Imagine if Oahu were a flat, weightless surface and all of its residents weighed the same. Based on point of residence, the island would balance perfectly 0.9 miles northwest of Red Hill Elementary School.

That hasn’t changed much in the past 20 years. However, neither have the population centers for the rest of the state.

After the census results are released, the state Department of Business, Economic Development and Tourism’s Office of Planning determines the population centers of each of the counties and the entire state.

This is determined by finding out how the area would perfectly balance by imagining the place as a flat, weightless and rigid map, and all of its residents as of identical weight. This is also called the mean center of the population.

The point of the exercise is to determine where the population migration shifts are taking place. Since the 18th century, the mainland’s population center moved from Maryland to Missouri, said Eugene Tian, acting state economist.

But Hawaii’s population center — in the Kaiwi Channel — has moved by far less than a single degree in latitude and longitude.

On Oahu, residents have moved toward the Leeward side, which shifted the island’s center by 0.1 miles since 2000. But Maui and the Big Island’s populations also have increased, offsetting any Ewa migration on Oahu when it comes to the state’s population center, Tian said.

The other population centers for 2010 are:

» Maui County: 1.7 miles southeast of the Wailuku post office.

» Hawaii County: 22.1 miles west of Hilo.

» Kauai County: 5.1 miles northeast of Knudsen Gap.

The state’s population center is 21.12 degrees north latitude and 157.48 degrees west longitude.