Shapefile: What it is?

Shapefile: What it is?

The shapefile is a popular geospatial vector data format for GIS (geographic information system) software. The shapefile was created by Esri, the software vendor that makes the ArcGIS suite of GIS software.  The shapefile was first introduced in the early 1990s with the debut of ArcView 2.

The shapefile format can spatially describe vector features: points, lines, and polygons, representing, for example, water wells, Tube well (points Shapefile) ; rivers, roads (lines Shapefile); and lakes, industrial Area (polygon Shapefile). Each item usually has attributes that describe it, such as name or temperature.

The term "shapefile" is quite common, but is misleading since the format consists of a collection of files with a common filename prefix, stored in the same directory.

What File Extensions are Associated with a Shapefile?

All shapefile files must have the same name but have different extensions. There are three required files that, at a minimum, make up a shapefile.  For example, a Shapefile of Lake must have the minimum set of files: Lake.shp, Lake.shx, and Lake.dbf.

These three minimum files are:

·         .shp — Main file (mandatory); a direct access, variable-record-length file in which each record describes a shape with a list of its vertices.

·         .shx — Index file (mandatory). In the index file, each record contains the offset of the corresponding main file record from the beginning of the main file. The index file (.shx) contains a 100-byte header followed by 8-byte, fixed-length records.

·         .dbf — dBASE Table file (mandatory); a constrained form of DBF that contains feature attributes with one record per feature. The one-to-one relationship between geometry and attributes is based on record number. Attribute records in the dBASE file must be in the same order as records in the main file.

Other file extensions that may be part of a shapefile are:

·         .sbn — Part 1 of spatial index for read-write instances of the Shapefile format. If present, essential for correct processing.

·         .sbx — Part 2 of spatial index for read-write instances of the Shapefile format. If present, essential for correct processing.

·         .atx — Created by ArcView 3.x for each instance of the Shapefile format or dBASE attribute index created in ArcCatalog. ArcView GIS 3.x attribute indexes for shapefiles and dBASE files are not used by later versions of ArcGIS as a new attribute indexing model has been developed for shapefiles and dBASE files.

·         .fbn — One of the files that store the spatial index of the features for instances of the Shapefile format that are read-only.

·         .fbx — The other file (besides .fbn) that stores the spatial index of the features for instances of the Shapefile format that are read-only.

·         .ain — One of the files that stores the attribute index of the active fields in a table or a theme’s attribute table.

·         .aih — The other file (besides .ain) that stores the attribute index of the active fields in a table or a theme’s attribute table

·         .ixs — Geocoding index for read/write shapefiles. If present, essential for correct processing.

·         .mxs –Geocoding index for read-write shapefiles (ODB format).

·         .prj — Projections Definition file; stores coordinate system information.

·         .xml — contains metadata, as used by ArcGIS.

·         .cpg — An optional file that can be used to specify the codepage for identifying the character set to be used.

In each of the .shp, .shx, and .dbf files, the shapes in each file correspond to each other in sequence (i.e., the first record in the .shp file corresponds to the first record in the .shxand .dbf files, etc.). The .shp and .shx files have various fields with different endianness, so an implementer of the file formats must be very careful to respect the endianness of each field and treat it properly.