This section describes the standard spatial data formats that are used to represent geometry objects in queries. They are:
Well-Known Text (WKT) format
Well-Known Binary (WKB) format
Internally, MySQL stores geometry values in a format that is not identical to either WKT or WKB format.
The Well-Known Text (WKT) representation of Geometry is designed to exchange geometry data in ASCII form. Examples of WKT representations of geometry objects are:
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 OGC specification document referenced near the beginning of this chapter.
The Well-Known Binary (WKB) representation for geometric values is defined by the OpenGIS specifications. It is also defined in the ISO “SQL/MM Part 3: Spatial” standard.
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.