ESRI Shapefile 文件格式(1)

ESRI Shapefile

.shp describe spatial data format

A sharp file stores nontopological(非拓扑) geometry(几何) and attribute information for the spatial features in a data set. The geometry for a feature is stored as a shape comprising a set of vector coordinates.

Shapefiles do not have the processing overhead of a topological data structure(shp格式文件没有处理拓扑格式结构的处理开销),they have advantages over other data such as a faster drawing and speed and edit ability. Shapefiles handle single features that overlap or that are noncontiguous. They also typically require less disk space and are easier to read and write.

Shapefiles support point, line, and area features. Area features are represented as closes loop. Double-digitized polygons(多边形). Attributes are held in a dBASE format file. Each attribute record has a one-to-one relationship

with the associated shaperecord.

Construct of shp file:

main file: a direct access, variable-record-length(可变记录长) file in which each record describes a shape with a list of its vertices.

index file：each record contains the offset of the corresponding main file record from the beginning of the main file.

dBASE the dBASE table contains feature attributes with one record per feature

The one-to-one relationship between geometry and attributes is based on record number. Attributes records in the dBASE file must be in the same order as records in the main file.(几何形状与其属性是一对一的关系，并有一个记录号确认，所以dBASE的顺序要与main file中的记录顺序一致)

文件的详细结构:

1 main file

contain a fixed-length file header followed by variable-length records.

Each variable-length record is made up of a fixed-length record header followed by variable-length record contents.

file Header (fixed-length) 100bytes
Record Header(fixed-length)8bytes	Record Contents (variable-length)
Record Header(fixed-length)8bytes	Record Contents (variable-length)
Record Header(fixed-length)8bytes	Record Contents (variable-length)
... 8bytes	Record Contents (variable-length)
... 8bytes	Record Contents (variable-length)
Record Header(fixed-length)8bytes	Record Contents (variable-length)

Figure1.main file

术语解释：

Byte Order （字节表单）(shp格式文件的内容可以被分成两个类属)

——数据相关

— Main file 记录内容

— Main file 头文件数据描述字段(Shape Type, Bounding Box, etc)

——文件管理相关

— 文件和记录长度

— 记录偏移

The Main File Header

Position	Field	Value	Type	Byte Order
Byte 0	File Code	9994	Integer	Big
Byte 4	Unused	0	Integer	Big
Byte 8	Unused	0	Integer	Big
Byte 12	Unused	0	Integer	Big
Byte 16	Unused	0	Integer	Big
Byte 20	Unused	0	Integer	Big
Byte 24	File Length	File Length	Integer	Big
Byte 28	Version	1000	Integer	Little
Byte 32	Shape Type	Shape Type	Double	Little
Byte 36	Bounding Box	Xmin	Double	Little
Byte 44	Bounding Box	Ymin	Double	Little
Byte 52	Bounding Box	Xmax	Double	Little
Byte 60	Bounding Box	Ymax	Double	Little
Byte 68	Bounding Box	Zmin	Double	Little
Byte 76	Bounding Box	Zmax	Double	Little
Byte 84	Bounding Box	Mmin	Double	Little
Byte 92	Bounding Box	Mmax	Double	Little

实际文件的格式

9994

0

0

0

0

0

0

8765 (16bytes)

1000

`````

Shape type

0	Null Shape	8	MultiPoint	18	MultiPointZ	28	MultiPointM
1	Point	11	PointZ	21	PointM	31	MultiPatch
3	PolyLine	13	PolyLineZ	23	PolyLineM
5	Polugon	15	PolygonZ	25	PolygonM