Well-known text representation of geometry

From Wikipedia, the free encyclopedia
Jump to navigation Jump to search

Well-known text (WKT) is a text markup language for representing vector geometry objects on a map. A binary equivalent, known as well-known binary (WKB), is used to transfer and store the same information on databases. The formats were originally defined by the Open Geospatial Consortium (OGC) and described in their Simple Feature Access.[1] The current standard definition is in the ISO/IEC 13249-3:2016 standard.[2]

Geometric objects[edit]

WKT can represent the following distinct geometric objects:

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 that 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.

The OGC standard definition requires a polygon to be topologically closed. It also states that if the exterior linear ring of a polygon is defined in a counterclockwise direction it will be seen from the "top". Any interior linear rings should be defined in opposite fashion compared to the exterior ring, in this case, clockwise.[3]

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

The following are some other examples of geometric WKT strings: (Note: Each item below is an individual geometry.)

GEOMETRYCOLLECTION(POINT(4 6),LINESTRING(4 6,7 10))
POINT ZM (1 1 5 60)
POINT M (1 1 80)
POINT EMPTY
MULTIPOLYGON EMPTY
TRIANGLE((0 0 0,0 1 0,1 1 0,0 0 0))
TIN (((0 0 0, 0 0 1, 0 1 0, 0 0 0)), ((0 0 0, 0 1 0, 1 1 0, 0 0 0)))
POLYHEDRALSURFACE Z ( PATCHES
    ((0 0 0, 0 1 0, 1 1 0, 1 0 0, 0 0 0)),
    ((0 0 0, 0 1 0, 0 1 1, 0 0 1, 0 0 0)),
    ((0 0 0, 1 0 0, 1 0 1, 0 0 1, 0 0 0)),
    ((1 1 1, 1 0 1, 0 0 1, 0 1 1, 1 1 1)),
    ((1 1 1, 1 0 1, 1 0 0, 1 1 0, 1 1 1)),
    ((1 1 1, 1 1 0, 0 1 0, 0 1 1, 1 1 1))
  )

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 next 4 bytes are a 32-bit unsigned 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
Circle 0018 1018 2018 3018
GeodesicString 0019 1019 2019 3019
EllipticalCurve 0020 1020 2020 3020
NurbsCurve 0021 1021 2021 3021
Clothoid 0022 1022 2022 3022
SpiralCurve 0023 1023 2023 3023
CompoundSurface 0024 1024 2024 3024
BrepSolid 1025
AffinePlacement 102 1102

Each data type has a unique data structure, such as the number of points or linear rings, followed by coordinates in 64-bit double numbers.

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

Format variations[edit]

EWKT and EWKBExtended Well-Known Text/Binary 
A PostGIS-specific format that includes the spatial reference system identifier (SRID) and up to 4 ordinate values (XYZM).[4][5] For example: SRID=4326;POINT(-44.3 60.1) to locate a longitude/latitude coordinate using the WGS 84 reference coordinate system.
AGF TextAutodesk Geometry Format 
An extension to OGC's Standard (at the time), to include curved elements; most notably used in MapGuide.[6]

Software support[edit]

Database engines[edit]

APIs[edit]

Protocols[edit]

See also[edit]

References[edit]

  1. ^ Herring, John R., ed. (2011-05-28), OpenGIS® Implementation Standard for Geographic information – Simple feature access – Part 1: Common architecture, Open Geospatial Consortium, retrieved 2019-01-28
  2. ^ Information technology – Database languages – SQL multimedia and application packages – Part 3: Spatial (5th ed.), ISO, 2016-01-15, retrieved 2019-01-28
  3. ^ See the OGC Implementation Specification for geographic information – Simple Feature Access, section 6.1.11.1. http://www.opengeospatial.org/standards/sfa
  4. ^ https://github.com/postgis/postgis/blob/2.1.0/doc/ZMSgeoms.txt
  5. ^ http://postgis.org/docs/ST_GeomFromEWKT.html
  6. ^ http://e-logistic-plans.gdfsuez.com/mapguide/help/webapi/da/dc0/group___agf_text.htm
  7. ^ Solr GEO support
  8. ^ Well-Known Text (WKT) Input Type in Elasticsearch documentation
  9. ^ https://docs.marklogic.com/guide/search-dev/geospatial
  10. ^ http://docs.memsql.com/docs/geospatial-guide
  11. ^ Well-Known Text (WKT) Format, MySQL documentation
  12. ^ https://neo4j-contrib.github.io/spatial/
  13. ^ H2 create spatial index documentation
  14. ^ "HP Vertica 7.1.x Release Notes". my.vertica.com. Retrieved 2018-03-21.
  15. ^ https://www.voltdb.com/company/press-releases/voltdb-adds-geospatial-query-support-industrys-innovative-fast-data-platform/

External links[edit]