13.4.3 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 14.16.6, “Geometry Format Conversion Functions”.

The following sections describe the spatial data formats MySQL uses:

• Well-Known Text (WKT) Format

• Well-Known Binary (WKB) Format

• Internal Geometry Storage Format

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 13.4, “Spatial Data Types”).

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 ST_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 ST_GeomFromText().

  mysql> SELECT ST_X(Point(15, 20));
  +---------------------+
  | ST_X(POINT(15, 20)) |
  +---------------------+
  |                  15 |
  +---------------------+
  
  mysql> SELECT ST_X(ST_GeomFromText('POINT(15 20)'));
  +---------------------------------------+
  | ST_X(ST_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)

  Spatial functions such as ST_MPointFromText() and ST_GeomFromText() that accept WKT-format representations of MultiPoint values permit individual points within values to be surrounded by parentheses. For example, both of the following function calls are valid:

  ST_MPointFromText('MULTIPOINT (1 1, 2 2, 3 3)')
  ST_MPointFromText('MULTIPOINT ((1 1), (2 2), (3 3))')

• 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 13.4, “Spatial Data Types”). 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 13.2 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-endian values.

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

• Only GeometryCollection can be empty. Such a value is stored with 0 elements.

• Polygon rings can be specified both clockwise and counterclockwise. MySQL flips the rings automatically when reading data.

Cartesian coordinates are stored in the length unit of the spatial reference system, with X values in the X coordinates and Y values in the Y coordinates. Axis directions are those specified by the spatial reference system.

Geographic coordinates are stored in the angle unit of the spatial reference system, with longitudes in the X coordinates and latitudes in the Y coordinates. Axis directions and the meridian are those specified by the spatial reference system.

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

mysql> SET @g = ST_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)