by Jean Roth , jroth@nber.org , 2014-06-11 http://www.census.gov/prod/cen2010/doc/sf1.pdf : ------------------------------------------------ 21. Latitude in degrees, to seven decimal places, of a point within the geographic area represented by the summary level. The character immediately preceding the first digit of the latitude of an internal point identifies the direction (hemispher e): a plus sign (+) indicates the Northern Hemisphere; a minus sign (–) indicates the Southern Hemisphere. See Appendix A, “Geographic Terms and Concepts,” for definition of this field. 22. Longitude in degrees, to seven decimal places, of a point within the geographic area represented by the summary level. The character immediately preceding the first digit of the longitude of an internal point identifies the direction (hemisphere): a plus sign (+) indicates the Eastern hemisphere; a minus sign (–) indicates the Western Hemisphere. A point on the 180th meridian is assigned to the Western Hemisphere (–180.0000000). See Appendix A, “Geographic Terms and Concepts,” for definition of this field 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. ------------------------------------------------ precision: http://www.scientificcomputing.com/article/2009/06/numerical-precision-how-much-enough https://astronavigationdemystified.com/the-accuracy-of-astronomical-celestial-navigation Difference in double vs. float for mi_to_zcta5 for within 1 mile is about 1.74 feet!