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MySQL 5.5 Reference Manual  /  ...  /  Supported Spatial Data Formats

11.5.3.1 Supported Spatial Data Formats

Two standard spatial data formats are used to represent geometry objects in queries:

  • 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. (Internal format is like WKB but with an initial 4 bytes to indicate the SRID.)

There are functions available to convert between different data formats; see Section 12.15.6, “Geometry Format Conversion Functions”.

The following sections describe the spatial data formats MySQL uses:

Well-Known Text (WKT) Format

The Well-Known Text (WKT) representation of geometry values is designed for exchanging geometry data in ASCII form. The OpenGIS specification provides a Backus-Naur grammar that specifies the formal production rules for writing WKT values (see Section 11.5, “Extensions for Spatial Data”).

Examples of WKT representations of geometry objects:

  • A Point:

    POINT(15 20)
    

    The point coordinates are specified with no separating comma. This differs from the syntax for the SQL Point() function, which requires a comma between the coordinates. Take care to use the syntax appropriate to the context of a given spatial operation. For example, the following statements both use X() to extract the X-coordinate from a Point object. The first produces the object directly using the Point() function. The second uses a WKT representation converted to a Point with GeomFromText().

    mysql> SELECT X(Point(15, 20));
    +------------------+
    | X(POINT(15, 20)) |
    +------------------+
    |               15 |
    +------------------+
    
    mysql> SELECT X(GeomFromText('POINT(15 20)'));
    +---------------------------------+
    | X(GeomFromText('POINT(15 20)')) |
    +---------------------------------+
    |                              15 |
    +---------------------------------+
    
  • A LineString with four points:

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

    The 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))
    
Well-Known Binary (WKB) Format

The Well-Known Binary (WKB) representation of geometric values is used for exchanging geometry data as binary streams represented by BLOB values containing geometric WKB information. This format is defined by the OpenGIS specification (see Section 11.5, “Extensions for Spatial Data”). It is also defined in the ISO SQL/MM Part 3: Spatial standard.

WKB uses 1-byte unsigned integers, 4-byte unsigned integers, and 8-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 by two hexadecimal digits:

0101000000000000000000F03F000000000000F0BF

The sequence consists of the components shown in the following table.

Table 11.1 WKB Components Example

Component Size Value
Byte order 1 byte 01
WKB type 4 bytes 01000000
X coordinate 8 bytes 000000000000F03F
Y coordinate 8 bytes 000000000000F0BF

Component representation is as follows:

  • The byte order indicator is either 1 or 0 to signify 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. MySQL uses values from 1 through 7 to 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 have more complex data structures, as detailed in the OpenGIS specification.

Internal Geometry Storage Format

MySQL stores geometry values using 4 bytes to indicate the SRID followed by the WKB representation of the value. For a description of WKB format, see Well-Known Binary (WKB) Format.

For the WKB part, these MySQL-specific considerations apply:

  • The byte-order indicator byte is 1 because MySQL stores geometries as little-ending values.

  • MySQL supports geometry types of Point, LineString, Polygon, MultiPoint, MultiLineString, MultiPolygon, and GeometryCollection. Other geometry types are not supported.

The LENGTH() function returns the space in bytes required for value storage. Example:

mysql> SET @g = GeomFromText('POINT(1 -1)');
mysql> SELECT LENGTH(@g);
+------------+
| LENGTH(@g) |
+------------+
|         25 |
+------------+
mysql> SELECT HEX(@g);
+----------------------------------------------------+
| HEX(@g)                                            |
+----------------------------------------------------+
| 000000000101000000000000000000F03F000000000000F0BF |
+----------------------------------------------------+

The value length is 25 bytes, made up of these components (as can be seen from the hexadecimal value):

  • 4 bytes for integer SRID (0)

  • 1 byte for integer byte order (1 = little-endian)

  • 4 bytes for integer type information (1 = Point)

  • 8 bytes for double-precision X coordinate (1)

  • 8 bytes for double-precision Y coordinate (−1)


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