TIGER/Line(TM) Files, 1992 Chapter 5: Data Quality Introduction This section provides detailed information on the lineage, positional accuracy, attribute accuracy, logical consistency, and completeness of the 1992 TIGER/Line files. Data users can use this information to help evaluate the adequacy and applicability of this geographic file for a particular use. Lineage Geometric Properties Generic source codes that specify the original digital source of complete chains in the TIGER/Line(TM) files are listed in Appendix H. These source codes are derivations of five separate categories of source codes in the Census TIGER data base: original source, computer operations, office operations, enumerator operations, and local official updates. The initial sources used to create the Census TIGER data base were the USGS 1:100,000-scale Digital Line Graph (DLG), USGS 1:24,000-scale quadrangles, and the Census Bureau's GBF/DIME-Files. The DLG coverage is extensive, albeit of variable currency, and comprises most of the rural, small city, and suburban area of the TIGER/Line(TM) files. GBF/DIME-File coverage areas were updated through 1987 with the manual translation of features from the most recent aerial photography available to the Census Bureau. In order to create a current geographic data base for the 1990 decennial census, the Census Bureau used various internal and external procedures through 1990 to update and maintain the Census TIGER data base. While the Census Bureau has made a reasonable and systematic attempt to gather the most recent information available about the features that this file portrays, the Census Bureau cautions users that the files are no more complete than the source documents used in their compilation, the vintage of those source documents, and the translation of the information on those source documents. The Census Bureau added enumerator updates compiled during 1988-1990 census operations to the Census TIGER data base. The updates came from map annotations that the enumerators made as they attempted to locate living quarters by traversing every street feature that appeared on a set of large scale TIGER-generated map sheets. The Census Bureau digitized the enumerator updates directly into the Census TIGER data base without geodetic controls or the use of aerial photography to confirm the features' existence or location accuracy. The Census Bureau also made other corrections and updates to the map sheets supplied by local participants in Census Bureau programs. Unconfirmed local updates originate from map reviews by local government officials or their liaisons. Maps were sent to the highest elected official of a governmental unit in 1989 and 1990 for use in various census programs, and some maps were returned with update annotations and corrections. The Census Bureau generally added the updates to the Census TIGER data base without extensive checks when the elected official approved the boundary or feature correction. The governmental unit supplied ordinance numbers if the changes affected a legal boundary. Changes made by local officials do not have geodetic control. Address Ranges and ZIP Codes(R) The 1992 TIGER/Line files contain potential address ranges and ZIP Codes(R) for most areas of the United States where city-style address ranges exist. The maps in Figure I-1 and Figure I-2 show the extent of address coverage for the lower 48 States and the District of Colombia. The address ranges in the urban core of metropolitan areas and some additional areas are the same as those in the 1990 Census TIGER/Line files. For the most part, these address ranges and ZIP Codes(R) have had only minor changes since the release of the 1980 GBF/DIME-Files. All other address ranges and ZIP Codes(R) (those added since the release of the 1990 Census TIGER/Line files) are derived from the 1990 decennial master list of addresses, the ACF. ZIP Codes(R) are derived from two sources: those already existing in the 1990 Census TIGER/Line files and those derived from the ACF. These ZIP Codes(R) will be updated and corrected by matching the Census TIGER data base with updated USPS ZIP+4 files in a future version of the TIGER/Line(TM) files and all subsequent versions of the TIGER/SDTS(TM). Source Codes For complete chain and landmark features, the TIGER/Line(TM) files identify a 1-alphanumeric character source code, see Appendix H. The TIGER/Line(TM) files use the address impute flag fields to identify the general source of the address range. * A value of 0 or 1 indicates that the address range was entered/ corrected in the Census TIGER data base from a precensus source including the 1980 GBF/DIME-Files, the extension areas, and corrections from the initial vender file match. * A value of 2 or 3 indicates that the address range was entered/ corrected based on the ACF. Record Locations: Source Codes RT Field Name Description 1 source Source code for the chain (excluding the geographic entity codes) 1 FRIADDL "From" address (left side of complete chain) 1 TOIADDL "To" address (left side of complete chain) 1 FRIADDR "From" address (right side of complete chain) 1 TOIADDR "To" address (right side of complete chain) 6 FRIADDL Additional "from" address (left side of complete chain) 6 TOIADDL Additional "to" address (left side of complete chain) 6 FRIADDR Additional "from" address (right side of complete chain) 6 TOIADDR Additional "to" address (right side of complete chain) 7 source Source code for the landmark feature Positional Accuracy General The Census Bureau's mission to count and profile the Nation's people and institutions does not require positional accuracy in its geographic products. Its files and maps are designed to show only the relative positions of elements. For features based on the DLG files, the positional accuracy of the information is no greater than the established National Map Accuracy standards for 1:100,000-scale maps from the USGS (approximately +/- 167 feet); thus, it is not suitable for high-precision measurement applications such as engineering problems, property transfers, or other uses that might require highly accurate measurements of the earth's surface. The Census Bureau cannot specify the accuracy of feature updates or features derived from the GBF/DIME-Files. Geographic elements should show positional consistency with elements in adjacent data sets; therefore, they should not require edge alignment. Visual comparison tests against source materials were made with check plots. Attribute Accuracy Geometric Properties The attribute accuracy of the TIGER/Line(TM) files is as accurate as the source used during the creation or update of the Census TIGER data base. Accuracy statements on the Census TIGER data base are based on deductive estimates. No field tests for attribute accuracy have been conducted on the files, although in most cases the operations and procedures followed by the Census Bureau ensure these attributes are as accurate as possible. For one source (USGS's DLG), the Census Bureau made a visual comparison with test plots. The Census Bureau overlayed element identification numbers on a graphic source and visually cross-referenced them with an attribute printout containing corresponding identification numbers. Experience suggests that attribute codes reflect the graphic source with less than a two-percent error. The feature network of complete chains (as represented by Record Types 1 and 2) is complete for census purposes. Data users should be aware that they may not be able to trace a specific feature by name or by CFCC as a continuous line throughout the TIGER/Line(TM) files without making additional edits. For example, State Highway 32 may cross the entire county. The TIGER/Line(TM) files will contain complete chains in the file at the location of State Highway 32, but the complete chains may individually have one of a collection of local names such as S. Elm Street, or Smallville Highway, with or without State Highway 32 as an alternate. The most frequent CFCC for a state highway is A21, but the complete chains at the location of State Highway 32 may have a variety of class codes such as A01, A40, or A21. Boundaries and Geographic Entity Codes The Census Bureau collects and tabulates information for both legal and statistical entities. Record Types 1, 3, A, and F identify the boundaries and codes for legal entities as reported to the Census Bureau to be legally in effect on January 1, 1990. Record Type G identifies the boundaries and code changes for counties and places with populations of at least 2,500, certified to be in affect on January 1, 1992, and occasionally later. They also contain the final 1990 census tabulation geographic entity codes for those entities. These boundaries are based on the annotations made by local officials in response to the Census Bureau's Boundary and Annexation Surveys. CSAC's generally define and delineate statistical entities following Census Bureau guidelines. However, there are several exceptions: UA's are defined strictly by the Census Bureau based on technical considerations; school districts, delineated by State Departments of Education; and VTD's, by an agency selected by the Governor of each State. Other attribute data in the TIGER/Line(TM) files were gathered from many sources. The Census Bureau's staff linked the attribute information to the spatial framework of features. Most procedures for gathering the needed attributes are clerical. The quality of these attributes was ensured by various tests conducted before, during, and after the time that the attribute information was entered into the Census TIGER data base. Tests included source material selection and evaluation checks, quality control checks on staff work, independent reviews by local and tribal leaders of maps produced from the Census TIGER data base, and staff reviews of computer-performed operations. Address Ranges and ZIP Codes(R) The conversion from the GBF/DIME-Files to the TIGER format involved neither verification of previously existing address ranges nor any significant updates or corrections (except as noted below). Prior to the release of this 1992 TIGER/Line files, the address ranges for an area were generally the same as those in the corresponding 1980 GBF/DIME-File. Preparations for the 1990 census involved making some minor updates in selected areas, but generally did not include changes in address numbering systems during the decade. The 1992 TIGER/Line files include ACF address ranges for existing and new features identified during census operations. Users of the 1992 TIGER/Line file's address ranges should check for address range overlaps, gaps, odd/even reversals, and other situations that may be incorrect. Although an address range in the TIGER/Line(TM) files may be incorrect, the Census Bureau implemented procedures to ensure that the error did not adversely affect the accuracy or the quality of the 1990 census. For the geographic areas with the GBF/DIME-File and extension area coverage, the Census Bureau used the address ranges to perform an initial assignment of residential addresses (purchased from a commercial vendor) to the 1990 census tract and block numbers and made a number of corrections to the address ranges. Later during field operations, enumerators updated, verified, and corrected, when necessary, the addresses assigned to each block number by walking the perimeter and all interior streets of each census block. Logical Consistency General Node-line-area relationships satisfy topological requirements. These requirements include: 1. Complete chains must begin and end at nodes; 2. Complete chains must connect to each other at nodes; 3. Complete chains do not extend through nodes; 4. Left and right polygons are defined for each complete chain element and are consistent for complete chains connecting at nodes; 5. Complete chains representing the limits of a file are free from gaps. The Census Bureau performed automated tests to ensure logical consistency and limits of file. Some polygons in the TIGER/Line(TM) files may be extremely small such that the polygon internal point has been manually placed on a node that defines the polygon perimeter. These small polygons have been detected, and corrections will be incorporated in the Census TIGER data base in the future. Programs and software have been developed by the Census Bureau programmers. The Census Bureau uses its internally developed Geographic Update System to enhance and modify spatial and attribute data to the Census TIGER data base. Standard geographic codes, such as FIPS codes for States, counties, municipalities, and places are used when encoding spatial entities. The Census Bureau has done spatial data tests for logical consistency of the codes during compilation of the original Census TIGER data base files. Completeness General The GBF/DIME-Files and the USGS's DLG were the two main sources of spatial attribute data. For these two sources, data for a given category will contain attribute codes that reflect the information portrayed on the source. The digital line data for a given category of data contain at least the same level of content and detail as shown on the source. The TIGER/Line(TM) files also use the Census Bureau's internal coding scheme which parallels the FIPS codes in some cases. Information used to create the file is as complete as possible. The feature network of complete chains is complete for census purposes. Enumerators identified new and previously unreported street features for the entire Nation during a series of decennial census operations. In some areas, local officials reviewed the census maps and identified new features and feature changes. The TIGER/Line(TM) files contain limited point and area landmark data. Enumerator updates do not include landmark features except for possibly a few rare instances.