS data. The system is used by surveyors to accurately measure and chart the United States.

Since 1960, the NGS has marked the current population center with a commemorative survey marker. Until 1990, the mark was placed within a few centimeters of the actual point, Doyle said.

“That’s nice, but they’re out in the middle of a forest somewhere,” he said. Since 1990, the markers have been placed in the nearest incorporated community. This year, it will be next to the Post Office in Plato, Mo., (estimated population 1,430 in 2000), about three miles from the actual location of the population center.

 

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