# Do You Know Where Your Centroid Is?

#### National Geodetic Survey marks the geographic center of U.S. population

###### May 9, 2011

Plato, a small town in Missouri, is the new "centroid" of the U.S. population based on 2010 U.S. Census data. Listen to an interview about the designation with NOAA's Chief Geodetic Surveyor, Dave Doyle:

Making Waves podcast

If you filled out the 2010 Census form from the U.S. Census Bureau, then you had a part in defining the centroidthe point where the center of the U.S. population falls. Every 10 years, after the Census Bureau crunches the numbers and figures out where the centroid is, NOAA’s National Geodetic Survey (NGS) places a geodetic survey disk (also called a survey marker, monument, or bench mark) in the incorporated community closest to its exact geographic location.

### How does the Census Bureau calculate the centroid for population?

NOAA Chief Geodetic Surveyor Dave Doyle, who has helped place a survey marker at the centroid every 10 years since 1980, says first to think of a rectangular-shaped state like Colorado. “If you imagine Colorado as a perfectly flat plain without all the mountains and valleys, and you put it on a map, the place where this rectangular shape balances perfectly on a point is the centroid.”

“Then, think of the United States, including Alaska and the territories, as a flat plain and put it on a map. Imagine also that every individual in the population weighs exactly the same,” he continues. “The centroid is where the center of the population is located.”

The process that the Census Bureau uses to figure out the centroid for the U.S. population is more complicated, but that’s the basic idea.

### Why is it useful to know where the centroid is?

The Census Bureau has been tracking the centroid for a long time. Every 10 years, another point is plotted on the U.S. map. Connecting the dots forms a snapshot of how the U.S. population has shifted over time.

"What you notice,” says Doyle, “is that from roughly 1790 when the centroid was in Kent County, Maryland, to about 1920 when it was in Indiana, the centroid moved almost in a straight line to the west. Then, around 1930, the trend moved southwest, with the influx of people moving to places like Texas, New Mexico, and California.”

NGS got involved with the effort back in 1960, when the Census Bureau asked NGS’s predecessor agency to place a geodetic survey disk at or very close to the exact coordinates of the centroid.

“The data from that survey marker – its latitude, longitude, and elevation – are then integrated into the network of other survey control points that we refer to as the National Spatial Reference System (NSRS),” Doyle says. NOAA and its predecessors have maintained some version of this national reference framework ever since Thomas Jefferson established the Survey of the Coast in 1807.

The NSRS is a huge network of about 1.5 million monuments, or bench marks, around the nation. These reference points form the foundation for all of the nation’s geographic mapping needs, including nautical charting, aeronautical charting, and topographic mapping.

While this has always been important to NOAA and a host of other scientific and technical entities, Doyle notes that the advent of the Global Positioning System made the NSRS relevant to nearly everyone.

"Almost everybody has some sort of positioning capacity in smart phones and iPads, in their cars and trucks, or on their bikes or wristwatches,” he says. “The individual’s ability to position him or herself has become an important part of our national infrastructure. At the heart of that is this rather invisible piece of infrastructure – the NSRS – for which NOAA is largely responsible.”

### Where is the 2010 Centroid?

"The 2010 Centroid is located in the town of Plato, Missouri, in the south-central part of the state about 10 miles south of Fort Leonard Wood,” Doyle reveals.

He led a team out to the site for reconnaissance in April so that they could pin down the exact location. Plato’s a small place, with a population of approximately 109.

"The residents are pretty excited about this,” he says, “and that’s one of the great things about the project. Bringing this large and somewhat vague concept to an actual community helps people understand how important everyone is in the context of the entire country.”

The Greek philosopher Plato, for whom the tiny town was named — author of The Republic — would no doubt agree with Doyle on this point.

A formal dedication ceremony sponsored by the Census Bureau was held in Plato on May 9 at 1 p.m. Dave Doyle was there, together with the directors of the National Geodetic Survey and the U.S. Census Bureau. A good portion of Plato’s populace was present, too.