Esri Shapefile .SHP File Description

.SHP - shape format; the feature geometry itself

.SHX - shape index format; a positional index of the feature geometry to allow seeking forwards and backwards quickly

.DBF - attribute format; columnar attributes for each shape, in dBase III format

 

.PRJ - projection format; the coordinate system and projection information, a plain text file describing the projection using well-known text format

.SBN and .SBX - a spatial index of the features

.FBN and .FBX - a spatial index of the features for shapefiles that are read-only

.AIN and .AIH - an attribute index of the active fields in a table or a theme's attribute table

.IXS - a geocoding index for read-write shapefiles

.MXS - a geocoding index for read-write shapefiles (ODB format)

.ATX - an attribute index for the .dbf file in the form of shapefile.columnname.atx (ArcGIS 8 and later)

.SHP.XML- metadata in XML format

 

The main file .SHP contains the primary geographic reference data in the shapefile. The file consists of a single fixed length header followed by one or more variable length records. Each of the variable length records includes a record header component and a record contents component. A detailed description of the file format is given in the Esri Shapefile Technical Description.[1] This format should not be confused with the AutoCAD shape font source format, which shares the .shp extension.

 

The main file header is fixed at 100 bytes in length and contains 17 fields; nine 4-byte (32-bit signed integer or int32) integer fields followed by eight 8-byte (double) signed floating point fields:

 

Bytes	Type	Endianness	Usage
0-3	int32	big	File code (always hex value 0x0000270a)
4-23	int32	big	Unused; five uint32
24-27	int32	big	File length (in 16-bit words, including the header)
28-31	int32	little	Version
32-35	int32	little	Shape type (see reference below)
36-67	double	little	Minimum bounding rectangle (MBR) of all shapes contained within the shapefile; four doubles in the following order: min X, min Y, max X, max Y
68-83	double	little	Range of Z; two doubles in the following order: min Z, max Z
84-99	double	little	Range of M; two doubles in the following order: min M, max M

 

The file then contains any number of variable-length records. Each record is prefixed with a record-header of 8 bytes:

 

Bytes	Type	Endianness	Usage
0-3	int32	big	Record number
4-7	int32	big	Record length (in 16-bit words)

 

Following the record header is the actual record:

 

Bytes	Type	Endianness	Usage
0-3	int32	big	Shape type (see reference below)
4-	-	-	Shape content

 

The variable length record contents depend on the shape type. The following are the possible shape types:

 

Value	Shape Type	Fields
0	Null shape	None
1	Point	X, Y
3	Polyline	MBR, Number of parts, Number of points, Parts, Points
5	Polygon	MBR, Number of parts, Number of points, Parts, Points
8	MultiPoint	MBR, Number of points, Points
11	PointZ	X, Y, Z, M
13	PolylineZ	Mandatory: MBR, Number of parts, Number of points, Parts, Points, Z range, Z array Optional: M range, M array
15	PolygonZ	Mandatory: MBR, Number of parts, Number of points, Parts, Points, Z range, Z array Optional: M range, M array
18	MultiPointZ	Mandatory: MBR, Number of points, Points, Z range, Z array Optional: M range, M array
21	PointM	X, Y, M
23	PolylineM	Mandatory: MBR, Number of parts, Number of points, Parts, Points Optional: M range, M array
25	PolygonM	Mandatory: MBR, Number of parts, Number of points, Parts, Points Optional: M range, M array
28	MultiPointM	Mandatory: MBR, Number of points, Points Optional Fields: M range, M array
31	MultiPatch	Mandatory: MBR, Number of parts, Number of points, Parts, Part types, Points, Z range, Z array Optional: M range, M array

 

In common use, shapefiles containing Point, Polyline, and Polygon are extremely popular. The "Z" types are three-dimensional. The "M" types contain a user-defined measurement which coincides with the point being referenced. Three-dimensional shapefiles are rather uncommon, and the measurement functionality has been largely superseded by more robust databases used in conjunction with the shapefile data.

 

The shapefile index contains the same 100-byte header as the .SHP file, followed by any number of 8-byte fixed-length records which consist of the following two fields:

 

Bytes	Type	Endianness	Usage
0-3	int32	big	Record offset (in 16-bit words)
4-7	int32	big	Record offset (in 16-bit words)

 

Using this index, it is possible to seek backwards in the shapefile by seeking backwards first in the shape index (which is possible because it uses fixed-length records), reading the record offset, and using that to seek to the correct position in the .SHP file. It is also possible to seek forwards an arbitrary number of records by using the same method.

 

Attributes for each shape are stored in the xBase (dBase) format, which has an open specification.

 

The projection information contained in the .PRJ file is critical in order to understand the data contained in the .SHP  file correctly. Although it is technically optional, it is most often provided, as it is not necessarily possible to guess the projection of any given points. The file is stored in well-known text (WKT) format.

 

Some typical information contained in the .PRJ file is:

• Geographic coordinate system

• Datum (geodesy)

• Spheroid

• Prime meridian

• Map projection

• Units used

• Parameters necessary to use the map projection, for example:

• Latitude of origin

• Scale factor

• Central meridian

• False northing

• False easting

• Standard parallels

 

This is a binary spatial index file, which is used only by Esri software. The format is not documented, and is not implemented by other vendors. The .SBN file is not strictly necessary, since the .SHP file contains all of the information necessary to successfully parse the spatial data.

 

Topology and shapefiles

Shapefiles do not have the ability to store topological information. ArcInfo coverages and Personal/File/Enterprise Geodatabases do have the ability to store feature topology.

 

Spatial representation

The edges of a polyline or polygon are defined using points, which can give it a jagged edge at higher resolutions. Additional points are required to give smooth shapes, which requires storing quite a lot of data compared to, for example, bézier curves, which can capture complexity using smooth curves, without using as many points. Currently, none of the shapefile types support bézier curves.

 

Data storage

Unlike most databases, the database format is based on older xBASE standard, incapable of storing null values in its fields. This limitation can make the storage of data in the attributes less flexible. In ArcGIS products, values that should be null are instead replaced with a 0 (without warning), which can make the data misleading. This problem is addressed in ArcGIS products by using Esri's Personal Geodatabase offerings, one of which is based on Microsoft Access.

 

Mixing shape types

Each shape file can technically store a mix of different shape types, as the shape type precedes each record, but common use of the specification dictates that only shapes of a single type can be in a single file. For example, a shape file cannot contain both Polyline and Polygon data. Thus, well (point), river (polyline) and lake (polygon) data must be kept in three separate files.