什么是WKT和WKB

WKT是Well-Known Text的缩写形式。

      

The Well-Known Text (WKT) representation of Geometry is designed to exchange geometry data in ASCII form.

Examples of WKT representations of geometry objects:

A Point:

·         POINT(15 20)

Note that point coordinates are specified with no separating comma.

A LineString with four points:

·         LINESTRING(0 0, 10 10, 20 25, 50 60)

Note that point coordinate pairs are separated by commas.

A Polygon with one exterior ring and one interior ring:

·         POLYGON((0 0,10 0,10 10,0 10,0 0),(5 5,7 5,7 7,5 7, 5 5))

A MultiPoint with three Point values:

·         MULTIPOINT(0 0, 20 20, 60 60)

A MultiLineString with two LineString values:

·         MULTILINESTRING((10 10, 20 20), (15 15, 30 15))

A MultiPolygon with two Polygon values:

·         MULTIPOLYGON(((0 0,10 0,10 10,0 10,0 0)),((5 5,7 5,7 7,5 7, 5 5)))

A GeometryCollection consisting of two Point values and one LineString:

·         GEOMETRYCOLLECTION(POINT(10 10), POINT(30 30), LINESTRING(15 15, 20 20))

A Backus-Naur grammar that specifies the formal production rules for writing WKT values can be found in the OpenGIS specification document referenced near the beginning of this chapter.

WKB是Well-Known Binary的缩写形式。

The Well-Known Binary (WKB) representation for geometric values is defined by the OpenGIS specification.

WKB is used to exchange geometry data as binary streams represented by BLOB values containing geometric WKB information.

WKB uses one-byte unsigned integers, four-byte unsigned integers, and eight-byte double-precision numbers (IEEE 754 format). A byte is eight bits.

For example, a WKB value that corresponds to POINT(1 1) consists of this sequence of 21 bytes (each represented here by two hex digits):

0101000000000000000000F03F000000000000F03F

The sequence may be broken down into these components:

Byte order : 01

WKB type   : 01000000

X          : 000000000000F03F

Y          : 000000000000F03F

Component representation is as follows:

The byte order may be either 0 or 1 to indicate little-endian or big-endian storage. The little-endian and big-endian byte orders are also known as Network Data Representation (NDR) and External Data Representation (XDR), respectively.

The WKB type is a code that indicates the geometry type. Values from 1 through 7 indicate Point, LineString, Polygon, MultiPoint, MultiLineString, MultiPolygon, and GeometryCollection.

A Point value has X and Y coordinates, each represented as a double-precision value.

WKB values for more complex geometry values are represented by more complex data structures, as detailed in the OpenGIS specification.