Well-known text

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Well-known text (WKT) is a text markup language for representing vector geometry objects on a map, spatial reference systems of spatial objects and transformations between spatial reference systems. A binary equivalent, known as well-known binary (WKB) is used to transfer and store the same information on databases, such as PostGIS, Microsoft SQL Server and DB2. The formats were originally defined by the Open Geospatial Consortium (OGC) and described in their Simple Feature Access and Coordinate Transformation Service specifications. The current standard definition is in the ISO/IEC 13249-3:2011 standard, "Information technology -- Database languages -- SQL multimedia and application packages -- Part 3: Spatial" (SQL/MM).

Contents

Geometric objects [edit]

File:Geometrietypen.jpg
Standard OGC geometry types.

In total, there are 18 distinct geometric objects that can be represented:

Coordinates for geometries may be 2D (x, y), 3D (x, y, z), 4D (x, y, z, m) with an m value that is part of a linear referencing system or 2D with an m value (x, y, m). Three-dimensional geometries are designated by a "Z" after the geometry type and geometries with a linear referencing system have an "M" after the geometry type. Empty geometries which contain no coordinates can be specified by using the symbol EMPTY after the type name.

WKT geometries are used throughout OGC specifications and are present in applications that implement these specifications. For example, PostGIS contains functions that can convert geometries to and from a WKT representation, making them human readable.

Geometry primitives (2D)
Type Examples
Point POINT (30 10) SFA Point.svg
LineString LINESTRING (30 10, 10 30, 40 40) SFA LineString.svg
Polygon POLYGON ((30 10, 10 20, 20 40, 40 40, 30 10)) SFA Polygon.svg
POLYGON ((35 10, 10 20, 15 40, 45 45, 35 10),
(20 30, 35 35, 30 20, 20 30))		 SFA Polygon with hole.svg
Multipart geometries (2D)
Type Examples
MultiPoint MULTIPOINT ((10 40), (40 30), (20 20), (30 10)) SFA MultiPoint.svg
MULTIPOINT (10 40, 40 30, 20 20, 30 10)
MultiLineString MULTILINESTRING ((10 10, 20 20, 10 40),
(40 40, 30 30, 40 20, 30 10))		 SFA MultiLineString.svg
MultiPolygon MULTIPOLYGON (((30 20, 10 40, 45 40, 30 20)),
((15 5, 40 10, 10 20, 5 10, 15 5)))		 SFA MultiPolygon.svg
MULTIPOLYGON (((40 40, 20 45, 45 30, 40 40)),
((20 35, 45 20, 30 5, 10 10, 10 30, 20 35),
(30 20, 20 25, 20 15, 30 20)))		 SFA MultiPolygon with hole.svg

The following are some other examples of geometric WKT strings:

GEOMETRYCOLLECTION(POINT(4 6),LINESTRING(4 6,7 10))
POINT ZM (1 1 5 60)
POINT M (1 1 80)
POINT EMPTY
MULTIPOLYGON EMPTY

Well-known binary [edit]

Well-known binary (WKB) representations are typically shown in hexadecimal strings.

The first byte indicates the byte order for the data:

  • 00 : big endian
  • 01 : little endian

The second part is an integer for the geometry type, as described below:

Geometry types, and WKB integer codes
Type 2D Z M ZM
Geometry 0000 1000 2000 3000
Point 0001 1001 2001 3001
LineString 0002 1002 2002 3002
Polygon 0003 1003 2003 3003
MultiPoint 0004 1004 2004 3004
MultiLineString 0005 1005 2005 3005
MultiPolygon 0006 1006 2006 3006
GeometryCollection 0007 1007 2007 3007
CircularString 0008 1008 2008 3008
CompoundCurve 0009 1009 2009 3009
CurvePolygon 0010 1010 2010 3010
MultiCurve 0011 1011 2011 3011
MultiSurface 0012 1012 2012 3012
Curve 0013 1013 2013 3013
Surface 0014 1014 2014 3014
PolyhedralSurface 0015 1015 2015 3015
TIN 0016 1016 2016 3016
Triangle 0017 1017 2017 3017

Each data type has a unique data structure, such as the number of points or linear rings, followed by coordinates in double-precision floating-point format (double).

For example, the geometry POINT(2.0 4.0) is represented as: 000000000140000000000000004010000000000000, where:

  • 1-byte integer 00 or 0: big endian
  • 4-byte integer 00000001 or 1: POINT (2D)
  • 8-byte float 4000000000000000 or 2.0: x-coordinate
  • 8-byte float 4010000000000000 or 4.0: y-coordinate

Transformations [edit]

A WKT format is defined to describe the transformation methods and parameters used to convert coordinates between two different spatial reference systems.

Below are two examples of WKT transformation descriptions.

PARAM_MT["Mercator_2SP", 
    PARAMETER["semi_major",6370997.0], 
    PARAMETER["semi_minor",6370997.0], 
    PARAMETER["central_meridian",180.0], 
    PARAMETER["false_easting",-500000.0], 
    PARAMETER["false_northing",-1000000.0], 
    PARAMETER["standard parallel 1",60.0]]
PARAM_MT["Affine",
    PARAMETER["num_row",3],
    PARAMETER["num_col",3],
    PARAMETER["elt_0_1",1],
    PARAMETER["elt_0_2",2],
    PARAMETER["elt 1 2",3]]

RDBMS Engines that provide support [edit]

APIs that provide support [edit]

Protocols that provide support [edit]

Variations [edit]

  • EWKT and EWKB – Extended Well-Known Text/Binary – A PostGIS-specific format that includes the spatial reference system identifier (SRID).[2] For example: SRID=4326;POINT(-44.3 60.1) to locate a longitude/latitude coordinate using the WGS 84 reference coordinate system.
  • AGF Text – Autodesk Geometry Format – An extension to OGC's Standard (at the time), to include curved elements; most notably used in MapGuide.[3]

See also [edit]

References [edit]

  1. ^ GIS and Spatial Extensions with MySQL
  2. ^ http://postgis.refractions.net/docs/ST_GeomFromEWKT.html
  3. ^ http://www.osgeo.org/files/mapguide/docs/2.0/da/dc0/group___agf_text.htm

External links [edit]

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