IV. THE TIGER/LINE CENSUS FILES, 1990: A TECHNICAL DESCRIPTION The 1990 Census TIGER/Line file is an extract of selected information from the TIGER data base, organized as a topologically consistent line network. Each computer record in the 1990 Census TIGER/Line file represents features traditionally found on a paper map. Each line segment is classified by codes that describe the feature it represents. The normal geographic coverage for a 1990 Census TIGER/Line file is a county. The files can be combined to cover the whole Nation and its possessions. Each 1990 Census TIGER/Line file consists of 12 record types that collectively contain geographic information (attributes) such as address ranges and ZIP codes for street segments (only in areas covered by the 1980 GBF/DIME-Files), names and codes of feature types, codes for legal and statistical entities, selected 1980 census geographic area codes, latitude/longitude coordinates of linear and point features, landmark features, area landmarks, and area and polygon boundaries. The 12 record types are on the tape as 12 separate files. The 1990 Census TIGER/Line file Data Dictionary contains a complete list of all the fields in Record Types 1 through 12. Data users will need Record Types 1 and 2 to develop maps for a given area and Record Types 3 through 6 for geocoding requirements. The following is a description of the 1990 Census TIGER/Line file record types. Summary of Record Types Record Type 1 Basic Data Record Record Type 2 Shape Coordinate Points Record Type 3 Additional Decennial Census Geographic Area Codes Record Type 4 Index to Alternate Feature Names Record Type 5 Feature Name List Record Type 6 Additional Address Range and ZIP Code Data Record Type 7 Landmark Features Record Type 8 Area Landmarks Record Type A Additional Polygon Geographic Area Codes Record Type I Area Boundaries Record Type P Polygon Location Record Type R Record Number Range Record Type 1: Basic Data Record Record Type 1 provides a single record for each unique line segment in the 1990 Census TIGER/Line file. The end points of the line segments are expressed in latitude/longitude coordinate values in degrees and decimal fractions of a degree to six decimal places. This record also contains address ranges and ZIP codes (in selected areas) and geographic area codes for each side of the line segment. By convention, if one is standing at the "from" coordinate position facing the "to" coordinate position, data listed in the fields carrying a right qualifier would indeed be found to the right of the line segment. Data users can collect the necessary segments to construct polygons and features that intersect from the information contained in this basic record. Record Type 2: Shape Coordinate Points Record Type 2 provides an additional series of latitude and longitude coordinate values that describe the shape of each line segment that is not straight for the associated Record Type 1. All coordinate values are expressed in degrees and decimal fractions of a degree of latitude and longitude. The decimals are carried to six places to permit the representation of lines that are very close to one another. If the segment in Record Type 1 is a straight line, there will not be a Record Type 2. Record Type 3: Additional Decennial Census Geographic Area Codes Record Type 3 includes the 1990 voting district codes provided to the Census Bureau for the 1990 Census Redistricting Data Program. Record Type 3 also includes some 1980 census geographic area codes and 1990 census geographic area codes not included on Record Type 1. The 1980 census block numbers are available only for areas covered by the 1980 GBF/DIME-Files. During the conversion of the 1980 GBF/DIME-Files to the TIGER data base format, some 1980 census block numbers may have been deleted or changed. Users are advised to check all 1980 census geographic area codes, especially any 1980 block numbers, before using them in a planned application. The Census Bureau has not verified any of the 1980 census geographic area codes in these files. There will be discrepancies between the geographic area boundaries and codes in these files and the 1980 census maps, which are the basis for the 1980 census tabulations. The TIGER/Line files may contain 1980 block numbers for portions of the country where the Census Bureau did not tabulate 1980 census data by block or block group. These situations occur because these portions of the TIGER file originated from the 1980 GBF/DIME-Files that extended beyond the 1980 block-numbered area. Data users concerned about the validity of 1980 block numbers are advised to discard all 1980 block numbers that do not correspond to block numbers in the 1980 MARF or 1980 STF files. Record Type 4: Index to Alternate Feature Names Record Type 4 provides an index to alternate feature names associated with the basic line segment (Record Type 1). A Record Type 4 will not exist for a Record Type 1 that has only one name. A line segment can have more than one alternate name. For example, Record Type 1 may have the name Main Street, while Record Type 4 points to the segment's alternate names, such as 5th Street and State Highway 101, contained in the list of feature names (Type 5 records). If no Type 1 record has an alternate name, there will be no Record Type 4 file. There is no assurance that a feature that consists of multiple-line segments will consistently have the same name as the primary (on Record Type 1) and alternate names on the Record Type 4 index. Record Type 5: Feature Name List Record Type 5 contains a list of all unique feature names for line segments in the 1990 Census TIGER/Line file. Each name has a Feature ID code. Record Type 4 uses these Feature ID codes as a "shorthand" for the alternate names. Feature names for landmarks are in Record Type 7. Record Type 6: Additional Address Range and ZIP Code Data Record Type 6 provides additional address range information for a street segment 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). Additional ZIP Codes, if any, also appear in Record Type 6 for corresponding address ranges. Record Type 6 appears only for those counties that have address ranges and ZIP Code information in the TIGER data base. There is no assurance that the address ranges provided on Record Type 6 will be "shorter" than those appearing on Record Type 1. Data users must use Record Type 6 to obtain the complete picture of the potential address ranges along a segment. Record Type 7: Landmark Features Record Type 7 contains the area and point landmarks in the Census Bureau's TIGER data base. During the extraction of this data, we assigned a temporary Landmark Identification Number that we use to link the landmark attributes to the polygons that comprise the landmark. If there is no landmark in a county file, there will be no Record Type 7 or Record Type 8 for that county file. Record Type 8: Area Landmarks We use Record Type 8 to link the polygon identification codes with the area landmark identification codes. Thus, there will be from 1 to n Type 8 records for each area landmark, depending on the number of internal polygons in the Census Bureau's TIGER data base. If a county file does not have Record Type 7, it also will not have Record Type 8. Record Type A: Additional Polygon Geographic Area Codes Record type A exists for every polygon in the TIGER data base. We also are providing the basic 1990 census geographic area codes (State, county, county subdivision, place, American Indian/Alaska Native area, census tract/BNA, block) on this record type to assist those data users who are interested only in polygon information. We have reserved several fields for possible future use; we have not established a schedule for future versions of the TIGER/Line file. Record Type I: Area Boundaries Record Type I links the segments in Record Type 1 to the polygons that are provided in Record Type P. We provide a Record Type I for every Record Type 1. When Record Type I is linked to a single-sided (county boundary) Record Type 1, only the left or right polygon identifier is provided. Record Type P: Polygon Location Record Type P exists for every polygon in the TIGER data base. Because the file includes all lines in the TIGER data base in the TIGER/Line file as Record Type 1 records, there will be a one-to-one relationship between the polygons constructed from Record Types 1 and 2 and Record Type P. In constructing polygons from Record Types 1 and 2, users are cautioned to be sure their software can handle the necessary coordinate precision and does not "snap" lines that are close together. Record Type P that represents a polygon contains an internal point for that polygon. Some of the polygons (approximately 400 nationwide are so small that the internal point given may be identical to a point on one of the lines bounding the polygon or identical to one of the nodes. Depending upon the software or hardware employed by the data user, the internal point may be identified to the data user as being outside the correct polygon or a neighboring polygon may be identified as possessing 2 internal points. Record Type R: Record Number Range Record Type R contains the minimum, maximum, and current high values for the permanent Record Number within each census file partition. Because each TIGER/Line file can reside in multiple partitions, there will be one record type R for each CENID in the TIGER/Line file for any county or statistically equivalent entity. LINKAGE In the 1990 Census TIGER/Line file, the record number is the linkage between Record Types 1, 2, 3, 4, 6, and I. Linkage from Record Type 4 to 5 is on the feature ID code. Record Type I relates to Record Type 8 by CENIDL, CENIDR, POLYL, AND POLYR. Record Type 8 relates to Record Type 7 by LAND. Record Type P relates to Record Types 8 and I by POLYID and CENID. Record Types 1 and 3 contain the geographic keys-the 1990 census geographic area codes-to the Census Bureau's statistical data (the summary tape files). Data users can use these geographic area codes to move the data tabulations into a new file or for geographic information system processing and display.