Census GEO.ID prefixes versus county GEOIDs

A downloaded Census table can use a longer geographic identifier than the corresponding TIGER/Line geography file. Matching the entire strings without checking their structures can therefore fail. The Census Bureau gives Harris County, Texas, as an example. Its county GEOID is 48201: the two-digit state component 48 followed by county component 201. The table identifier 0500000US48201 additionally includes a summary-level prefix, geographic variant/component information, and US. Those leading characters are not part of the five-digit county code. [S1] For an ordinary census tract, the documented geographic structure is state plus county plus six-digit tract code. Different geography types have different structures. [S1] Before joining files, identify the geography type, document the relevant prefix, and compare the resulting key lengths. Keep identifiers as text so formatting is not accidentally lost. Do not apply one fixed trimming rule to every Census download. These are practical parsing checks; the linked Census documentation is the authority for the actual field definition. Additional source notes This note describes identifier structure. It does not guarantee geographic continuity across years: matching characters can still require a separate check of the geography's vintage and definition.