A vector map, with points, polylines and polygons. |
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| 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]
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.
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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 mandatory to store the core data that comprises a shapefile: ".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, but alone is incomplete for distribution, as the other supporting files are required.
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 filename convention (8 character filename prefix, period, 3 character filename suffix such as shapefil.shp) in order to be compatible with past applications that handle shapefiles, though many recent software applications accept files with longer names. For this same reason, all files should be located in the same folder.
Mandatory files :
Optional files :
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 an implementor of the file formats 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.
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 (1-based) |
| 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.
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 forwards an arbitrary number of records by using the same method.
Attributes for each shape are stored in the dBase format. An alternative format that can also be used is the xBase format, which has an open specification, and is used in open source Shapefile libraries, such as the Shapefile C library.[2]
The information contained in the .prj file specifies the geographic coordinate system of the geometric data in the .shp file. Although optional, it is usually provided, as it is not necessarily possible to guess the coordinate system of any given points. The file is created in well-known text (WKT) format when generated with ArcGIS Desktop versions 9 and later. Previous ArcGIS versions and some third-party software generate it in another format, shown here:
Older projection file format example:
Projection UTM
Zunits NO
Units METERS
Spheroid CLARKE1866
Xshift 0.0000000000
Yshift -4000000.0000000000
Parameters
-108 0 0.000 /* longitude
36 0 0.000 /* latitude
New WKT format example:
GEOGCS["GCS_North_American_1927",DATUM["D_North_American_1927",SPHEROID["Clarke_1866",6378206.4,294.9786982],PRIMEM["Greenwich",0],UNIT"Degree",0.0174532925199433
The information contained in the .prj file specifies the:
This is a binary spatial index file, which is used only by Esri software. The format is not documented by Esri. However it has been reverse-engineered and documented [3] by the open source community. It is not currently 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
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.
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.
The maximum size of either .shp or .dbf component files cannot exceed 2 GB (or 231 bits). This translates to, at best, about 70 million point features.[4] The maximum number of feature storage for other geometry types varies depending on the number of vertices used.
The attribute database format for the .dbf component file is based on an older dBase standard. This database format inherently has a number of limitations, including:[4]
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.