gaz2015zcta5centroid set up by Jean Roth , jroth@nber.org ,  4 Dec 2015
Data source : http://www2.census.gov/geo/docs/maps-data/data/gazetteer/2015_Gazetteer/2015_Gaz_zcta_national.zip
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from http://www.census.gov/prod/cen2010/doc/sf1.pdf :
Internal point-
The Census Bureau calculates an internal point (latitude and longitude
coordinates) for each geographic entity. For many geographic entities,
the internal point is at or near the geographic center of the entity.
For some irregularly shaped entities (such as those shaped like a
crescent), the calculated geographic center may be located outside
the boundaries of the entity. In such instances, the internal point
is identified as a point inside the entity boundaries nearest to the
calculated geographic center and, if possible, within a land polygon.