The most common vector file format is the shapefile. Shapefiles, developed by ESRI in the early 1990s for use with the dBASE III database management software package in ArcView 2, are simple, nontopological files developed to store the geometric location and attribute information of geographic features.
Shapefiles are incapable of storing null values, as well as annotations or network features. Field names within the attribute table are limited to ten characters, and each shapefile can represent only point, line, or polygon feature sets. Supported data types are limited to floating point, integer, date, and text. Shapefiles are supported by almost all commercial and open-source GIS software.
Despite being called a “shapefile,” this format is actually a compilation of many different files. Table 5.1 "Shapefile File Types" lists and describes the different file formats associated with the shapefile. Among those listed, only the SHP, SHX, and DBF file formats are mandatory to create a functioning shapefile, while all others are conditionally required. As a general rule, the names for each file should conform to the MS-DOS 8.3 convention when using older versions of GIS software packages. According to this
convention, the filename prefix can contain up to eight characters, and the filename suffix contains three characters. The more recent GIS software packages have relaxed this requirement and will accept longer filename prefixes.
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File Extension Purpose
SHP* Feature geometry
SHX* Index format for the feature geometry
DBF* Feature attribute information in dBASE IV format
PRJ Projection information
SBN and SBX Spatial index of the features
FBN and FBX Read-only spatial index of the features
AIN and AIH Attribute information for active fields in the table
IXS Geocoding index for read-write shapefiles
MXS Geocoding index for read-write shapefiles with ODB format
ATX Attribute index used in ArcGIS 8 and later
SHP.XML Metadata in XML format
CPG Code page specifications for identifying character encoding * Indicates mandatory files
The earliest vector format file for use in GIS software packages, which is still in use today, is the ArcInfo coverage. This georelational file format supports multiple features types (e.g., points, lines, polygons, annotations) while also storing the topological information associated with those features. Attribute data are stored as multiple files in a separate directory labeled “Info.” Due to its creation in an MS-DOS environment, these files maintain strict naming conventions. File names cannot be longer than thirteen characters, cannot contain spaces, cannot start with a number, and must be completely in lowercase. Coverages cannot be edited in ArcGIS 9.x or later versions of ESRI’s software package. The US Census Bureau maintains a specific type of shapefile referred to as TIGER
orTIGER/Line (Topologically Integrated Geographic Encoding and Referencing system). Although these open-source files do not contain actual census information, they map features such as census tracts, roads, railroads, buildings, rivers, and other features that support and improve the bureau and improve the Bureau’s ability to#8217;s ability to collect census information. TIGER/Line shapefiles, first released in 1990, are topologically explicit and are linked to the Census Bureau’s Master Address File (MAF),
therefore enabling the geocoding of street addresses. These files are free to the public and can be freely downloaded from private vendors that support the format.
TheAutoCAD DXF (Drawing Interchange Format or Drawing Exchange Format)is a proprietary vector file format developed by Autodesk to allow interchange between engineering-based CAD (computer-aided design) software and other mapping software packages. DXF files were originally released in 1982 with the purpose of providing an exact representation of AutoCAD’s native DWG format. Although the DXF is still commonly used, newer versions of AutoCAD have incorporated more complex data types (e.g., regions, dynamic blocks) that are not supported in the DXF format. Therefore, it may be presumed that the DXF format may become less popular in geospatial analysis over time.
Finally, the US Geological Survey (USGS) maintains an open-source vector file format that details physical and cultural features across the United States. These topologically
explicit DLGs (Digital Line Graphics) come in large-, intermediate-, and small-scale depending on whether they are derived from 1:24,000-; 1:100,000-; or 1:2,000,000-scale USGS topographic
quadrangle maps. The features available in the different DLG types depend on the scale of the DLG but generally include data such as administrative and political boundaries, hydrography, transportation systems, hypsography, and land cover.
Vector data files can also be structured to represent surface elevation information.
A TIN (Triangulated Irregular Network) is an open-source vector data structure that uses contiguous, nonoverlapping triangles to represent geographic surfaces (Figure 5.10 "Triangulated Irregular Network (TIN)"). Whereas the raster depiction of a surface represents elevation as an average value over the spatial extent of the individual pixel (see Section 5.3.2 "Raster File Formats"), the TIN data structure models each vertex of the triangle as an exact elevation value at a specific point on the earth. The arcs between each vertex are an approximation of the elevation between two vertices. These arcs are then aggregated into triangles from which information on elevation, slope, aspect, and surface area can be derived across the entire extent of the model’s space. Note that term “irregular” in the name of the data model refers to the fact that the vertices are typically laid out in a scattered fashion.
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Figure 5.10 Triangulated Irregular Network (TIN)
The use of TINs confers certain advantages over raster-based elevation models (see Section 5.3.2 "Raster File Formats"). First, linear topographic features are very accurately represented relative to their raster counterpart. Second, a comparatively small number of data points are needed to represent a surface, so file sizes are typically much smaller. This is particularly true as vertices can be clustered in areas where relief is complex and can be sparse in areas where relief is simple. Third, specific elevation data can be incorporated into the data model in a post hoc fashion via the placement of additional vertices if the original is deemed insufficient or inadequate. Finally, certain spatial statistics can be calculated that
cannot be obtained when using a raster-based elevation model, such as flood plain delineation, storage capacity curves for reservoirs, and time-area curves for hydrographs.