TIGER/Line(TM) Files,1992 Chapter 1: An Overview and Geographic Concepts Overview What Is TIGER? The Census Bureau's Census TIGER System automates the mapping and related geographic activities required to support the decennial census and sample survey programs of the Census Bureau starting with the 1990 decennial census. The Census TIGER System provides support for the following: * Creation and maintenance of the digital geographic data base that includes complete coverage of the United States, Puerto Rico, the Virgin Islands of the United States, American Samoa, Guam, the Commonwealth of the Northern Mariana Islands, the Republic of Palau, the other Pacific entities that were part of the Trust Territory of the Pacific Islands (the Republic of the Marshall Islands and the Federated States of Micronesia), and the Midway Islands. * Production of maps from the Census TIGER data base for all Census Bureau enumeration and publication programs. * Ability to assign individual addresses to geographic entities and census blocks based on polygons formed by features such as roads and streams. The design of the Census TIGER data base adapts the theories of topology, graph theory, and associated fields of mathematics to provide a disciplined, mathematical description for the geographic structure of the United States and its territories. The topological structure of the Census TIGER data base defines the location and relationship of streets, rivers, railroads, and other features to each other and to the numerous geographic entities for which the Census Bureau tabulates data from its censuses and sample surveys. It is designed to assure no duplication of these features or areas. The building of the Census TIGER data base integrated a variety of encoding techniques such as automated map scanning, manual map "digitizing," standard data keying, and sophisticated computer file matching. The goal was to provide automated access to and retrieval of relevant geographic information about the United States and its territories. TIGER Data Base Extracts In order for others to use the information in the Census TIGER data base in a geographic information system (GIS) or for other geographic applications, the Census Bureau releases periodic extracts of this data base to the public, including the TIGER/Line(TM) files. Various versions of the TIGER/Line(TM) files already have been released, the previous one being the 1990 Census TIGER/Line files that accompanied the 1990 decennial census data products. The current 1992 TIGER/Line files were produced following a requirement by the U.S. Department of Education; it will contain all updates and revisions since the 1990 Census TIGER/Line files were produced. Relationship of the TIGER/Line(TM) to 1990 Census Statistical Data What makes the TIGER extract products particularly valuable to the GIS environment and the data user community is the direct linkage between the 1990 decennial census data products and the TIGER data base extracts. TIGER's digital description of the Nation's legal and statistical entities includes Federal Information Processing Standards (FIPS) codes and the Census Bureau codes so that these can be matched easily with the 1990 census data. Please refer to the Census Bureau Publication, 1990 Census of Population and Housing Tabulation and Publication Program for a description of the Public Law (PL) 94-171 data files, Summary Tape Files (STF's), and other sources of data from the 1990 census. Related Files The TIGER Geographic Names File(TM) provides the bridge between the geographic entity codes (i.e., State, county, minor civil division [MCD], etc.) found in TIGER/Line(TM) files and their official names. It is included on each of the 1992 TIGER/Line files CD-ROMs and also is available on magnetic tape or tape cartridge from Customer Services (see Acknowledgements for information). The STF's provide 1990 statistical data for a wide range of subject headings and geographic entities compatible with the TIGER/Line(TM) files. These files are available from Customer Services on tape and CD-ROM. The PL 94-171 Program data files provide selected population data for small area geography (State, county, county subdivision, place, census tract/block numbering area [BNA], block group [BG], and block) and are compatible with the TIGER/Line(TM) files. These files are available on tape and CD-ROM from Customer Services. TIGER SDTS(TM) is a relational data file following the FIPS SDTS. These files provide data equivalent to the TIGER/Line(TM) files with additional relational data linkages and data content more similar to the Census TIGER data base. Prototypes of the file have been released. For more information, contact the Geographic Base Development Branch of the Geography Division. The TIGER/UA Limit File(TM) contain just the features that form the boundaries of the 1990 census urbanized areas (UA's) along with the codes in a reduced TIGER/Line(TM) file format. These files are available from Customer Services. The TIGER/Line(TM) 103rd Congressional District File contains just the features that form the boundaries of the districts of the 103rd Congress. The files follow the format of Record Types 1 and 2 of the TIGER/Line(TM) files; each set of files covers one State. These files are available from Customer Services. County-Based Files The geographic coverage for a TIGER/Line(TM) file is a county or statistically equivalent entity. (See Appendix A for a list of State and county codes and Chapter 4 for a description of county equivalent entities). The county files have a coverage area based on their January 1, 1990 legal boundaries obtained in response to the Census Bureau's Boundary and Annexation Survey. Any corrections or further changes to the county boundaries will appear in Record Type F, which identifies corrected 1990 legal boundaries and Record Type G, which identifies current geographic entity changes; they will not affect the file's coverage area. Even though the Census TIGER data base represents a seamless national file with no overlaps or gaps between parts, the county-based TIGER/Line(TM) files are designed to stand alone as an independent data set. The files can be combined to cover the whole Nation and its territories (see the Single-Side Flags section in Chapter 3). The Data Content of the TIGER/Line(TM) Files The TIGER/Line(TM) files contain data describing three major types of data: Line features including: roads railroads hydrography Miscellaneous transportation features and selected power lines and pipe lines boundaries Landmark point landmarks such as schools and churches area landmarks such as parks and cemeteries Polygon geographic entity codes for areas used to tabulate the 1990 census statistical data locations of area landmarks. The line features and polygon information form the majority of data in the TIGER/Line(TM) files. Some of the data describing the lines include coordinates, feature identifiers (names), feature classification codes address ranges, and geographic entity codes. Chapter 3 details these data items and Chapter 4 defines the geographic entities and codes. The TIGER/Line(TM) files contain point and area labels that describe landmark features. These features provide locational references for field staff and map users. Area landmarks consist of a feature name or label and feature type assigned to a polygon or a group of polygons. Landmarks may overlap or refer to the same set of polygons. For more details on landmark data, see Chapter 3. Topology and Spatial Objects in the TIGER/Line(TM) Files Spatial Objects in the TIGER/Line(TM) Files The Census TIGER data base uses a collection of "spatial objects," points, lines and polygons, to model or describe real world geography. The Census Bureau uses these spatial objects to describe features such as streets and assigns attributes to these features to identify and describe specific features such as the 500 block of Market Street in Philadelphia, Pennsylvania. The TIGER/Line(TM) files contain information about the spatial objects distributed over a series of record types. Users of the TIGER/Line(TM) files may need to link information from several record types to find all the attributes of interest that belong to one spatial object. The final section of this chapter includes a description of the record types. Topology Spatial objects in the Census TIGER data base are interrelated. A sequence of points define line segments and lines segments connect to define polygons. The Census Bureau uses topology as the foundation for organizing spatial objects in the Census TIGER data base to explain how points, lines, and areas relate to each other. The Census TIGER data base uses these points, lines, and areas to provide a disciplined, mathematical description of the earth's surface features.Topology provides a basic language for describing geographic features. The TIGER System relates information to points or 0-cells, lines or 1-cells, and polygons or 2-cells. The number preceding the "cell" identifies the dimensionality of the object; for instance, a line segment has a single dimension: length. Each of these objects builds on the others to form higher-level objects. The 0-cells form the end points of 1-cells. The 1-cells connect at 0-cells and form closed figures that partition space into polygons or 2-cells. Terminology The terms point, line segment, and polygon are familiar but general terms that may have different meanings to data users working with a variety of different applications and and data sets. The TIGER/Line(TM) file documentation uses the specific terminology from the SDTS. Since the first release of the TIGER/Line(TM) files, the U.S. Geological Survey (USGS) has coordinated the development and release of the SDTS, now a FIPS standard (see FIPS PUB 173). The SDTS specifies a series of terms and definitions for spatial objects. Appendix B lists the SDTS definitions for spatial objects. Why use the SDTS terminology? Even though the TIGER/Line(TM) files do not follow the SDTS format, the TIGER/Line(TM) documentation will use these terms and definitions in order to promote a common language for describing geographic data and to facilitate the transition to the SDTS. The spatial objects in the TIGER/Line(TM) files embody both geometry (coordinate location and shape) and topology (the relationship between points, line objects, and polygons) and therefore belong to the "geometry and topology" (GT) class of objects in the SDTS. In the SDTS, nodes represent point objects (0-cells) that identify the start and end position of lines or 1-dimensional objects (1-cells) called chains. The chains in the TIGER/Line(TM) files are complete chains because they also form the polygon boundaries and identify the polygon identification numbers and geographic entity codes for these polygons. Topological chains that do not reference the polygons are network chains. Data users may choose to not use the polygon or geographic entity codes and consider the TIGER/Line(TM) files as a source of network chain data. Figure 1-1 illustrates the relationship between nodes and complete chains. The figure shows two complete chains forming a central road; a start and end node define each complete chain. Complete chains that meet at an intersection share the same node. As the figure suggests, complete chains may consist of one or more line segments that describe the shape and position of the complete chain. Shape points define the line segments and are not part of the topology of the TIGER/Line(TM) files. Shape points and the resulting line segments are attributes of the complete chains. When complete chains link node to node and form a closed figure (a 2-cell), a GT-polygon results. The GT-polygon containing Friendship Park in Figure 1-1 is bounded by five complete chains that share five nodes. GT-polygons are elementary units; they are not subdivided into smaller polygons. They are space filling and do not overlap. The geographic entities and area landmarks in the TIGER/Line(TM) files are associated with one or a set of GT-polygons. The TIGER/Line(TM) files contain point landmark data that are not included in the Census TIGER data base topology. Point landmarks are entity points that mark the location of points of interest and are not connected to complete chains or GT-polygons. The following table summarizes the terms for spatial objects in the TIGER/Line(TM) files: Point Line Polygon (0-cell) (1-cell) (2-cell) Topology Node Complete Chain GT-polygon or Network Chain Non-topology Entity Point Attribute Shape Point Features The TIGER System uses the term "feature" to informally describe spatial objects at a level higher than the spatial objects identified above. For instance, Main Street is a feature that may consist of a series of complete chains with the same name. The TIGER System identifies complete chains but does not identify features or link complete chains to features. Left- and Right-Side Data Fields If one is standing on a complete chain at the "start" node facing the "end" node, data listed in the fields carrying a right qualifier would be found to the right of the complete chain. Note the position of the start and end nodes for the road in the central section of Figure 1-1; the right- side of the complete chain corresponds to GT-polygon 1 and the left-side corresponds to GT-polygon 2. Data users can collect the necessary complete chains to construct polygons and features that intersect from the information contained in this basic record. Single-Layer Topology All spatial objects in the TIGER/Line(TM) files exist in a single data layer that includes roads, hydrography, railroads, boundary lines, and miscellaneous features; they are topologically linked. For instance, nodes mark the intersections of roads and rivers. Subsurface features such as tunnels or above surface features such as bridges also create nodes when they cross surface features even though there is no direct connection. Introduction to the TIGER/Line(TM) File Structure Basics The TIGER/Line(TM) 1992 are extracts of selected information from the Census TIGER data base, organized as topologically consistent networks. The records in the TIGER/Line(TM) Files, 1992 represent features traditionally found on a paper map. Each complete chain is classified by codes that describe the type of feature it represents. The TIGER/Line(TM) 1992 consists of 14 record types that collectively contain geographic information (attributes) such as address ranges and ZIP Codes(R) for street complete chains, names, and codes of feature types, codes for legal and statistical entities, selected 1980 census geographic entity codes, latitude/longitude coordinates of linear and point features, landmark features, area landmarks, and area and polygon boundaries. A separate file exists for each of the 14 record types for each county or county equivalent only where data exists for that record type and county. The TIGER/Line(TM) 1992 data dictionary contains a complete list of all the fields in Record Types 1 through 14 (see Chapter 6). Separate chapters cross list the fields by feature attribute and geographic entity type. The following section provides a summary level description of the TIGER/Line(TM) Files, 1992 record types. Description of the TIGER/Line(TM) 1992 Record Types Record Type 1 -- Basic Data Record for Complete Chains Record Type 1 provides a single record for each unique complete chain in the 1992 TIGER/Line files. The basic data record contains the end nodes for the complete chain. This record also contains address ranges and ZIP Codes(R) (for most areas of the country where a street name/house numbering system existed at the time of the 1990 census) and the 1990 census geographic entity codes for each side of the complete chain. Record Type 2 -- Shape Point Coordinates Record Type 2 provides an additional series of latitude and longitude coordinate values that describe the shape of each complete chain that is not a straight segment. Record Type 3 -- Additional 1990 and 1980 Decennial Census Geographic Entity Codes Record Type 3 includes the 1990 voting district (VTD) codes provided to the Census Bureau for the 1990 Census Redistricting Data Program. Record Type 3 also includes some 1980 census geographic entity codes and 1990 census geographic entity codes not included on Record Type 1. Record Type 4 -- Index to Alternate Feature Identifiers Record Type 4 provides an index to alternate feature names associated with the complete chain (Record Type 1). A Record Type 4 will not exist for a Record Type 1 that has only one name. A complete chain can have more than one alternate name. Record Type 5 -- Feature Identifier List Record Type 5 contains a list of all unique feature names for complete chains in the 1992 TIGER/Line files. Each name (or feature identifier) has an identification code number (FEAT). Record Type 6 -- Additional Address Range and ZIP Code(R) Data Record Type 6 provides additional address range information for a street complete chain when the information cannot be presented as a single address range (e.g., the house/building numbers are not uniformly arranged to form an address range). Record Type 6 appears only for those counties that have address ranges and ZIP Code(R) information in the Census TIGER data base. There is no assurance that the address ranges provided on Record Type 6 will cover fewer addresses than the address ranges appearing on Record Type 1. Data users must use Record Type 6 to obtain the complete picture of the potential address ranges along a complete chain. Note that the address ranges used for geocoding along corporate corridors and corporate limit offsets appear only in Record Type 6. Record Type 7 -- Landmark Features Record Type 7 contains the area and point landmarks from the Census TIGER data base. If a county file has no landmarks, no Record Types 7 or 8 will exist for that county file. Record Type 8 -- Polygons Linked to Area Landmarks Record Type 8 links the polygon identification codes with the area landmark identification codes. If a county file does not have any Record Type 7's, it also does not have Record Type 8. Record Type A -- Additional Polygon Geographic Entity Codes Record Type A exists for every polygon in the Census TIGER data base. The Census Bureau provides the basic 1990 census geographic entity codes (State, county, county subdivision, place, American Indian/Alaska Native Areas [AI/ANA's], census tract/BNA, block) on this record type to assist those data users who are interested only in polygon information. The Census Bureau has reserved several fields for possible future use; however, it has not established a schedule for future versions of the TIGER/Line(TM) file. Record Type F -- Corrected Geographic Entity Codes for the 1990 Census Record Type F contains the corrected 1990 census and FIPS codes for governmental units. This record contains 5-character-field block numbers. Record Type G -- 1992 Geographic Codes and Entity Changes Record Type G provides updated 1992 census and FIPS codes for governmental units based on the 1992 Boundary and Annexation Survey. Record Type I -- The Link Between Complete Chains and Polygons Record Type I links the complete chains in Record Type 1 to the polygons that are provided in Record Type P. A Record Type I exists for every Record Type 1. When Record Type I is linked to a single-sided (county boundary) Record Type 1, it will provide only the left or right polygon identifier. Record Type P -- Polygon Internal Point Record Type P exists for every polygon in the TIGER/Line(TM) files and identifies an internal point for each polygon. The TIGER/Line(TM) files include all complete chains and polygons in the Census TIGER data base. The topology of the Census TIGER System will ensure that a one-to-one relationship exists between the polygons constructed from Record Types 1 and 2 and Record Type P. Record Type R -- Record Number Range Record Type R contains the range of unique complete chain record numbers assigned to a census file in a nationwide scheme. Record Type R has the lowest (minimum allowable), and the highest (maximum allowable) record numbers for the range. Numbers are assigned to complete chains beginning at the lowest value. The current number is the highest record number for the census file used. The Census Bureau assigns a range of permanent record numbers to each partition of the Census TIGER data base. Partitions are based on initial county and county equivalent boundaries existing prior to boundary corrections and updates made during decennial census operations. Because a county/county equivalent may reside in multiple partitions and a county/county equivalent forms the coverage area of a TIGER/Line(TM) file, Record Type R has one record for each census file partition that contains parts of the county. The Relationship Between Spatial Objects and TIGER/Line(TM) Record Types Note that the TIGER/Line(TM) files do not have specific record types for each of the spatial objects. A record type is not available for individual nodes; node coordinates appear in Record Type 1. A full definition of a complete chain requires information from Record Types 1, 2, and I. Record Types 1 and 2 alone describe the set of network chains. GT-polygons require the combined information of Record Types 1, 2, I, and P. Linkages Between Record Types Figure 1-2 shows the record linkages between the 14 record types. All the record types, except Record Type R, contain fields (such as TLID, FEAT, CENID, POLYID, and LAND) that are used to link data from the record types together. Chapter 2 discusses the identification codes in detail. Some of the links are direct while others are indirect requiring a connection through an intermediate record type. Chapter 3 discusses how to link data about different types of spatial objects. Record Types 1, 3, and A contain the geographic keys -- the 1990 census geographic entity codes -- to the Census Bureau's statistical data (the PL 94-171 data and the several STF's). Data users can use the geographic area codes to move the data tabulations into a new file or into a GIS for processing and display.