2008 TIGER/Line Shapefiles Technical Documentation Chapter 2: Structure and Format 2.1 Structure of the Files The TIGER/Line Shapefiles and associated relationship files are offered in a compressed format. One zipped file is available for each layer, with a file extension of .zip. Each zipped shapefile consists of the following five files: .shp - the file that stores the feature geometry .shx - the file that stores the index of the feature geometry .dbf - the dBASE (database) file that stores attribute information .prj - the file that stores the coordinate system information .shp.xml - the file that stores the metadata Each zipped relationship file consists of the following two files: .dbf - the dBASE (database) file that stores additional attribute information .dbf.xml - the file that stores the metadata 2.2 Organization of the Files Geographic entities tabulated by the Census Bureau generally are hierarchical; that is, in a progression from the nation to the block level. The organizational structure of the TIGER/Line Shapefiles is based on this hierarchical framework. This structure is derived from the legal, administrative, or areal relationships of the entities. The smallest level of census geography is the census block. Census blocks are defined within block groups, block groups are defined within census tracts, census tracts are defined within counties and statistically equivalent entities, counties and statistically equivalent entities are within states and statistically equivalent entities, states and statistically equivalent entities are within the nation. American Indian areas, Alaska Native areas, Native Hawaiian areas, metropolitan areas, urban areas, and ZIP Code Tabulation Areas (ZCTAs(TM)) are within the nation and comprised of census blocks. Within states and statistically equivalent entities and comprised of census blocks are congressional districts, Alaska Native Regional Corporations (ANRCs), school districts, Urban Growth Areas (UGAs), state legislative districts (SLDs), and places. Within counties and statistically equivalent entities and comprised of census blocks are county subdivisions, voting districts (VTDs), and Traffic Analysis Zones (TAZs). In Puerto Rico, subbarrios are within county subdivisions and are comprised of census blocks. American Indian Tribal Subdivisions (AITSs) fall within American Indian/Alaska Native/Native Hawaiian areas (AIANNHs). Shapefiles are released in one of four types of hierarchical coverages—nation-based, American Indian Area-based, state-based, and county-based. Some shapefiles are released in multiple coverages to enable flexibility in downloading files. Below are descriptions of each coverage. Table 2.2.1 provides an overview of which file types are available under each hierarchical coverage. Nation-Based Files—Entities that are defined independently from states and counties, such as American Indian Areas, are available in nation-based shapefiles that encompass the entire country. Though defined within states, county boundaries are available in nation-based as well as state-based shapefiles. American Indian Area-Based Files—Entities that are defined within American Indian Areas (AIA), are available in AIA-based shapefiles. State-Based Files—Entities such as school districts and congressional districts that are defined within states and can cross county boundaries are represented in state-based shapefiles. Though defined within counties, blocks, block groups, census tracts, and county subdivisions are available in state-based as well as county-based shapefiles. Though defined within states, counties are available in state-based as well as nation- based shapefiles. County-Based Files—Entities that are defined within counties and do not cross county or state lines such as census tracts and voting districts are represented in county-based shapefiles. Though defined within counties, blocks, block groups, census tracts, and county subdivisions are available in state-based as well as county-based shapefiles. Table 2.2.1: 2008 TIGER/Line Shapefile Layers Organizational Structure Layer Nation- Based Files State-Based Files County- Based Files 108th Congressional District (Congress elected in 2002) X 110th Congressional District (Congress elected in 2006) X 1-Percent Public Use Microdata Area X 5- or 10-Percent Public Use Microdata Area X 3-Digit ZIP Code Tabulation Area X 5-Digit ZIP Code Tabulation Area X Address Range-Feature Name Relationship File X Address Ranges Relationship File X Alaska Native Regional Corporation X All Lines X American Indian Tribal Subdivision X (Also available by AIA) American Indian/Alaska Native/Native Hawaiian Area X Area Hydrography X Area Landmark X Block X X Block Group X X Census Tract X X Combined New England City and Town Area X Combined Statistical Area X Commercial Region X Consolidated City X County and Equivalent X X County Subdivision X X Elementary School District X Feature Names Relationship File X Metropolitan Division X Metropolitan/Micropolitan Statistical Area X Military Installation X New England City and Town Area X New England City and Town Area Division X Other Identifiers Relationship File X Place X Point Landmark X Secondary School District X State and Equivalent X State Legislative District—Lower Chamber X State Legislative District—Upper Chamber X Subbarrio X Topological Faces (2-Cells With All Geocodes) Relationship File X Topological Faces-Area Hydrography Relationship File X Topological Faces-Area Landmark Relationship File X Traffic Analysis Zone X Unified School District X Urban Areas X Urban Growth Area X Voting District X 2.3 File Naming Conventions The name of each file is: tl_2008__. Where: tl = TIGER/Line 2008 = the version of the files = entity ID code of variable length The entity ID code identifies the geographic extent by specific entity for which the file contains data. It is of variable length depending on the type of file: Nation-based: 2 characters, “us” State-based: 2-digit numeric state FIPS code County-based: 5-digit numeric state-county FIPS code American Indian area-based: 4-digit American Indian area census code = layer tag of variable length The layer tag specifies the type of geography or feature the file contains. If “00” appears at the end of the layer tag, the file contains Census 2000 geography. If “ec” appears at the end of the layer tag, the file contains Economic Census geography. If neither “00” nor “ec” appears, the file contains current geography. = the file extension Examples: Nation-based shapefile: Current New England City and Town Area (NECTA) shapefile tl_2008_us_necta.shp State-based shapefile: Economic Census County and Equivalent shapefile for Maryland tl_2008_24_countyec.shp County-based shapefile: Census 2000 Block shapefile for Worcester County, MA tl_2008_25027_tabblock00.shp American Indian Area-based Shapefile: Census 2000 American Indian Tribal Subdivision shapefile for Bois Forte Reservation tl_2008_0335_aitsaia00.shp 2.4 Terminology The modernization of the MAF/TIGER system has resulted in some changes in terminology used in TIGER/Line files before 2007. Edge—Supersedes complete chain and 1-cell; refers to both visible and non-visible linear topological primitives. An edge extends from a designated start node and continues to its end node. The order of these nodes determines the from-to orientation and left/right sides of the edge. Face—Supersedes GT-polygon; refers to areal (polygon) topological primitive. A face is bounded by one or more edges; its boundary includes not only the edges that separate it from other faces, but also any interior edges contained within the area of the face. Node—a point location representing a point in space defined by a coordinate pair. A node can be associated with one or more edges (a connected node), either as the end point of an edge in space or as the intersection point between one or more other edges. A node can also represent a point feature that is not connected to any edge (an isolated node). MAF/TIGER Feature Class Code (MTFCC)—Supersedes the Census Feature Class Code (CFCC). The MTFCC is a 5-digit code intended to classify and describe geographic objects or features. MTFCC definitions are available in the metadata files that accompany each shapefile and relationship file and in Appendix F of this document. A crosswalk between CFCC and MTFCC codes can be found on the 2008 TIGER/Line Shapefiles webpage. 2.5 Datum Each shapefile contains a .prj file that contains the GIS industry standard well-known text (WKT) format to describe the coordinate system/ projection/datum information for each shapefile. This enables users to easily import the shapefiles into their local coordinate system. All Census Bureau generated shapefiles are in GCS NAD83 and each .prj file contains the following: GEOGCS["GCS_North_American_1983",DATUM["D_North_American_1983",SPHER OID["GRS_1980",6378137,298.257222101]],PRIMEM["Greenwich",0],UNIT["Degree" ,0.017453292519943295]] 2.6 Metadata A metadata file in XML (Extensible Markup Language) format is provided along with each shapefile and relationship file. Metadata files associated with shapefiles have the extension .shp.xml, and those associated with relationship files have the extension .dbf.xml. The metadata files comply with Federal Geographic Data Committee (FGDC) standards, and can be read in any text editor. Users should refer to the metadata files for extensive documentation about the contents of the shapefiles and relationship files.