Shapefile

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Shapefile
Image:Simple vector map.svg
A vector map, with points, polylines and polygons.
Filename extension .shp, .shx, .dbf
Developed by ESRI
Type of format GIS
Standard(s) Shapefile Technical Description

The ESRI Shapefile or simply a shapefile is a popular geospatial vector data format for geographic information systems software. It is developed and regulated by ESRI as a (mostly) open specification for data interoperability among ESRI and other software products.[1] A "shapefile" commonly refers to a collection of files with ".shp", ".shx", ".dbf", and other extensions on a common prefix name (e.g., "lakes.*"). The actual shapefile relates specifically to files with the ".shp" extension, however this file alone is incomplete for distribution, as the other supporting files are required.

Shapefiles spatially describe geometries: points, polylines, and polygons. These, for example, could represent water wells, rivers, and lakes, respectively. Each item may also have attributes that describe the items, such as the name or temperature.

Contents

[edit] Overview

A shapefile is a digital vector storage format for storing geometric location and associated attribute information. This format lacks the capacity to store topological information. The shapefile format was introduced with ArcView GIS version 2 in the beginning of the 1990s. It is now possible to read and write shapefiles using a variety of free and non-free programs.

Shapefiles are simple because they store primitive geometrical data types of points, lines, and polygons. These primitives are of limited use without any attributes to specify what they represent. Therefore, a table of records will store properties/attributes for each primitive shape in the shapefile. Shapes (points/lines/polygons) together with data attributes can create infinitely many representations about geographical data. Representation provides the ability for powerful and accurate computations.

While the term "shapefile" is quite common, a "shapefile" is actually a set of several files. Three individual files are normally mandatory to store the core data that comprises a shapefile. There are a further eight optional files which store primarily index data to improve performance. Each individual file should conform to the MS DOS 8.3 filenameing convention (8 character filename prefix, fullstop, 3 character filename suffix such as shapefil.shp) in order to be compatible with past applications that handle shapefiles. For this same reason, all files should be located in the same folder.

Mandatory files :

  • .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

Optional files :

  • .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
  • .cpg — used to specify the code page (only for .dbf) for identifying the character encoding to be used

In each of the .shp, .shx, and .dbf files, the shapes in each file correspond to each other in sequence. That is, the first record in the .shp file corresponds to the first record in the .shx and .dbf files, and so on. The .shp and .shx files have various fields with different endianness, so as an implementor of the file formats you must be very careful to respect the endianness of each field and treat it properly.

Shapefiles deal with coordinates in terms of X and Y, although they are often storing longitude and latitude, respectively. While working with the X and Y terms, be sure to respect the order of the terms (longitude is stored in X, latitude in Y).

[edit] Shapefile shape format (.shp)

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 little 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.

[edit] Shapefile shape index format (.shx)

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 length (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 forward's an arbitrary number of records by using the same method.

[edit] Shapefile attribute format (.dbf)

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

[edit] Shapefile projection format (.prj)

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:

[edit] Shapefile spatial index format (.sbn)

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.

[edit] Limitations

[edit] 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.

[edit] Spatial representation

The edges of a polyline or polygon are defined using points. The spacing of the points implicitly determines the scale for which the data are useful. Exceeding that scale results in jagged representation of features. Additional points would be required to achieve smooth shapes at greater scales. For features better represented by smooth curves, the polygon representation requires much more data storage than, for example, splines, which can capture smoothly varying shapes efficiently. None of the shapefile types supports splines.

[edit] 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.

[edit] Mixing shape types

Because the shape type precedes each record, a shape file is physically capable of storing a mixture of different shape types. However, the specification states, "All the non-Null shapes in a shapefile are required to be of the same shape type." Therefore this ability to mix shape types must be limited to interspersing null shapes with the single shape type declared in the file's header. A shape file must not contain both Polyline and Polygon data, for example, and the descriptions for a well (point), a river (polyline) and a lake (polygon) would be stored in three separate files.

[edit] References

  1. ^ a b Environmental Systems Research Institute, Inc. (July, 1998). ESRI Shapefile technical description. http://www.esri.com/library/whitepapers/pdfs/shapefile.pdf. Retrieved on 2007-07-04. 

[edit] External links

Retrieved from "http://en.wikipedia.org/wiki/Shapefile"
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