The mean center of population, traditionally referred to as the center of population, is provided for each census in the United States since 1790. The mean center of population is the point at which an imaginary, flat, weightless, and rigid map of the United States would balance if weights of identical value were placed on it so that each weight represented the location of one person. The mean center of population based on the 2000 census results is located in Phelps County, Missouri. For a complete list of the mean center of population for each census since 1790, and for a more detailed description of how these values are calculated, see http://www.census.gov/geo/www/cenpop/calculate2k.pdf.
GCMD keywords can be found on the Internet with the following citation:
Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 220.127.116.11.0