Esri Shapefile .SHP File Description

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:

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

Following the record header is the actual record:

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

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 Three-dimensional shapefiles are rather uncommon, and the measurement functionality has been largely superseded by more robust databases used in conjunction with the shapefile data.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:

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.