The US Constitution obligates the US Census Bureau to administer a nationwide census every ten years. In preparation, the bureau reviews previous census tract boundaries. When a tract’s population grows to over 8,000 persons, it may be split into two or more smaller census tracts. Oppositely, if the population in adjacent tracts drastically declines they may be combined into one new tract.
Here in the Coachella Valley, eight 2010 census tract boundaries were split, as shown in the map here. The eight previous tracts have been replaced with nineteen new ones. The bureau uses a decimal numbering system for tract id’s. The whole digits before the decimal retain the previous tract’s number. The new fractional or decimal digits are added sequentially to the end of the previous census tract list. For example, tracts 449.33 and 449.44 were added to the 449 series and their 449.15 “parent” is retired for this census. This facilitates more easily comparing census tract statistics over previous decades.